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January 31, 2008

HMT Forecast 1/31/2008 19:43 UTC

This morning we are seeing the start of the next system to impact the ARB with light precipitation to the north, BLU reporting 25F and S- and Truckee with 23F and cloudy. Most locations in the ARB are reporting cloudy conditions this morning.

Offshore we have two major circulation systems in the satellite water vapor, one at 50N 169W and the second at 48N 133W, the latter is responsible for circulating the current moderate to weak moisture plume to the ARB south of its location. Last night's GFS has this plume containing about 2.5 cm TPW sweeping by the ARB by 6UT 2/1. This morning's SSMI measurements at 1/31 11ut shows this to be a reasonable estimate of TPW with a TPW max offshore ranging up to about 3cm. The GFS has a second plume of about equal intensity as the one we are looking at this morning passing by the ARB at about 15UT 2/3. Each of these is about equal in intensity according to the model, but the second appears to sweep by the region at a faster clip reducing the precipitation.

The system that is upon us today begins with mostly southerly winds ranging from 10 kts increasing to a peak of 50 kts at 03UT 2/1. It appears that we have a trof passage about 09UT 2/1 and this coincides with a sharp decrease in the precipitation rate which has a peak of 0.63 inches in 3h at 06 UT 2/1. Total precipitation for this first event appears to be about 2.13 inches (NAM) 1.72 inches (GFS). The HMT high res ensemble precipitation amounts forecast lower amounts for this system with a regional total closer to the GFS of about 1.5 inches and more to the north of the ARB and less to the south. SREF probabilities have the maximum probability of precipitation at 03UT 2/1 (about 3h earlier than this morning's NAM) and it has the event ending at about the same time (18UT 2/1). Temperatures for this episode appear mostly cold with the freezing level starting at about 900 hPa (3400ft) and remaining there throughout the event.

The next precipitation event for the area is forecast to begin 00UT 2/3 (Saturday evening local time) about 27 hours after the current episode ends. It is shorter in duration ending about 21UT 2/3 late Sunday. Temperatures for this event appear a bit warmer overall, but with freezing level starting colder than the current event, at about 950 hPa (2300ft) and rising in altitude to about 850 hPa (5000ft) at 15UT 2/3. It appears that for the mountain portions of the ARB, both this and the prior system will be snow events. The event ends with trof passage about 21UT 2/3. SREF puts the maximum of the event at about 06 UT 2/3 with the NAM showing this later with two consecutive 3h periods of 0.5 inch accumulations (liq) at 12 and 15UT 2/3. The NAM indicates this system will bring a total of about 2 inches of precipitation to the region (NAM) (1.8 GFS) maybe with a heftier punch than the current episode now starting. This is borne out in the winds which start from the SW at 30 and increase to 55kts over a deeper extent of the atmosphere peaking about 12UT 2/3. Of the two events, I would say this second one, though bringing less overall precipitation, might be more interesting of the two to study even though it is of shorter duration. I say this because the winds are at a more favorable direction for orographics and the wind speeds are greater.

After this second episode ends at 21UT 2/3, another moisture plume possibly passes through the area around 06UT 2/6, but the GFS shows the ARB may just be grazed by this event, mainly passing to its north. A strong plume then is forecast to form out west of the coast at 11UT 2/11, but appears to remain offshore. If something were able to bring this into the region, it might prove to be a very good case, but as of this time, it remains offshore. For the most part the region appears to remain dry at least as far out as 384h (2/16) at which time the GFS ensemble hints at possibly some new precipitation for the area.


Dan Birkenheuer ESRL/GSD

January 30, 2008

HMT forecast discussion for Wednesday 30 January 2008 (entered 20z/Thu)

Mean low-amplitude upper level ridge near 150W with fast-moving systems heading across the Pacific in a zonal flow pattern. Some subtropical moisture lurking near 180W, then east of this is a weak upper low near Hawaii, with another area of tropical moisture from Hawaii eastwards near and south of 20N. The systems moving quickly across the Pacific are tapping some of the moisture west of Hawaii through the one this weekend, but only as they move across. So given their limited moisture and rapid speed ending up with systems Thu-Fri and Sat-Sun that will be cold in the ARB with snow at the BLU radar site, and precip perhaps approaching max amounts of 3" for the first event and probably less for the second. Maybe a tap into the moisture east of Hawaii for a possible wave midweek next week, but by then the upper level ridge is forecast to build a bit and shift eastward slightly, sending this system more to the north and east of the ARB in the current model runs. Possible changes though towards the following weekend (9-10 Feb) and longer term.

The current system is ending with the ESRL gages showing the following totals: 1.5" at Big Bend and 1.2" at Alta, not sure of the 0.6 or so registered at BLU (telcon suggested this may be an unreliable amount with a heated tipping bucket gage). The next system is still on pace to start about 12z/Thu and go until about 18z/Fri. The NAM is pretty moist but with a strong terrain gradient, putting out a total of 2.5" at BLU and about 0.5 at SAC, vs. 0.6 at SAC and 2.25 at BLU from the 12z/GFS. In the 12z/ECMWF 1.2" at BLU and 0.5" at SAC. So a stronger system than the current one, but still cold, with all snow at the BLU radar site, though likely a bit warmer than the current one with the high on Thursday near 32 according to the latest forecast from the SAC WFO. HPC forecasts have a max close to 3" just to the nw of the ARB, in a similar spot to where the NAM is predicting over 3" of precip. About 2-2.5" max in our ensemble mean forecasts from the 3 km runs, and like the NAM, 1-2" more to the nw of the ARB in the higher terrain there.

The next system in line starts up about 18z/Sat and winds down the next day around 21z/Sun, with the GFS putting out ~1.4" at BLU and 0.6" again at SAC. In the 12z ECMWF 1.1" at BLU and 0.5" at SAC. So not quite as strong in the current forecasts as the Thu-Fri storm, and also cold again, with the snow level below BLU. Barring some change this would probably be considered not appropriate for an IOP. HPC is forecasting far less with this event, only 0.66" max amounts in the ARB.

Longer range into next week still looking like the upper level ridge inches a little closer to the West Coast, sending the next wave more to the east, but brushing the ARB with some precip perhaps on Wed/6 Feb. Note though that this system is forecast to have a possible tap into the moisture that is now east of Hawaii, and so if this should come in a little farther south, could be a storm of interest in the ARB, but right now this does not look to be the case.

Looking further into the longer range, by 240h (Sat/9 Feb) the ridge breaks down with a very zonal flow across the Pacific, which could lead to warmer systems hitting the West Coast. While this could still result in fast-moving systems affecting the ARB, perhaps as early as 9-10 Feb, the temperature with these would be warmer, so a better chance of getting rain down below the BLU radar site. There is a considerable amount of spread in the 10-15 day longer range period (ending Fri/15 Feb), however.

ed szoke NOAA/CIRA/GSD

January 29, 2008

HMT forecast discussion for Tuesday 29 January 2008 (entered 20z/Tue)

Now that the upper level trough/low that was lurking off the California coast all of last week has moved inland the pattern has changed to a more zonal flow across the Pacific with now a low-amplitude upper level ridge well off the West Coast north of Hawaii, with fast-moving systems that approach the ARB from the wnw with limited moisture. A very weak closed low has been left behind near Hawaii, and associated with this is a pool of tropical moisture east of the upper low and Hawaii. The other persistent tap to the tropics is near the Dateline, with each of the fast-moving waves tapping into some of this moisture, but then losing this connection as they approach the West Coast. The one forecast for Thu/Fri of this week is doing this right now as the wave itself moves just south of the Aleutians. All of the upcoming events will be fast-moving and marginal for an IOP, and all look at this point to be all snow at the elevation of the BLU radar. The most promising wave in terms of total precip may be the Thu-Fri one of this week, with more spread on the weekend storm, and then even more spread for anything of substance at all for next week.

Precip from the next system just now beginning to move into the ARB, but it is a colder storm with the 15z temp at KBLU at only 22F. The freezing level is quite low, near 2000 ft or so, rising perhaps to 4500-4800 ft or so by late Tue before droping again, and this will be all snow at BLU. The 00z GFS predicts 0.9" at BLU with the precip ending by 18z/Wed with 0.3" at SAC, and the latest 12z/GFS is similar in both timing and precip. Less in the 00z/ECMWF and the 12z/ECMWF with 0.23 at SAC and 0.5-0.6" at BLU with similar timing to the 00z/GFS. More at higher elevations in the 12z/NAM with 1.25" at BLU but only 0.25" at SAC. The ensemble mean of our 12z 3 km runs has a few points up to an inch for the 24-h ending 12z Wed.

The next wave follows quickly behind the current storm. In the 00z GFS light precip begins in the higher elevations of the ARB about 15z/Thu, and really doesn't pick up until 00z/Fri 1 Feb and continues until about 15z/Fri though lingering light precip longer, with 1.4" at BLU and 0.4" at SAC. The 12z/GFS has 1.7" at BLU and 0.5" at SAC with similar timing to last night's run. More also than today's storm in the 00z/ECMWF run with 0.65" at SAC and 1.3" at BLU for totals with similar timing to the GFS. The 12z ECMWF is quite similar to the 00z run, with a heavy precip period 00-12z/Sat. Even more in the 00z/Global Canadian model with about 2" max in the ARB and the precip lingering longer on Friday. The 12z/NAM has similar timing to the GFS with ~1.75" total at BLU, and about 0.5" at SAC. The ensemble mean of our 12z 3 km runs has areas of 1-1.5" amounts in the ARB for the 24-h ending at 12z/Fri/1 Feb, so also better than the current event, with individual runs around 2" by 12z/Fri. Precip begins in our 3 km runs as early as 09-10z/Thu in the higher elevations of the ARB. Looking at the NAM BUFR soundings this system also starts out cold with the freezing level at 3500 ft at SAC, rising to around 5000 ft or so by Thu evening (though lower at BLU with upslope cooling), so again barring something unexpected this system should also be all snow at the radar site at BLU. WSW flow picks up to about 50 kts at 700 mb by late Thu which provides good orographic forcing for the higher elevations of the ARB. There was discussion as to whether this event would be IOP-worthy during the telcon, with general agreement on the scenario above, but Dave felt that the snow could get wet enough to potentially cause radar problems, and being it was a fast-mover with limited precip, would not be of interest for an IOP.

The 3rd wave is stronger with the precip in the 00z GFS beginning 12-15z/Sat in the ARB. The wave takes a perfect track in the 00z/GFS with the heaviest precip 18z/Sat to 09z/Sun, and precip ending totally later on Sunday/3 Feb. Totals from the 00z/GFS for this event are 0.7" at SAC and 1.5" at BLU. There is much less though in the ECMWF 00z run with only about 0.25" at BLU, with the system not quite as concentrated or tracking as perfectly as in the GFS. Likewise the 00z/Global Canadian model is much weaker, even more than the EC run with a more southern track. Right now this system also looks to be snow at BLU unless it deepens more. The 12z/GFS just in is now following this much weaker trend, with a more southern track to the upper level wave and very little precip in the ARB from this event, so quite a change from last night's run. Consensus on the telcon was that this system will probably now be weaker than the Thu-Fri event.

Following the weekend storm, in the 00z/GFS the next wave dives a little farther inland Tue-Wed/5-6 Feb as the upper level ridge builds off the West Coast, so most of the precip misses the ARB. Similar in the 12z/GFS, with just a tad of precip Mon/4 Feb and Wed-Thu/6-7 Feb as the waves pass more to the east into a mean trough position near the central CONUS. Then dry through most of the rest of the 15-day forecast as fast-moving waves continue to miss just to the north of the ARB until Day 15 (13 Feb) when another fast-moving wave takes a more southerly track In the 12z/GFS also dry through 13 Feb but with the pattern changing to a deeper trough over the Pacific. It would not take much of a shift, however, to keep the ARB in the track of fast-moving waves from the nw, and this is seen in the 00z ECMWF and Global Canadian models with a little more precip for a couple of waves after the weekend (2-3 Feb) storm, but right now the chance of any of these waves being IOP-worthy would not be great. Likewise, while there is variation in the ensembles, nothing to indicate any huge event in the longer range.

ed szoke NOAA/CIRA/GSD

January 28, 2008

HMT forecast discussion for Monday 28 January 2008 (entered 2030z Monday)

An interesting day and evening on Sunday in the ARB. At BLU managed to get about a half inch of rain ~08-16z with the temperature in the mid-30s with what looked like the western portion of the band of subtropical moisture that had been moving rapidly northward on Saturday. Then just enough cooling to change the precip to snow by 17z and never changed back with temps hovering near freezing into the evening. In fact at 05z KBLU METAR rose to 33F with wind 190 gusting to 40 kts (peak gust was 200 at 45 kts at 07z). Rapid cooling after 07z at KBLU with the approach and passage of the system from the northwest, and this morning (15z) the temp is down to 21F with a light sw wind and S- with showery conditions on the radar. At KBLU METAR 24-h total of 1.10", with 0.25" as of 12z/Sun, for METAR total of 1.35". This compares to the ESRL gage amount of ~1.3" after 14z/Sun (note sure of the amount before then, think it was around 0.5"; note this is the tipping bucket gage so could be snow issues). According to the automated snow gage, about 6" of snow (will be interesting to see how this compares to obs). There is a nice signal of the northern system passage with a rapid temp fall beginning at 08z. Some other amounts from the ESRL gages: at Alta just under 2" with most between 19z/Sun-08z/Mon; at Big Bend about 2.3" on the hot plate gage between 18z/Sun and 08z/Mon, then since light precip; also a nice cold front signal at 08z. It was a close call for getting more rain at the radar site but just enough cooling between the first wave of subtropical moisture and the precip with the approaching remnants of the old upper low to change things over to heavy wet snow. Dave indicated in the telcon about 6" of heavy wet snow at BLU radar site, max precip up to 3" at Big Bend with 12" of snow, with Alta about 2.25" total. So local and other models not too far off, and pretty decent forecast from the ensemble mean.

Precip is now down to light snow showers that will continue today. A series of fast-moving systems will reach the ARB this week, all at this point potential IOPs (after Tue one) but also potentially not meeting the criteria. Each wave taps a bit of the subtropical moisture in the south-central Pacific as they move eastward to varying degrees. The next system still on tap for ~18z/Tue to 18z/Wed is a fast-mover with about an inch max precip in the 00z GFS and a cold system overall with the snow level below BLU radar site. In the 12z/NAM about 1-1.25" max in the ARB for this event, timing similar to 00z GFS. The GSD 3-km runs have up to 2" for this event but this may be on the high side.

Later in the week the next in line in the series of fast-moving storms brings precip beginning ~18z/Thu/31 Jan and ending about 21z/Fri/1 Feb. Slightly warmer at first with this one with maybe some rain into BLU Thu night, but too early to tell. The 12z GFS has total precip of about 1.4" at BLU and 0.9 at SAC for this one, while the 00z ECMWF has ~1.2" with this event and with similar timing, and the 12z EC close to an inch at BLU. NOGAPS and Global Canadian model solutions look similar for this event and generally agree with the GFS and EC solutions. It appears this has a little better tap into the Pacific as it moves eastward midweek.

Another system right behind the Thu-Fri one starts precip by about 06z/Sat/2 Feb, with more precip (~1.5" max on the 12z/GFS) through midday Sunday. Right now the 0C line at 850 mb gets close to the ARB late on Saturday, so probably a snow event at BLU but will need to watch as we get closer. This event is much weaker than the GFS in the 00z ECMWF forecast, and in the latest 12z ECMWF run just in BLU only gets about 0.25" with this storm, so a big difference between this run and the GFS, even though it is actually a deeper upper low in the EC solution. The 12z Canadian Global model looks similar to the GFS for this storm and the NOGAPS somewhat more like the EC run. The 12z GFS ensembles do show some variation with this wave, with some members looking to drop the system a bit farther south as in the EC solution.

Longer range the 00z deterministic GFS has it dry after the weekend storm through 384h or 12 Feb with upper level ridge inching closer to the West Coast and systems missing to the north, then towards the end of the run more zonal flow. As noted yesterday though some variation on this ridge position so not all the ensemble runs are dry. For instance, the 00z ECMWF has a potential system on Wed/6 Feb that the GFS keeps north and east, and this is true of the 12z ECMWF also. Overall though, there seems to be a little better agreement on a drier forecast in the long range, at least in the 00z ensembles, then there was yesterday. Having noted that, the 12z GFS ensembles show a little more variation after this coming weekend, though the majority of members do keep it generally dry in the ARB.

ed szoke NOAA/CIRA/GSD

January 27, 2008

HMT forecast discussion for Sunday 27 January 2008 (entered 20z/Sunday)

Highlights: brief shot of rain last 6h or so at BLU with about a half inch so far as it looks like we may have gotten some of the southern plume. Temporary lull until the remnants of the upper low come this afternoon, with heavy precip likely. Then northern stream wave passes ~09z/Mon preceded by heavy precip that will be wet snow going to lower density snow towards morning and tapering off to snow showers that will likely continue all day on Monday at BLU. IThe 18z obs at BLU is at 34 and they went over to snow, confirmed by the radar crew that reports almost an inch on the ground (and, unfortunately, now accumulating on the radar). Heaviest precip period at BLU should be ~21z/Sun through 06z/Mon. While only limited cooling is expected this afternoon as the first wave approaches, the consensus from the telcon was that the precip at BLU will now stay as snow, heavy wet snow with the temp near or just above freezing. WFO SAC indicated 1 to 2 feet of snow would be likely by Monday morning at the radar site. Issues with the heavy wet snow on the radar may force the radar operations to shut down later this afternoon.

A review of this morning: rain started about as thought, amounts not substantial so far, but SAC did report 0.25" between 06-12z, while at BLU also 0.25" but in the form of rain, with a burst of heavier rain around 12z (with another 0.22" since 12z). Currently (1630z) at BLU +2C (35) with the wind varying from S to SSE at about 10 kts. The ESRL BLU tipping bucket gage (hotplate not available) has received just over a half inch total, and the temperature has been on a very slow downward trend with the current reading at 1C. At the Big Bend site by 18z about 0.8" with the temp now approaching 0C (elevation 1739 m/5704 ft). The burst of rain this morning was likely the western side of the southern moisture plume that we have been mentioning the last couple of days, the one that is pounding the LA area. The bulk of this plume is heading off to the east and south of the ARB, and next in line will be the remnants of the upper low. This precip is just reaching the Central California coast south of SFO as a concentrated curl of heavy rains, with the circulation still off the coast and lifting to the northeast. The 500 mb trough axis from the offshore wave will pass the ARB near 00z/Mon, while the next wave now offshore but approaching from the northwest still looks to pass around 09z/Mon. The models all show precip increasing in the ARB gradually the next few hours then really picking up mid to late afternoon with the remnants of the cutoff low coming through, then more again this evening as the northern wave approaches. The tricky part will be when it goes to snow at BLU; max mix at times or even be snow at times this afternoon as the warmest temps have likely passed at this point, but could remain mostly rain or mixed until closer to 22z or so when the remnants of the upper low get close. The really strong cold advection should come by about 06z as the northern wave approaches and passes. Ideally it will be rain this afternoon and if so could be some heavy totals; our ensemble mean has 1.5-2" from the 06z run ending at 06z/Mon at BLU, with the latest 12z ensemble 24-h forecast maxing in the 2-3" range in the ARB, a little more in the 12z NAM, and 3-4" with point values higher by 06z/Mon in individual 3 km runs from 12z and in the 06z NCEP hi-res window runs. So it should be an active afternoon, could be troublesome for the radar if it goes to snow early, and certainly will go to snow by later in the day. Heaviest precip period at BLU should be ~21z/Sun through 06z/Mon, then going to more orographic cold snow showers that may not totally end at BLU before the next system arrives.

The next wave is still on track approaching from the nw and will bring about 24h of the main period of precip to the ARB from 18z/Tue to 18z/Wed. The 12z NAM has 1-1.5" total for the ARB, the 12z GFS not quite as much, rough total from the 09z SREF ensemble mean is about an inch max. Appears to be around an inch in the Global Canadian model as well. A colder system with the snow level staying well below BLU through the event. This is the beginning of a series of fast-moving waves, with an upper level ridge well off the West Coast and CA in strong northwest flow with waves digging into the west-central CONUS. The next wave on the 12z GFS passes near 12z/Fri, then another follows with a trough passage near 06z/Sun, but that MAY be the end for awhile as the upper ridge is forecast on the GFS to then shift to near CA keeping waves farther to the north. The GFS has 0.5-1" or so max for the ARB with the Tue-Wed and Sat-Sun systems, but remains more robust with 1-2" for the Friday/1 Feb wave, and this has been consistent in the GFS forecasts for this system which is still a fast-mover but may be a little warmer than the others. The Global Canadian model is also producing more precip with the Friday storm. A look at the U of Hawaii GFS Pacific PW forecast shows that the Friday system gets a slightly better tap into the big area of tropical moisture west of Hawaii as the system crosses the Pacific. Note that when it hits the California coast on Friday this tap is no longer a direct one.

Although the 12z GFS deterministic forecast indicates the Sat-Sun/2-3 Feb wave might be the last one through the rest of the forecast period ending 12 Feb, there is considerable uncertainty on this when looking at the 12z and 00z ensembles. This is because it only takes a slight shift of the upper level ridge to the west to keep the ARB vulnerable to continuing to be in the track of fast-moving waves coming across the Pacific and then down the east side of the ridge. None of the members that brings shots of precip to the ARB ever has a prolonged system, so at this point unless something changes we would be looking at relatively fast-moving waves coming out of the wnw. However, one positive is an eastward shift in the plume of moisture across and then east of Hawaii by the 2nd week in February, which would bring it closer to the CA coast.

ed szoke NOAA/CIRA/GSD

January 26, 2008

HMT forecast discussion for Saturday 26 January 2008 (entered 22z/Sat)

There was some precip across the ARB last night as the persistent band made inland progress but now the upper low has sunk south and elongated, resulting in a retrogression of the band to mostly off the coast and near the coast north of SFO. So this morning things are dry in the ARB; BLU at 15z is 35 with a SE wind at only 5 kts and 24-h precip of 0.40", while SAC is 52 with a light SE wind and 24-h precip of 0.46". It looks like any digging should be over as the strong jet aloft has rotated now around the upper low. There appears to be wave forming se of the upper low near 30N/130W with associated cloud enhancement. Another area of enhanced moisture is associated with the subtropical jet and is approaching LAX/SAN where Flash Flood Watches have been hoisted for tonight and Sunday. The models this morning are consistent with last night's runs overall and generally with the picture from yesterday, except perhaps drier today with the precip not beginning until late tonight. In brief summary (details below), the upper low gradually weakens and ejects to the ene, passing the ARB on Sunday at 700 mb between 18-21z with the remnants of the band of precip now off the coast hitting the ARB. Ahead of this though could get into warmer temps aloft and the northern fringe of the southern moisture. Then the next wave quickly follows and enhances precip late Sunday into Monday with falling temps and a transition to cold advection precip. Still uncertainties of course, but I think Sunday should prove to be interesting.

As discussed in the telcon, there was agreement that the start of the main precip in the ARB will be later than thought yesterday, about 09z/Sun at BLU, but there were a few optimistic signs in regards to the ARB getting into the subtropical southern moisture plume tomorrow. These included a more favorable looking 12z ECMWF run that pulled the moisture northward in a more concentrated fashion than the GFS, and the movement observed on satellite of the moisture itself. Here is an update as of 22z/Sat; the 18z run of the NAM is a little slower now in bringing the offshore upper low past the ARB on Sunday, although still has trouble bringing a lot of the southern plume moisture into the ARB. It has about the same timing as the 12z run with the northern wave, passing the ARB about 09z/Mon. Precip begins near BLU about 07-08z/Sun, and increases after 15z, with ~0.5" at SAC and almost an inch at BLU by 00z/Mon, then by 12z/Mon 2.25" at BLU and ~0.9 at SAC. It gradually diminishes but never completely stops in the higher elevations of the ARB, with a total near 3" at BLU by 00z/Tue, then almost ending but picking up by 15-18z Tue with an inch or so new by 00z/Wed with the next system (which would be still ongoing at that time). The 18z GFS has similar timing to the 18z NAM, but more precip (~1.25 at both BLU and SAC) by 00z/Mon, even though it looks like most of the southern plume moisture stays to the south and east. About 2.25 at BLU by 12z/Mon and 1.4 at SAC, then only light precip Mon with a break until about 18z/Tue. The next system comes in as 2 shortwaves, with precip ending by 00z/Thu. Totals for the Tue-Wed wave are about 0.5 at SAC and 1.0 at BLU, so somewhat marginal, and a colder event. The latest satellite loops suggest more moisture from the southern plume is heading straighter to the north than perhaps suggested by the models, as noted on the telcon by WFO RNO. Finally, the timing in the latest 18z runs is supported by our local 18z runs as well, with precip forecasts of over 4" by 12z/Mon within the ARB. The freezing level in the 3 km run hovers around 800 mb or 2 km until a sharp drop about 02z/Mon.

Some details from the 12z NAM. SSW 700 mb flow increases dramatically ahead of the remnants of the upper low wave it looks like some of the southern plume of moisture (the one hitting LAX tonight) briefly makes it into the ARB on Sunday ahead of the wave, which itself will carry what is left of the band of precip now off the coast. Together this should yield an intense period of precip (unless Dave Reynold's 40 N rule holds, but since it is an open wave when it comes through think we are okay) from ~15z Sun through ~06z Mon. Kind of complicated as the first part could be delayed IF the southern moisture never makes it far enough north. Two parts to the moisture after this, one with the remnants of the upper low, the other with the next wave which quickly passes after this. The first trof passage at 700 mb as noted would be ~20z in the ARB, with ssw flow at 700 mb forecast to increase to 60 kts with the temp rising to about -6C (~+2C at 850 mb) at 18z Sun. Not much cooling and only a slight wind shift (more a decrease) with the first wave passage (remnants of the upper low), but then more cooling begins by 00z/Mon with the next wave approaching, and this passes the ARB at 700 mb at roughly 09-12z Monday. In the NAM precip begins at BLU ~09z/Sun, increasing by ~15z, peaking 21z/Sun-06z Mon, but with a secondary max at BLU in the colder air on Monday morning, and precip does not really end with the next system picking things up again by 18z Tue and continuing through the end of the NAM on 00z/Wed (with good orographic flow but colder, -12 or so at 700 mb). Precip totals: by 00z/Mon 1.1 at BLU (2.5 just to the nw) and 0.45 at SAC, then by 12z Mon total precip 2.7 at BLU and .77 at SAC, but 3.25 at BLU by 18z Mon and then it just slowly piles up, 4.2 by 12z/Tue (but over 5" nearby), and then increases on Tue, with nearly 5" by 00z/Wed. This run would suggest no real solid break between the events, a definite snow event for Tue into Wed, and perhaps a brief changeover to rain on Sunday before going to snow Sunday night at the radar site.

In the 00z ECMWF run there is a break between the 2 systems with about 12h of now precip from ~06z-18z Tue, but then it starts up again for about 18-24 h but with only about 0.5" or so in the EC model for the Tue-Wed wave. Then the EC has another system Fri-early Sat with 0.5-1", then another fast mover Sunday night-Mon (3-4 Feb) with about the same amount. The 12z EC is similar, with yet another system lined up for about Wed/6 Feb. The 12z GFS is similar with the break, with not much from 21z/Mon through 18z Tue, then similar in structure and amount to the EC Tue-Wed, so very marginal in both these global runs. The Friday event is juicier in the GFS with over an inch at BLU but is still a shortwave trough that is relatively fast moving so too early to tell.

In terms of the immediate storm, the GFS is similar in timing to the NAM, but most of the precip is done by 12z Mon. The GFS does not get much of the southern plume into the ARB, but still has the heavier precip on Sunday and sunday night. Totals are not quite as high as the NAM but still over 2" at BLU by 12z Mon, 3" to the nw of the ARB. Our local 3 km runs have precip beginning near BLU by 09z, then a nice burst of heavy echoes moving up from the south by 15z Sun, with 3" max amounts in the ARB by the end of Sunday. So a lot depends on the amount of the southern plume getting into the system, but all in all still looks like a worthy day to sample on Sunday.

ed szoke NOAA/CIRA/GSD

January 25, 2008

HMT forecast discussion for Friday 25 January 2008 (entered 2030z/Friday)

A complex situation for the next 72 h with many uncertainties, as discussed in the telcon. Bottom line is that IOP6 was called based on the potential for a significant period of precipitation across the ARB beginning late Saturday or Sunday morning and continuing into Monday, with the chance that the snow level could rise to above Blue Canyon on Sunday with increasing southerly to sw flow.

The upper low that has persisted all week off the central California coast has strenghthened and retrograded this morning as a new shortwave has dug southward on the east side of the upper level ridge centered near 155W. The current upper low sits west of SFO near 37N/130W with a slight drift to the west. The result is a new round of heavy precipitation that has moved into Northern California this morning and is pushing towards the ARB, but stalling JUST to the west of SAC at this time and may not make much further eastward progess over the next 24-36h. This band of heavy precip is relatively narrow, with the heaviest precip at 20z over SFO where there is even some lightning, then extending offshore to the wsw. The upper low is forecast to elongate off the coast over the next 24-48 h and then move inland ahead of the next wave which is at this time forecast to progress across Northern California late Sunday into Monday. The combination of these systems could bring some pretty heavy precipitation to the ARB very late Saturday into Monday; certainly IOP-worthy amounts. Following these 2 systems the pattern is forecast to change to a much more progressive one with a series of storms beginning ~Tue/29 Jan and conintuing into next weekend. Each system appears to tap into some tropical moisture, but also is relatively fast-moving. The details are below, but I think it should be noted that there are some significant uncertainties, in my view, in how this system will evolve, with the possibility that a heavy precip period could occur late Saturday through early Monday.

There are several questions worth considering that were discussed in the conference call. First, up to this point, the upper low that has been hanging off the California coast this week has not had any real tap into an extensive moisture plume. There is a chance that could change over the weekend, as there is currently a surge of high PW air moving northward towards the Southern California coast. This does not factor into the precip for the next 36h, but even so, the latest 12z NAM produces nearly 6" of rain north of SFO and up to 5" of accumulated precip just north of the ARB, but with a sharp gradient and thus only 1.05" at SAC and 1.25 at BLU for the 36-h ending 00z/Sun. The latest trend in what is actually occurring suggests this may be an over-forecast for this period, with the ARB staying JUST east of the heavy precip. The current snow level is below BLU, with KBLU this morning at 32 in S- and a light ese wind. The snow level is forecast to remain below BLU by WFO SAC through the weekend, but as was noted in the telcon there is the possibility it could rise on Sunday. As the upper low drops farther south and elongates the flow becomes more southerly and increases in strength. In the NAM 12z analysis at KBLU 850 mb flow is se at 10 kts with a temp of -2C, at 700 mb -11C with a ssw wind at 35 kts. By 00z/Fri (this evening) the 700 mb flow is forecast to increast to ssw at 55 kts, with a temp rising to ~-7C, at 850 mb the values are +2C and ssw-s at 20 kts. Then hovering between 0 to +2 for the next 24 h at KBLU with s-sse 850 mb winds of 20 kts (but much stronger to the west, up to s at 50-60 kts at 850 mb at KSAC), and at 700 mb -7 to -8C at KBLU with ssw winds 50-55 kts. The wild card comes into the Sunday period. The aforementioned moisture plume now far to the south is working northward on Saturday with the increasing southerly flow, but is forecast to stay just east and south of the ARB on Sunday as the upper low is predicted by the NAM to weaken rapidly and push inland as an open wave, passing the ARB about 18z on Sunday as the next wave approached from the north but takes an inland track. If this upper low is slower to move inland then i would think the surge of warmer and more moist air would move farther norhtward on Sunday with the potential for some really heavy precip totals in the ARB. This whole pattern has been hard for the models to predict certainly in the long range. Some of the SREF members from the 09z run are in fact slower moving the trough inland though they all eventually do....something to discuss would be how confident we all are that this will occur. The SREF mean still has 3-4" total for the ARB area by 00z/Tue with up to 7" on the coast. Through the same period the 12z NAM has ~1.5 at KSAC and ~4.5 at KBLU (up to 7-8" to the nw and along the coast). Note that the last ~24h of the system are quite chilly with dropping snow levels as the wave passes from the wnw, assuming it takes this track. An alternative on the dry side, as brought up by Dave Reynolds during the telcon, is that the ARB never gets into a good moisture plume, with the upper low passing too far south (the old "south of 40N rule"). Indeed, it is a risk that the main moisture could never make it far enough to the north and the main precip would be with the second shortwave, so the system also remains cold and all snow at KBLU. On the other end of the spectrum, though, is the possibility of a heavy event, with amounts ending up per the HPC ~7" range in portions of the area by the end of Monday.

The 00z ECMWF model was in general agreement with the evolution of the systems described above, but has some distinct differences in the precip forecast. By 00z/Sun, for example, the accumulated precip max of just over 5" is found SOUTH of SFO, with 3.3" at SAC and 2.9 at BLU. The weakening upper low passes near SFO at 12z/Sun, with the next wave from the nw quickly following this and passing the ARB by or before 12z/Monday, with precip ending. Total accumulation by late Monday in the 00z/ECMWF run then is ~4.5" at both SAC and BLU. Timing in the latest 12z ECMWF is similar to what is noted above, but it should be mentioned that it looks like the EC has too much precip in the first 36 h too far east, compared to the latest trend and the other models. The 12z GFS is best described as slower in moving everything inland, and keeps the heavy precip just west of the ARB until later in the weekend. As a comparison then, by 00z/Sun in the GFS the accumulated precip is 0.6" at SAC and 0.8" at BLU, then the precip increases as the weakened upper low passes to the south followed quickly by the next wave, with only cold advection precip after 15-18z on Monday. Final totals by 00z/Tue are 1.4" at SAC and 3.1" at BLU. The max of 4-5" is on the coast from just north to south of SFO, and to the north of the ARB. So at this point none of the models are stalling the upper low off the coast, though the GFS is slower, allowing for some of the plume to make it into the ARB on Sunday just ahead of the next wave.

Following this system, as noted earlier, are some faster-moving waves approaching from the west-northwest. In the latest GFS the first of these is ~21z/Tue through ~21z/Wed, with about 1" of precip at BLU, and likely snow as it looks fairly cold. A weaker wave on Thu-Fri, then maybe something more significant on the weekend (2-3 Feb). The weekend trough looks to be a slower one again, but it may dig farther inland and not be a big precip producer along the coast. At the moment, it is not clear that any of these will be worthy of an IOP, but it is of course too early to tell.

ed szoke NOAA/CIRA/GSD

January 24, 2008

HMT Forecast Thu 1/24/2008 2005UTC

Once again we've had heavier precip in the central valley in the past 24 hours
(approaching 1") with lighter amounts over the ARB closer to .1".

Our next cutoff low is sliding well off OR coast this afternoon, and off the
NRN CA coast tonight. It starts to elongate towards the south Friday into
Saturday, helping to provide heavy precip just W of the ARB. Some tropical
moisture gets into the act and feeds into a heavy precip event for SoCal early
Sunday before the system is kicked out. The 12Z ESRL WRF-NMM gives up 1" of
precip in the NRN ARB by 12Z Fri with lighter amounts to the south. 12Z Fri
to 12Z Sat the amounts range from 2" in the northern ARB to .5" in the south.
The 54hr GFS snow accumulation is 20-40" of snow over the ARB ending 18Z Sat.
Orographic enhancement is limited however with the S and SSE winds for the event.
There is a break in the action beginning Sat 18Z.

On Sunday the next replacement trough is coming in from the north to help
pick up the precip over the ARB. The first part of this event continues to
look on the warm side. Light rain/snow begins about 03Z Sunday and picks up
with a total of 1.3" by 21Z Sun at which time the 850mb temp drops from about
+3C to below zero. For comparison the ESRL WRF Ensemble also shows about +3C
at 850mb on 12Z Sun with a wet-bulb zero about 1600-1700m. The Ensemble snow
accumulation level is 1400-1500m at 12Z Sun. This suggests either wet snow or
rain at Blue Canyon for the first part of the event. The radar site being at 1610m
would favor a wet snow scenario. The winds on Sunday morning are from the SSE so
upslope enhancement would be limited.

After 21Z Sunday we get an additional 1" of snow liquid equivalent that tapers
off Monday afternoon with 850mb temps dropping to -6C. The tropical moisture
feed will be shunted away from the cold part of the storm. Monday night through
Tuesday morning represents a brief dry break.

The ECMWF looks similar to the GFS over the 96hr fcst period.

The system for next Tuesday continues to bear watching in the GFS. Between
18Z Tue and 14Z Wed we get about .8" of cold precip with 850mb temps in the -3C
to -7C range. There is however only a limited feed to sub-tropical moisture.
The ECMWF has a flatter appearance for this trough.

Later next week we continue a more progressive pattern with a wave train in the
NRN PAC. The PAC JET sags southward a bit in the WRN PAC during the period.

Steve Albers

January 23, 2008

HMT Forecast Wed 1/23/2008 2000UTC

Current system has given generous precip around 1" in the SAC area the past 24
hours with lighter amounts over the ARB, particularly above the snow line.
The GFS gives us about 0.5" of remaining precip with the current cutoff low,
mostly as it starts to kick out overnight Wednesday. The ESRL WRF-NMM continues
the precip longer than the GFS, with some light amounts up to 12Z Friday.

The next cutoff low is sliding down offshore on Thursday. This is a rather deep
system and interestingly it digs a bit southward towards Friday and starts to
entrain some tropical moisture just as it gets kicked out by the weekend
short wave. The GFS gives about 0.7" over the ARB between 00Z Friday and 06Z
Saturday. The precip then migrates westward in response to the southerly
elongation of the cutoff. The ESRL WRF-NMM keeps the Friday precip mainly to
the west of the ARB.

As the cutoff starts to kick out (albeit a bit slower in the ECMWF) and the
weekend short wave moves in we get and additional 2" between 00Z Sunday and
18Z Monday. We thus may reach the lower end of the IOP criteria. Access to
the tropical moisture will be somewhat limited once the previous system shunts
it more to the east. The initial southerly trajectory also increases upstream
orographic shielding of the plume. We will start out with warmer precip than
previously expected, with 850mb temps ranging from +4C down to -5C near the end.
The heaviest snow should be Sunday evening. The ECMWF shows a similar pattern
with this short wave.

In the longer range we will want to watch the system coming in about next
Tuesday to see how strong of an atmospheric river connection it will have. GFS
IWV plots show a moderate potential ATTM. We will have to see if it moves
southward into CA from WA/OR for a possible IOP.

Even longer term we may see an atmospheric river moving from BC down to CA
in the Feb 5-8 time frame.

Steve Albers

January 22, 2008

HMT Forecast Tue 1/22/2008 2000UTC

Large-scale pattern continues to show E PAC ridge gradually weakening over
the next week with a series of short waves moving through the flow. Over time
they will come in less from the north and more from the west to get a
somewhat better connection with tropical moisture.

The past 24 hours has seen the heavier precip staying W of the ARB.
This ongoing system keeps giving about .1" precip every 6 hours over the ARB
until a brief finale with .2" on Thursday morning. Some heavier amounts are
forecast in a band extending to the south just N of LAX. This is rotating
around the persistant cutoff low centered just W of SFO. ESRL WRF-NMM gives
totals of 0.5-1.0" in the lower elevations of the ARB.

After an 18 hour break the next kicker system drops in from the north and
the GFS brings about .6" total between 06Z Friday and 06Z Saturday.
We again have a cutoff low positioning itself just W of SFO. The ECMWF keeps
this cutoff more offshore compared with the GFS, then kicks it out later on
a more northerly trajectory. The ESRL WRF-NMM run is more generous with the
precip giving roughly 1" during the initial 6-12 hours. This is another cold
system with 850 temps slowly rising from about -3C to -1C. Possibly a marginal
IOP event.

Following this we see the weekend storm that begins to get a better moisture
feed, starting about 06Z Sunday with just over 2" total by 00Z next Tuesday.
The GFS gives about 1" of this between 12Z Sunday and 00Z Monday. So this
might reach IOP criteria. A similar cold system with 850mb temps from
-4C to -2C. The ECMWF has a sharper trough coming in that also has good
precip amounts.

There's yet another short wave in the pipeline for the middle of next week
that has a bit better moisture feed. This one has about 2-3" total in the GFS
with most of the action next Tuesday. At this time it looks to be a better
IOP than the weekend storm though it may be a faster moving system with a
sharper open wave configuration.

Steve Albers

January 21, 2008

HMT Forecast Mon 1/21/2008 2015UTC

For this week we will have a couple of cutoff lows to watch producing light
to moderate intensity precip that is fairly persistant and adds up over time.

For today and the next couple of days we have the E/W oriented trough becoming
a bit more cutoff just offshore from CA. Light precip over the ARB should
continue with LAPS analysis showing 1-3" of snow over the past 6 hours.
The peak precip in the GFS over CA is just under 1"/6h near Santa Barbara
Wednesday afternoon. As the low is kicked out to the east we do get some
moderate precip (GFS has .28"/6h) over the ARB Wednesday evening. This is
unsupported in the ESRL WRF-NMM with the higher totals staying mostly near
the coast.

Next shortwave of interest currently near 165E longitude again rides to the
north by the Alaska and BC coasts before approaching the ARB Thursday evening.
Even though its a fairly strong system, there is little in the way of a
trajectory that feeds into any atmospheric rivers. Thus we would be marginal
for an IOP, particularly as the GFS is keeping the main precip to the
west of the ARB closer to the CA coast. Amounts reach .45"/6h west of the ARB
on Friday morning. The ECMWF is a bit more cutoff and this would also keep the
precip to the west.

Later in the weekend and early next week the long wave ridge off the west
coast gradually breaks down with each passing shortwave. The PacJet that
develops may initially aim towards WA. The wave passage on Sunday may be worth
watching for an IOP within this context. The ECMWF is about 12 hours slower
than the GFS timing. The GFS is currently generating .6"/6h near the ARB, with
about 2" accumulating over a 42 hour period.

Longer range GFS IWV plots show continuing atmospheric river potential in the
Jan 30 to Feb 6 time frame.

Steve Albers

January 19, 2008

HMT Discussion for Saturday 19 January 2008

Nothing to get excited about in the short term. Bottom line: earliest date for the next IOP is next Friday, January 25. IOP seems likely between 25 January to 1 February.

GFS ensemble moves the ridge westward in earnest with a wave in the Gulf of Alaska at 00 UTC Thursday 24 January. Quite a spread as the wave approaches ARB 00 UTC Saturday 26 January. Even those that have a direct hit in which the wave lingers show little evidence of a high PW plume.

Canadian ensemble isn't completely sold on a regime shift this soon, though it is the majority opinion. The ECMWF deterministic run also has a later regime shift.

PSD retrospective ensemble also shows the regime shift well underway at 00 UTC Saturday 26 January with a large trough positioned offshore of CA in the ensemble mean, suggesting next Saturday as a potential IOP. The retrograded position of the ridge, which has an apex over the Aleutian Islands, stays in place well beyond 240 hours. This may not be the prime scenario for atmospheric rivers, as the mean position of the storm track is quite a bit north of the central Pacific.

Chris Anderson

January 18, 2008

HMT Forecast Discussion for 1/18/2008

A wave is expected to go over the HMT area 00 UTC 12/20. IWV values in the GFS are about 15 mm, and the system is moving out of the north at a rapid rate. Furthermore, the Pacific water vapor imagery does not show any swirls or convective activity with the potential to undercut the ridge. However, a really great atmospheric river extends from Hawaii to the Aleutian Islands.

A change in the large scale pattern is evident in many ensemble members at 7-8 days. The ridge in place over the Gulf of Alaska is forecast to retrograde in the 144-192 hour period. This occurs in concert with building heights in the western Tropical Pacific, as evident in the PSD reforecast ensemble. The pattern of troughs that will emerge from the regime shift is anyone's guess at this point. Ed Berry noted the timing of this pattern shift is roughly consistent with the briefing they gave last week.

Chris Anderson

January 17, 2008

HMT forecast discussion for Thursday 17 January 2008 (entered 20 UTC/17 Jan)

A high-amplitude upper-level ridge continues to sit off the West Coast, keeping things dry across the ARB, with a deep trough downstream over the west-central CONUS. A strong shortwave trough will ride over the ridge and plunge south over the weekend along the West Coast, retrograding the trough and bringing a chance of some precip and very cold temperatures to the ARB. Overall though the ridge is expected to remain through most of next week, preventing significant moisture from reaching the ARB. Longer term, however, this pattern is forecast to break down some, at least temporarily, which could (with considerable uncertainty!) usher in a chance of significant systems as early as next weekend (26-27 Jan) and into the last week of January. But as noted below and on the telcon, there is a lot of uncertainty in the forecasts beginning next weekend (26-27 Jan) and through to 2 Feb.

The first issue is whether the weekend system is of interest for a potential IOP, and the consensus on the telcon is no. As noted above the main feature continues to be a high amplitude ridge positioned off the West Coast. Westward across the Pacific a strong jet is present from the Dateline westward, south of a broad trough centered near 160E. A cutoff low that had been lurking to the northeast of Hawaii is opening up now and will combine with a shortwave trough in the northern stream to become the system that rides over the ridge on Friday and then plunges southward into early next week. There is good model agreement on this, with the system bottoming out near west-central CA early next week (mon-tue/21-22 Jan). As this happens extremely cold air will plunge southward into much of the nation, and some of this will reach far west to the ARB. So this will be a very cold wave, with snow levels possible at or below 2000 ft, but moisture limited with no real tap to anything substantial in the Pacific. So it is difficult to imagine a lot of snow out of this, though perhaps it could pile up some if the system stalls awhile. The 00z ECMWF run has less than a tenth of an inch total through Tue/22 Jan with this system in the ARB, and the latest 12z EC is similar. The latest 12z GFS has a little more, but not much, even though the upper level system closes off south of the ARB by Tuesday. There is up to a third of an inch from Sun night through Tue east of the ARB, and up to 0.7 much farther south in the south-central CA mountains. These amounts are generally in agreement with the longer range forecasts from the HPC.

The models and ensembles generally agree that the upper level trough should shift eastward by midweek (Wed/23 Jan), with at least a temporary change in the pattern over the CONUS to less of a deep trough, and a retrogression to the upper level ridge, shifting it further to the west. In a number of the ensemble members and the deterministic forecast from the ECMWF and moreso the latest GFS, this allows the next wave to dive off the West Coast and, at least in some members, close off as a deep upper low next weekend (26-27 Jan). In a good number of the 00z ensemble members this trough/closed low then eventually makes its way to the coast by early to midweek of the last week in January, with a potentially good precip event for the ARB. This is seen nicely in the 00z NAEFS meteogram for Sacramento, with precip chances increasing from 26 Jan onwards. There is, however, a lot of uncertainty in this whole scenario, with another set of ensembles simply shifting the ridge back to off the West Coast late January into the first couple of days in February, so as to end up with a pattern similar to what is present now. In fact, this is precisely the forecast from the latest GFS 12z deterministic run, with the aforementioned closed low staying too far off the coast, and then the upper level ridge building up, so that the forecast by day 15 (Fri/1 Feb) has a pattern that looks a lot like what we have today, and it gets there without ever having much precip hit the ARB. Comparing this GFS run to the new 12z ECMWF shows this diversity in solutions off the West Coast; instead of a closed low peeling off the main flow and settling in near 35N/145W with an upper level ridge to the north into Alaska in the GFS by the last weekend in January, the ECWMF has zonal flow with a low-amplitude trough moving the Gulf of Alaska. The 12z GFS ensembles show a lot of spread at 240 h, and a number of members by Day 15 (2 Feb) are right back to a pattern looking much like what is present right now.

ed szoke NOAA/CIRA/GSD

January 16, 2008

HMT Forecast 1/16/2008 19:42 UTC

Yesterday's forecast could almost be used again today. We have clear conditions for the most part in the ARB with perhaps some gustier light winds. Temps range from 20 to 40F. The Pacific low pressure is now located near 34N 149W and the water vapor loops are slowing definite ridging occurring north of this location with the top of the ridge around 53N 153W, deflecting any moisture well away from the ARB area.

The ARB remains dry until a potential cold outbreak that is shown to start with a short wave in the EC model coming down the side of the sharp ridge off the CONUS coast. This is just north of the ARB 1/20 (Sunday) 00ut and moves through by 00utc Monday (1/21). Today's EC is not quite as vigorous as earlier runs but still maintains a 500 cutoff over central CA after Monday with a persistent duration of light precipitation marginally over and mostly south of the ARB. The GFS 00ut run has the precip starting in the ARB Monday 1/22 at 06utc through Wednesday 1/23 06utc and possibly beyond, with accumulation of roughly 0.5 inches. The ensembles still indicate that the ARB will be at the north side of this activity with most of the action to the south; this is possibly associated with moisture coming over the ridge that has by this time pulled somewhat offshore so most Pacific moisture will be on the coast or to the west of the ARB. After this episode the ensembles are still indicating the weak possibility of another precipitation event on or about 1/29-1/30. Ensemble 500 heights show the ridge continues to slowly retrograde over the next 384 hours.

Dan Birkenheuer ESRL/GSD

January 15, 2008

HMT Forecast 1/15/2008 19:40 UTC

Again there is no precipitation activity in the ARB this morning; temperatures have warmed to the upper 30s and 40s throughout the area. Today a short wave is progged to pass to the north skipping the area and denying it active weather, the best the SREF can do is 5% chance of flurries in the high terrain. The low pressure system at 31N 148W is very symmetrical on the water vapor imagery and has a dry band now extending the flow from the ARB into it across the Pacific. Thus the ARB is in sort of a negative atmospheric "river" flow regime. At 45N 158W there is a hint of a circulation associated with a moisture plume extending to the tropics, the one that we have watched for days. This moisture is still now entraining in active convection west of the large Pacific low circulation that was noted yesterday. Moisture supply from the Pacific is essentially turned off.

Once again, our focus remains on the long-term for hopes of precipitation. As of this morning it does not appear that we will see any possibility of significant precipitation in the region until possibly Sunday 1/20 at the earliest. We have been watching this scenario since this last weekend as there appears to be a potential of a cold air outbreak that might reach far enough west to affect the ARB. Starting with yesterday's 18ut GFS model run, the GFS is now looking more like the EC model did on Sunday where a strong moist surge out of Canada extends to the SW CONUS and actually cuts off some cold air and hangs over CA. Earlier EC model runs had the associated 1000-500 thickness down to 529, but today's (06ut) GFS run has this "cutoff" with a 534 thickness appearing on Sunday 1/20 at about 12ut and dropping about a quarter inch of moisture in the upper elevations of that ARB, likely in the form of snow. This morning's GFS 12ut now has much less precipitation but extended for a longer duration but only over the higher terrain (1/20 - 1/22) and a colder thickness (527) that reaches the CA coast on or about Tuesday of next week, on the other hand last night's EC agrees with a longer duration episode and with more total precip (~0.44 inches in the higher terrain). However, the GFS 12ut ensembles tend to put more of the emphasis on S CA and the ARB is only at the edge of the main precip area. Right now I would not get too excited about this potential event since it is still changing quite a bit in its overall nature run to run, but bears close watching.

Following this episode, there is a hint of something in the ensembles around 1/28-1/29. The mean ridge pattern appears to retrograde comparing the mean ridge position at the current time with the 384 hour ensemble positions. Therefore, I see no major breakdown evident in the current dominant ridge pattern but it might move in the anticipated direction.

Dan Birkenheuer ESRL/GSD

January 14, 2008

HMT Forecast 1/14/2008 19:45 UTC

This morning has evidence of good radiational cooling over the ARB with fog in some areas and temperatures ranging over a wide extent from 49F at KBLU to +5F at KTRK. No precipitation was likely received since yesterday at this time. The moisture situation in the Pacific has taken a turn for the worse as we have lost many of the strong circulations this morning and associated moisture plumes are dissipating. The progged low off the CA coast at 31N 145W remains in place, the circulation center at 41N 180W has dissipated with the major plume from Indonesia now drawn up into an area of convection in the mid-Pacific. Its potential for reaching the CONUS now looks to be doomed.

In the near term, the current potential event that will be in the area tomorrow morning is still expected to miss the ARB passing mainly to its north. No model brings precipitation into the area and the SREF (03ut) even has 0% probability of rain or snow over any portion of the ARB . The NAM shows short wave passage roughly at 18UTC 1/15 and one is hard pressed to even identify a wave in the GFS or EC models.

Beyond this, the area looks to be ruled by severe dry conditions until possibly the Sunday 1/20 timeframe when the arctic cold outbreak that was discussed in detail yesterday still shows a possibility of wringing out some moisture (it certainly does not advect any into the area), but today the models are pretty stingy on bringing snow to the region. It would be snow if it occurred as the EC model brings in a 526 thickness over the ARB on 1/22. Along with this, the off shore ridge appears to adopt a strong positive tilt that directs any Pacific moisture rivers well up into AK and down into central Canada steering them well away from the ARB.

Beyond this, the ensembles have no precip in the ARB until 1/21 (Monday) 12utc at the earliest and in this case it is only a couple runs. There appears to be another potential cold air outbreak on or about 1/30, again not an optimal situation for the experiment if it even does bring precip into the ARB. Today's ensemble 500-height forecasts however, show less tendency for the ridge to retrograde, keeping it pretty much set up off the CONUS coast with high amplitude out for the full 384 hours (1/30) with a very strong cold low pressure circulation stationary over Hudson's Bay.

Dan Birkenheuer ESRL/GSD

January 13, 2008

HMT Forecast 1/13/2008 19:40 UTC

This morning the weather in the ARB is primarily clear with some fog in spots with early morning temperatures ranging from the 40s in the valley areas to teens in the mountains.

The prognosis for precipitation this coming week looks to be near zero with a slim potential on or about Tuesday 1/15 12utc when the models show a precipitation system passing to the north of the area. Ensembles show that this event should miss the ARB. There will be a cold arctic surge impacting the area on or about 1/17 00UTC (Thursday) but it now appears dry for the research area.

This morning's water vapor imagery over the Pacific shows a low has developed as forecast near 11N 129 W. A plume of moisture has reached British Columbia's coast, a small vorticity center at 32N 156W, another center at 50N 171E with a moisture plume with no tropical connection ridging between the two low centers. The water vapor imagery shows subsidence over the ARB at the current time.

For the intermediate forecast, the EC model has a short wave reaching the CONUS coast at about 00ut 1/15 (Tuesday) and the GFS moves any moisture associated with this wave north of the region and has it pass by around 15ut 1/15. So this is a very fast moving impulse but the models have the ARB remaining dry throughout with the SREF indicating less than 10% chance of snow near Reno at 21utc 1/15. At 18utc 1/15 the EC model builds the off-shore ridge and it reaches its maximum this week by 00utc Friday 1/18. It appears to be a stable persistent feature with the GFS showing a 585 ridge axis at 12utc 1/16 (Wednesday).

Looking to the long-range, the ensembles are now showing a fair chance of a cold air outbreak that brings precipitation (likely snow) to the ARB on or about 1/21 (Monday). This is manifest by a 500 low in N Canada at 55N 106W, a surface high pressure of 1034 in the great basin, and a 1044 high to its north in central to northern Alberta. The GFS 00ut run puts 0.25 inches of precipitation in the ARB in the first 6 hours of frontal impact, it appears that the 12utc run today is not as likely to bring precipitation into the area but the 12UT GFS ensembles have 7 of 12 frames showing precip over the ARB, we will have to watch this during the week.

Beyond this, the 00ut GFS ensemble now shows the potential for a moist plume to come up over Baja well to the south of the ARB in the 1/26 timeframe, something that showed up on ensembles a few days ago, then disappeared but now has returned. It looks to be associated with a closed upper low at the base of the high pressure ridge opening a moisture path to the tropics far to the south of the ARB. One can hope that if the ridge retrogrades this may put the ARB into a more favorable target zone for moisture plumes.

Dan Birkenheuer ESRL/GSD

January 12, 2008

HMT Forecast 1/12/2008 19:40 UTC

The minor system we have noted in earlier forecasts is now passing north of the ARB region. This morning, minor precipitation was observed primarily north of I80 with radar echoes 35-40 dbz at times with surface reports of light snow in the higher elevations of the ARB and light rain to the north of the ARB in valley areas. With temperatures right around freezing at the higher elevations of the ARB, I would not be surprised to see the snow switch over to rain/drizzle as the day progresses. All model guidance shows the passing system will simply graze the area leaving trace to minor precipitation amounts and end by this evening (03utc 1/13, Sunday).

Looking at the water vapor imagery over the Pacific we see the current system is related to the wave observed yesterday nearest the coast. This is now associated with a good vorticity center located at 49N 130W. The second circulation center mentioned yesterday has now occluded with a circulation at 35N 167W and is spinning up an associated rotation at 41N 154W. This has a good topical connection, as it has all along, extending back to Indonesia. This plume of moisture might impact the ARB in a minor way next week as we will see in the discussion below.

The models and the SREF probabilities show no precipitation after 03utc 1/13 with the current event leaving us with a dry ARB for certain out to 00utc 1/16 (Wednesday). During this interval the GFS builds a strong ridge off the coast and the moisture plume that was mentioned yesterday is seen to approach the coast with progressive short wave on or about 06utc 1/15 Tuesday only to have it retreat out to sea (drawn into a 554 low the EC model puts at 32N 147W) before it can reach the ARB leaving the area dry. The ridge continues to build in the GFS with a 584 ridge axis at 44N 130W extending down to 25N 124W, this appears to reach its maximum level on or about 09utc 1/16 (Wednesday). At about this same time a cold air surge is forecast to drop from Canada and push west possibly approaching the ARB from its east. Again, this should have no impact on HMT operations.

Looking out to the long range, the GFS shows a possible surge of moisture coming down the coast from the north, likely from the second circulation now out in the Pacific that has been driven over the high ridge, traversed just south of the Aleutians, around the Gulf of AK and then down the west coast. It is not anticipated that this will be an "optimal" moisture plume if this scenario pans out and is downplayed in the GFS 12ut run over the earlier 06ut run. Beyond that, the ensembles present a roughly 50% chance of precipitation in the ARB on or about 1/22 (Tuesday) associated with a cold air outbreak (both Canadian and GFS ensembles). The bottom line is that after today's minor event, models are consistent in forecasting an extended dry period with perhaps some minor activity associated mainly with cold air outbreaks. The near term is dominated by a sharp off-shore ridge that shuts off the possibility of atmospheric "rivers." Ensembles show the possibility of the ridge retrograding but staying persistent out as far as the end of this month.

Dan Birkenheuer ESRL/GSD

January 11, 2008

HMT Forecast 1/11/2008 20:10 UTC

This morning's conditions in the ARB are basically clear and good radiational cooling has dropped temperatures at upper elevations overnight into the teens in the mountains. The last surface observation of precipitation occurred north of the ARB at 03utc 1/11. We can say with certainty that the latest precipitation event has ended.

In the Pacific we still have two major circulation centers apparent in the water vapor imagery, one located at 40N 143W with the greatest moisture as far east as 41N 135W. Today there may be a reestablished connection to the tropics as it appears that moist air is entrained in the flow. The second maximum is now at 38N 173W and as progressed since yesterday. It still has a very notable tropical connection.

The near term forecast has a moist plume reaching the CONUS coast by about 12ut Saturday 1/12 but the main moisture stays off shore. This corresponds to a weak system progged to move through the ARB starting early Saturday (~00utc) and continuing through 1/13 06utc. The NAM produces no precipitation over the ARB with the major amounts staying near the coast and to the north. The GFS 00UTC run optimistically generates a total of 0.03+ inches of liquid equivalent, while the 12UT run only delivers 0.01, most of this falling at higher elevations (if at all) in the 12-18UTC timeframe on Saturday. The SREF gives a 20% chance of this to be snow early on and then increases the likelihood of rain even at higher elevations to 35-40% later. Thus, the forecast time trend (dProg/dt) of the GFS is towards less precipitation. The GFS has the upper wave corresponding to this weak event departing the ARB by 06UTC on Sunday 1/13. This system is deemed to be too minimal in precip amount to warrant research activity.

The ARB remains dry until a second moist plume reaches the extreme northern coast of the CONUS near the Monday timeframe and works its way south. It reaches the ARB latitude on or about Tuesday 1/15 at 06UTC but a developing low pressure system well out in the pacific appears to circulate the moisture away from the CONUS back to sea and it never reaches the ARB. Thus the potential for precipitation in the 03-12utc 1/15 timeframe (associated with a weak upper level wave passage) appears nil in models at this time.

The long range prospects appear to maintain dry conditions for the next several days with the GFS ensembles now showing no activity over the ARB through 1/27 with the exception of one precipitation event mainly affecting the Reno side of the high terrain as a cold airmass descends from Canada associated with the strong ridge that forms off the coast in the Tuesday - Wednesday time frame next week. The EC model develops a very strong ridge off the coast at 136 to 130 W and a 558 low center at 34N 148W for most of next week. This ridge then appears to break down to more of a zonal pattern to perhaps allow moisture into the area starting Saturday 1/19 - Monday 1/21. This seems to be perhaps the earliest we will see any potential of moisture reaching the ARB and will watch this in the coming days.

Dan Birkenheuer ESRL/GSD

January 10, 2008

HMT Forecast 1/10/2008 19:46 UTC

Current conditions as of this morning are light precipitation and fog in the ARB with temperatures 28-34F in the higher terrain and 40 degree temperatures in valley areas. Precipitation has been light snow in the high terrain and light rain along the coastal areas.

Water vapor satellite imagery shows a moisture plume well on shore to the north of the ARB extending back to the HI Islands. The imagery also shows a definite break in the moisture at the point with the tropical source south of HI. In other words, the plume is "disconnected" from the tropics. In addition, a good vorticity center appears near 163W 33N and a second circulation is noted further west at 186W 40N. The second one (186W) has a better moisture fetch to the tropics extending back well into the Indonesia area and would be well worth watching. The source nearer the CONUS does not appear to be that impressive at this time.

Near term model forecasts show that the current system should produce minor precip over the ARB ranging from 0.03- 0.17 in ending by 06utc Friday. The probability of precipitation (SREF) increases precipitation probabilities over higher terrain by 1/12 18utc (Saturday) through 1/13 06 UTC Sunday. However, all models show that the total precipitation expected during this interval will be light. The GFS shows no moisture over the ARB while the EC model produces only minor precipitation well under the threshold for an IOP. Also during this time a ridge builds over the coast in both the GFS and EC models. At 1/12 00UTC, a weak wave moves over the ridge coinciding with the higher precip probabilities in the SREF reaching the coast at 1/12 18UTC but the GFS keeps the precipitation north of the ARB. The EC model has this wave propagating faster and even though it brings in the prospect of some precipitation into the ARB, it moves through very rapidly.
Following the end of this weak precipitation potential 1/13 06UTC (Sunday) we enter what appears to be an extended dry period with the exception of the EC model (more on that below). The GFS ensemble runs show basically a dry CA and ARB from late Sunday (1/13) until the end of the ensemble forecasts ending 1/26 (Saturday) with maybe a hint of something on 1/21. A strong 500 hPa ridge builds off the CA coast by Tuesday 1/15 18UTC with a closed 584 high pressure center at its base centered at 128W 23N. With the exception of the EC model no precipitation is forecast to enter the ARB in this timeframe with the EC bringing a very fast, but weak precip event into the ARB 1/15 03UTC (Tuesday). However, the system might only result in increased cloudiness to the region and may not produce measurable precipitation. At this time it appears that the chances of this becoming an IOP worthy event is near zero.

If the large ridge builds as forecast off the coast by Tuesday, it will be very difficult for moisture plumes to find their way to the ARB until it breaks down or allows the circulation of moisture to be brought up under the ridge, but the closed high pressure center does not appear to strengthen after Tuesday, which would likely reduce the likelihood of the "undercut" possibility. The fundamental message at this time is that with the exception of some minor disturbances moving through the ARB in the near term, we will be entering an extended period of dry conditions for the ARB.

Dan Birkenheuer ESRL/GSD

January 9, 2008

HMT forecast discussion for Wednesday 9 January 2008/1945 UTC

A few snow showers over the higher terrain of the ARB this morning ending at KBLU at 1530z, so the end of IOP5. The KBLU METAR recorded a 24-h total of 0.97" at 12z (quite possibly an underestimate since it is a tipping bucket gage), with off and on S- accumulating another 0.01" in the 3h ending at 15z. Precip remained as snow through the event with temperatures at KBLU rising to -1C then peaking briefly at 0C at 23z. Precip timing during the event was about as forecast, lingering maybe a few hours longer with S- (0.18" was recorded by KBLU between 06-12z), but otherwise the heaviest period was as expected, with KBLU have 0.28" 18-21z, 0.23" 21-00z, then 0.13" over the next two 3-h periods. The automated mesonet at BLU was not reporting, but other ESRL mesonets were available. The one at Big Bend (elevation 1739 m = 5706 ft) showed precip (on the hot plate sensor) beginning to accumulate ~17z, tapering off at ~08z and pretty much over by 12z, with ~2 inches total. Temperatures at Big Bend rose to close to 0C just before 00z/Thu, then rising pressure thereafter. Not sure if the surface front passes around this time or not. The KBLU observations did not have any really notable wind shift, but rather a gradual turning from 170 to 190 from 23z through 04z. At Alta (elevation 1085 m = 3600 ft) the hot plate was not working, apparently, but the tipping bucket has precip beginning near 18z, with a steady climb to ~1.4" by 07z/Thu, then light precip till 13z with 1.6" total. Note that the 24-h totals from the ALERT gages were somewhat different, at BLU only 0.77", with a max at Huysink of 1.52", and 0.20 at SAC (0.32" from the KSAC METAR). Radar loop indicated the heaviest band, with reflectivities up to 50 dBZ (perhaps contaminated by bright band?), slipped just south of BLU around 23z. Trough passage at OAK, per the special soundings, looks to be ~01z at 700 mb, while at Sloughhouse it occurs at 04z/Thu. Max PW from the OAK RAOBs was 0.79" on the 18z/Tue sounding. So it appears the NAM forecast amounts were not too bad for this event, with the GFS/EC just a little low but close. Overall, a pretty good forecast for this event by the models.

As for the next wave, looping of water vapor satellite imagery with 500 mb analyses in the Pacific indicates a gradual weakening of the approaching wave, which in the analysis is a broad shortwave trough centered near 45N/155W (north of Hawaii) this morning, with the lead cloud shield approaching the OR/Northern CA coast. There is a moisture tap back to nw of Hawaii, but the plume is not as substantial as earlier in the week. Another, weaker wave is behind the trough centered near the Dateline. It is a wave behind this system that is forecast to split off from the main flow and become a cutoff low that lingers well off the West Coast (near 30-33N/150W) next week. Overall the models are consistent in forecasting a less organized and overall weaker system for Wed night through Thu, with precip amounts most likely remaining below IOP criteria. The 12z NAM begins the precip about 03/Thu/10 Jan, with 0.03" from 03-06z, 0.27" total by 12z/Thu, then 0.4" total by 00z/Fri, but lingering cold advection precip for 0.72" by 12z/Fri. More to the north with >1.3", and ~0.85 nearby to the west of ARB, but the highest amounts of 2.5" near OR/CA border on coast. 09z SREF: mean in 0.25-0.5" range for the entire event through 12z/Fri/11 Jan; individual members vary from 0.1-0.25 to 0.5-0.75" range. 00z Canadian Global, ~0.5" w/similar timing to NAM. The latest GFS is quite disorganized with the precip, with similar timing to the NAM but only 0.24" at KBLU through 12z/Fri. The 00z ECMWF run has ~0.3" at KBLU for the entire event. NOGAPS looks similar. Our local models have ~0.3 to near 0.5" by late Thu for this event. This event will be warmer, although certainly beginning as snow at KBLU elevation, then likely over to rain showers on Thursday with the freezing level possibly rising to near 8000 feet, at least on the NAM BUFR soundings. It is likely the real snow level will be lower, perhaps in the 6000-6500 ft range. General telcon agreement that this system should remain below IOP criteria, with only an outside chance the jet combined with the moisture plume could sneak out more precip than expected.

The trailing trough now near the Dateline mainly stays north of the ARB through the weekend, with the 12z Canadian Global model pretty much the only one bringing some light precip to the ARB. No ensemble members though have any precip over the weekend.

In the longer range, the 12z GFS continues the forecast of a high-amplitude ridge building just off the West Coast for all of next week, with the cutoff low staying well to the west, and a deep trough over the CONUS. The 00z ECMWF and Global Canadian model forecasts are pretty similar to the latest GFS. The 00z NOGAPS brings the upper low MUCH closer to the coast by midweek, but even with that the precip stays just off the coast. The NOGAPS solution is seen in some of the GFS and Canadian 00z ensemble members, and in several of the GFS 12z ensemble members, but for the most part only a few members bring precip onto the West Coast. This closed low will have to be watched since these things are generally difficult to forecast. Otherwise, the models keep it dry with the mean trough over the CONUS, retrograding by the following week (20-24 Jan) in at least some of the ensemble members. If this retrogression were to be even more prominent then that could be another way to eventually get the ARB back into precipitation. Alternatively, some members do the opposite, gradually shifting the CONUS trough eastward by the week of 20-24 Jan, with a shift eastward of the upper level ridge as well.

ed szoke NOAA/CIRA/GSD

January 8, 2008

HMT forecast discussion for Tuesday 8 January 2008

Echoes are beginning to increase in the ARB area and will continue to do so as an initial broad sw-ne band of echoes moves into the area through the next couple of hours. At KBLU very light snow was first observed at 14z, then again in the 16z obs. 18z KBLU obs has 0.25 miles in moderate snow with south wind gusting to 27 kts, and the temp dropping now to 27F. Only 0.02" melted so far as of 18z. A heavier band of rain is now hitting the Northern California coast, extending north into coastal Washington. The very latest radar at 20z shows a strong band just to the sw of the ARB with reflectivities of 50 dBZ drifting east (some could be melting band but does not look like this is the only part of this band). No cloud to ground lightning has been noted so far. The 500 mb shortwave trough axis is just off the Northern California coast, and is still forecast to pass across the ARB around 01-02z/Wed. The Sloughhouse profiler has shown increasing southerly flow through the lowest km+ over the last couple of hours as the trough approaches. No ESRL BLU mesonet data being displayed yet. Visible satellite imagery overlaid with surface reports indicates the surface front is still just off the Northern California coast.

Some model forecasts are up a bit this morning on forecast precip for this event so this is a good sign. Latest 12z/NAM is fairly robust, but already a bit too much so, with 0.18" at KBLU by 18z (when there is likely to be only a few hundreths at most), with about a third of an inch just to the west. It has the heaviest precip coming after 18z, with 0.5 at KBLU 18-21z, then another 0.58" 21-00z. Still another 0.2" by 03z but precip rapidly winding down. Storm totals then are near 2" at KBLU, and ~2.3" just to the west. Our local 3 km models are also a little fast with the precip by 18z. These models also show a good period of precip from 18z through 02z, with the main frontal band passing KBLU near 22z. Max amounts end up in the 2-3" range in the ARB, likely too much, but overall agrees with other indications that this should be a decent event. Lower resolution models like the GFS also have good precip, with 1.4" at KBLU, with similar timing. A close call at KBLU with the freezing level...our local models shift it above KBLU (to ~800 mb) this afternoon before trof passage, but the official forecast keeps it near 4000 feet with ~10" of snow forecast. The morning OAK sounding had a very high freezing level, at 9500 ft, just below 700 mb, but is much lower as one heads east into the higher terrain. In the 18z special OAK sounding the freezing level lowered to near 7400 ft, with a deep layer below this not a lot above freezing (at about 2C) all the way down to 900 mb (~3200 ft). This is the layer that cools heading into the higher terrain leading to the freezing level below KBLU. BUFR soundings from the NAM indicate the freezing level does get close to going near or above KBLU during the afternoon, with the closest by 22z, when sw winds increase to 50 kts near 800 mb, then wsw at 55 kts higher up thru 700 mb. Cooling in the BUFR soundings above 750 mb after 22z, but no real wind shift aloft until around 03z/Wed. This timing agrees with other models. Per the conference call WFO/SAC reports snow down to near 3000 feet, and the Alta mesonet is right near freezing at 3560 ft.

The next event for Wed night and Thu still looks marginal for an IOP. In the 12z NAM light precip begins ~06z/Thu/10 Jan, with a third of an inch at KBLU by 12z/Thu. Then precip at the rate of ~0.15"/3h through 21z then diminishing. Storm totals just west of KBLU still approach an inch over an 18 h period ending 00z/Fri, but a good portion of this would be in cold advection following a 500 mb trof passage near 15z/Thu, with the main trough farther north where more precip occurs. The GFS and ECMWF (00z run) are similar with a weaker system, but with less precip than the NAM. The 12z GFS only has about 0.15" total at KBLU, falling from ~06z/Thu thru 18z/Thu, but much more (about an inch) closer to the coast in Northern California. The 09z SREF run looks to be in general agreement on a lower end precip event. On the positive side, there is still a plume around to be tapped by the wave, and it looks to be a warmer event with the precip likely to be mainly rain at KBLU, if they were to get a decent amount of precip. HPC forecasts keep most of the precip farther north in WA and OR for this event.

Longer range still looking dry, with ridge building on the West Coast and persisting. One dramatic change was that the wave forecast by most models recently on Saturday (12 Jan) to pass north of the ARB suddenly dips south in the 00z/ECMWF run to produce a quick inch of precip in the ARB. However, the 12z ECMWF just in keeps this wave farther north, in agreement with other model runs. Otherwise, there remains general agreement in the ensembles of a dry period through the end of the runs on 23 Jan. However, there is more spread in regards to positioning of the overall CONUS mean trough, and this is even reflected in the forecasts for next week from the deterministic runs of the 12z ECMWF and GFS. This uncertainty may be a result of the positioning of a broad upper level low in the east-central Pacific next week (developing as a wave breaks off from the mean flow), something that would likely have considerable variation in different model runs. It is positioned farther east (but still far enough from California to keep precip away) in the GFS, and subsequently the CONUS trough is also farther east, while in the ECMWF both are shifted west. In some ensemble members this upper low drifts close enough to perhaps be a threat for precipitation sometime in the 15-22 Jan range, but this would be a minority forecast at this time.

ed szoke NOAA/CIRA/GSD

January 7, 2008

HMT forecast discussion for Monday 7 January 2008

KBLU METAR is back and last bits of snow from this long system are falling this morning with some light snow still at 19z and a temp of 28. According to the KBLU obs 0.44" of melted in the last 24 h.
Reports from the radar site indicate about 2 feet of snow fell from the event. A quick overview for this week. The two waves of interest are moving rapidly eastward across the Pacific as a strong jet of up to 150 kts+ continues, with the max from near 40N/140W back to 30N/160E. The first wave located near 45N/155W is a tight spiral at lower levels with upper level cloudiness well in advance of the upper level shortwave, while the second system looks stronger at this time aloft and is found near the Dateline at 40N. A nice plume of moisture remains in place north of Hawaii and extending back to the sw, and this is being tapped by the first system at this time, while the second wave is forecast to tap into the plume as well before it hits midweek. SSMI imagery indicates about 1.5" or so of PW in the plume now feeding into the lead wave. The main issue with both waves that will limit their potential for heavy precip is that they are fast moving for one, and that the second wave tends to hit the coast farther to the north, and weakens considerably from its current status. The latest 12z GFS is in good overall agreement with last night's ECMWF run on these two systems.

Some details: on the first system, timing still looks to have light precip beginning near 12z/Tue. The precip picks up by 18z, with the heaviest period about 18z/Tue to ~02z/Wed. The 00z ECMWF put out a 0.85" max just south of KBLU in the 12h ending 00z/Wed, then another 0.26" max in the next 12-h period. The latest 12z GFS is roughly similar but with the heavier precip shifted more to the nnw of KBLU. At KBLU, very light precip begins near 12z, by 18z the GFS has 0.17" total, then by 00z/Wed another 0.7", followed by 0.22" by 06z/Wed which is about the end of the precip from this system. The high resolution models run by NOAA have slightly different timing. The WRF/NMM has the same start time but only light precip until about 21z when it picks up dramatically, then continues until 03z and tapers off after 06z. Totals for this period then are 1.75-2" max. Only slightly less in the WRF/ARW. Both could be too much, as we have somewhat limited verification of these hi-res models, but clearly the max in these runs is tied to the terrain which is better resolved in the hi-res runs. The latest runs of our own 3 km models indicate similar timing to the NCEP hi-res runs; some echoes begin to appear in the ARB area at 13z, increasing in coverage down from the highest terrain by 17z, then really picking up by 21-22z as the main rainband with the shortwave moves inland, with ~0.7" by 00z/Wed (end of the current runs available from 12z). The 12z NAM has similar timing; precip beginning around 12z/Tue is quite light, with around a tenth of an inch or so by 18z, but then begins to increase, at KBLU 0.13" 18-21z, then 0.5" 21-00z (with up to 0.75" to the nnw), another 0.4" by 03z, then decreasing by 06z with only light showers thereafter. This adds up to ~1.5-1.75" for max amounts near or in the ARB area for the 12z/NAM run. The 12z Canadian Regional model run, which also has higher resolution, supports these higher totals. Latest HPC graphics indicate about an inch or so forecast for this first wave in the ARB region. It looks like this would be a worthy event, especially since the crew is already there.

NAM BUFR forecast soundings indicate good warm advection from ~800-650 mb tonight, then on Tuesday lowering as the wave approaches, with wsw flow increasing to around 50 kts in the 700-750 mb layer ~21z/Tue to ~03z/Wed. Upper level (500 mb) trough passage is around 04z/Wed at KBLU, with a rather ill-defined low level wind shift. The temperature profile never rises above the freezing point at KBLU, supporting the idea by WFO/SAC yesterday that the freezing level would remain lower than KBLU (lower than 5282 feet). At SAC the freezing level in the BUFR sounding forecast rises to near 6800 ft, so upslope cooling must be lowering the level as we head up towards KBLU in the model forecasts. The WFO/SAC forecast for KBLU has rain and 34 for tomorrow, so it looks close. A look at cross-sections from our 3 km runs indicates a rise in the freezing level for a time just above the BLU elevation tomorrow, then lowering below it as the precip increases. Consensus from the telcon was a freezing level near the radar site but probably staying below 5000 feet, so mainly snow at KBLU, but a close call.

The next system brings some light precip to the ARB by 06z/Thu, with the GFS then having 0.14" by 12z/Thu, then not much more, with the heaviest precip staying near the coast, so only about a quarter of an inch for the ARB area. The 12z NAM is quite weak with this second wave, having only very light precip by 12z/Thu, then only another ~0.33" or so shifted toward the higher terrain west of KBLU by the end of the run at 00z/Fri/11 Jan. Right now this looks like a very marginal event. If we can somehow delay a decision until the wave gets closer then we should know more tomorrow. The main potential for better rainfall would be a better tap into the moisture plume then the current model forecasts are showing. Possibly a little milder temperatures than the next system with this wave.

Looking ahead, still looks dry in the longer range forecasts beginning after the Thursday system, as an upper level ridge builds along the West Coast through midweek next week (Wed/16 Jan). Then the ridge
retrogrades a bit in most of the ensemble forecasts but things remain dry through the next week (ending 23 Jan). Pretty good agreement right now in the ensembles that this will be a prolonged dry period.

ed szoke NOAA/CIRA/GSD

January 6, 2008

HMT forecast discussion for Sunday 6 January 2008

Lull underway across the area this morning but the next and final wave in this system has echoes just now reaching the coast. Freezing level remains low, at ~3300' in this morning's OAK sounding. The snowfall in the forecast for BLU area out of WFO/SAC is in the order of 6" or so more with the snow ending late tonight. Snow should diminish according to the latest models by 12z/Mon with little further accumulation on Monday. Still not sure of total maximum precip and snow amounts; KBLU still not reporting. Huysink gage (west of KBLU) had 1.64" in 24-h ending at 7am local time this morning. SAC area around 0.5". Heavenly ski area at Lake Tahoe reported 108" storm total so far, with a breakdown of amounts of 12"/7am 4Jan, 36"/7am 5Jan, and 60"/7am 6 Jan, this on the RSN site, on their own homepage they list "only" 64-96" storm totals. WFO/SAC estimates from reports a little higher than BLU that perhaps in the order of 4-5 ft of snow could have fallen so far at the radar site.

Biggest change in the model forecasts is that all models generally predict less precip for both events this week. In fact the forecast amounts make them marginal for IOPs. There still is a nice pool of moisture west of Hawaii and both systems tap this, but the likely reason for less precip appears to be the systems are both fast-moving and weaker than in yesterday's runs. Perhaps playing into any decision on what to do would be how soon the radar site gets plowed out, but also the long range forecast that looks quite dry after these two storms, at this point through ~20 January.

The trend to lesser precip amounts was found in pretty much all the 00z model runs. For the first wave the focus for heavier precip was more in Northern California, with the ARB area in the GFS getting ~1-1.25" max. A little more is forecast by the 12z NAM, but the 12z GFS has a little less, just under an inch, with a slightly weaker shortwave than the NAM. SREF 09z forecasts have a mean precip amount of about 0.75" for event 1. Models generally agree on light precip beginning by 12z/Tue, but the main part of the system is from 18-21z/Tue thru 06z/Wed, then ending.

Clearing is then forecast between this and the next storm, which is forecast to be even a weaker wave in both 00z runs and the latest 12z forecasts. The 12z GFS has precip with storm 2 beginning near 00z/Thu with max in the 12-h period ending 18z/Thu/10 Jan, but only about 0.5" or so. This is similar to the 00z GFS, but the Canadian was juicier with a longer duration event, and also to a lesser extent the NOGAPS. The ECMWF (did not see precip amounts tho for this) does not seem too impressive with the event.

There could be a close call with another wave on Saturday into Sunday (12-13 Jan), but this looks unlikely overall. Then rather good agreement of a ridge building along the West Coast with mean trough over the CONUS. This CONUS trough then gradually retrogrades, with the ridge shifting west a bit, but not enough to get the ARB into any precip of note through the 15 day period (ending 22 Jan). Latest 12z GFS ensembles have a few members with more uncertainty for the week of 14-20 Jan but most members remain dry in the ARB region. This is also seen in the ECMWF 12z run (and also the latest GFS deterministic 12z run) that both have a detached closed low slowly moving towards central California by day 10 (12z/16 Jan).

ed szoke NOAA/CIRA/GSD


January 5, 2008

HMT forecast discussion for Saturday 5 January 2008

The event continues across the ARB with a moist westerly flow now in place behind yesterday's strong wave. A brief review of yesterday's storm: the power to the ETL gage at BLU went down just as the precip began to pick up, totals at the KBLU METAR indicate 2.56" of melted, but not sure how trustworthy that would be. WFO/SAC in the telcon mentioned the reports of more than 4 feet of snow at nearby ski areas and that I80 was shut down for a time late yesterday and overnight. They noted that some spotters at 6000 ft reported nearly 4 ft of snow, with an automated sensor near Lassen Peak reporting 9" of liquid precip. Automated gages include 1.51 in 24-h at SAC, and a max near the ARB of 4.92 at Huysink in the 24-h ending at 7am local time. The front passed KBLU a couple of hours faster than yesterday's forecast, according to the METAR obs FROPA was near 22z, with temperature dropping to 34 quickly and S+ by 2235z. Overnight KBLU reported some freezing rain and sleet at times, if this is accurate not sure. Peak wind gust yesterday at KBLU was 180/63 kts at 2113z. This morning KBLU has been down since ~1330z, the last ob just before then had 1 mi in S-, 210/14 g30kt, and a temp of -3C (27F).

Current analyses show a very strong jet is still in place extending from 100 kts+ in sw UT west across central CA, then wnw to an elongated jet near 40-45N stretching back across the Pacific with a 170-180 kt jet max from ~140W to the Dateline, the the jet bends to the sw into the south-central Pacific, where there is a moisture plume. This area of moisture to the nw and w of Hawaii does get tapped by the systems next week. At 500 mb an upper low remains over Alaska with the trough extending south off the West Coast. Strong w-wnw flow impinging on the coast with embedded shortwave troughs. One of these is a potent one now approaching the Northern California coast with an organized area of echoes that will hit the ARB this afternoon and tonight. Snow level will be well below KBLU through this with the NWS predicting about 2 feet more by Sunday morning. One last wave later Sunday into Monday with precip finally decreasing after 12z/Mon and some clearing possible.

The next wave does move rapidly across the Pacific, arriving across the ARB on Tue/8 Jan. Precip totals from this event vary some but it looks like max amounts in the 1.5-2.5 inch range would be likely. Startup of precip varies some, the GFS and NAM have some precip beginning a little before 12z/Tue, but other models are a tad later. Latest ECMWF model looks like it has roughly similar timing for this and the next wave to what the GFS is forecasting, and the Canadian Global is also in general agreement. At this time too tough to call, the plowing out of the radar site may determine when folks could get there. This system winds down by ~06z/Wed, then the next wave comes in right behind it with precip beginning around 00-06z/Thu/10 Jan, lasting again about 15-18h. More uncertainty with this wave, but an intriguing aspect of it is that it too could tap the elongated moisture plume. Snow levels with both waves should rise considerably from where they are now, but at least for the first one KBLU could be close to the transistion zone, and this discussion in the telcon indicated that the GFS probably brings the snow level above Blue Canyon, but right now thinking is it could be quite close to KBLU.

After the potential Thursday event ridging builds up near the West Coast with a mean trough over the CONUS. One last potential wave is possible before this occurs though on Sat/12 Jan, though ensembles indicate this has only about a 30% chance of bringing anything of interest to the ARB. The ensembles generally favor an extended dry period then from 13 Jan through 20 Jan. There is a chance, shown in some members (especially in the Canadian ensemble system) that the CONUS trough could retrograde enough to begin to extend back across the West Coast during the week of 14-20 Jan, which could make things interesting again, but this is quite uncertain at this time.

ed szoke NOAA/CIRA/GSD

January 4, 2008

HMT Forecast for 4 Jan 2008

Wind and precip are on the increase as scheduled as a very strong jet
and associated wave approach the Northern California coast this morning.
Temperatures have been rising at BLU and are now above freezing as the
heavy precip gets underway, and winds are picking up as well. Last
night's precip was mainly snow but not as much as had been thought
possible. But today will be the big day as the strong jet
approaches...peak winds in satellite obs this morning are a whooping 230
kts near 45N/160W at 34kft, with this strong jet dipping to the
southeast on the backside of the strong upper level system centered near
45N/135W this morning. This is providing enormous diffluence/divergence
aloft that is approaching the ARB and helping to squeeze out the
moisture that had been lurking off the coast. Several extensive ssw-nne
bands of echoes across the northern half of CA and this will come across
the ARB over the next 12 h. At 17z KBLU reporting 37F with S winds
gusting to 46 kts. Strong S winds at lower elevations as well with KSFO
gusting to 57 kts. At 11z some lightning was observed off the coast.
Central pressure in the surface low off the coast of WA is near 960 mb,
with the surface cold front extending to the se to just off the Northern
CA coast (it may have just passed at 17z a buoy to the north of KSFO).
At 1744z KBLU ob was R+, +4C, gust 180/61 kt, generally KBLU thru the
latest 1930z ob has been maintaining R+ with S winds gusting to near 60
kts. The latest obs do show the surface front has progressed inland
some across and north of KSFO with a shift to a more westerly direction
in the winds.

A look at the short-range forecast shows things pretty much on track as
discussed yesterday. Good call by the local folks on the snow level
getting back above KBLU, where it will remain as the really heavy precip
continues this afternoon. The 700 mb flow at ~55 kts at KBLU at 12z is
forecast to peak near 90 kts by 21z with 700 mb temps remaining around
-2C, then the 700 mb front passes near 00z with only a slight shift in
the winds but a wind decrease and temperatures down to -10C by 03z,
according to the latest 12z NAM run. A look at the hourly BUFR
soundings from the 12z NAM shows a sharp drop in temperature in the 800
to 700 mb layer between 00z and 01z. Surface temperatures lower
significantly by 02z, and on the BUFR soundings get to 0C by 06z/Sat.
Given the strong drop aloft it appears precip at KBLU should transistion
to snow between 01-03z. By 12z/Sat the freezing level drops to around
3800 ft, possibly at tad lower at times before a final drop towards 3000
ft on Sunday. Peak precip period is 18-21z with ~1.25" forecast just
west of KBLU (in the NCEP hi-res 5 km runs up to 2" in this period),
close to another inch at KBLU in the next 3h (ending 00z/5 Jan), then a
decrease to 0.3-0.5"/per 3-h period through 12z/Sat, followed by
continued great orographic snows all day Saturday at the rate of about a
third of an inch per 3-h at KBLU, with wsw 700 mb winds 40-50 kts and
temps around -12C. Some predicted precip totals from the models: by
06z/Sat at KBLU NAM has ~3.8", with max of 4.8" to the west, while the
GFS max of ~4" is at the elevation of KBLU (this location difference
could simply be a terrain resolution issue); 06z run of the WRF/NMM ~7"
at KBLU, while the WRF/ARW has ~8". Total precip by the time things
totally taper off ~06z/Mon/7 Jan: ~10" at KBLU in the NAM (max 12.3"
just to the west); ~7.2" max (located near KBLU) in the GFS. The GFS
amounts are more in line with the current HPC forecasts. Per the
conference call the NWS is predicting about 5 ft of snow by early Monday
at KBLU, with up to 10 ft at higher elevations to the West. Discussion
of road conditions (as relates to getting the radar crew out if they
stayed through the event) indicated that folks expected I80 would be
opening up by Monday given the current forecast. Decision though was
ultimately made that the radar crew should leave by ~00z (later today
before dark).

A slight lulll is possible 18z-22z/Sat but then things begin to pick up
again with the approach of "part 3", a shortwave riding se in the flow.
A change from yesterday appears to be that this wave (or perhaps
another right behind it) will not finally clear past the ARB until
~00z/7 Jan (late Sunday), when winds finally shift to the nw at 700 mb
and orographic snows at KBLU decrease to a rate of ~0.1"/3h, with this
continuing through Monday along with a slow rise in 700 mb temps. So
another change in thinking, at least in the NAM, would be a lull in the
precip from ~00z/7 Jan through the next 24 h (so all of Monday daytime).
Overall the above timing looks similar to what is in the 12z/GFS, which
shows a pretty good decrease to the precip after 00z/7 Jan, and
especially by 12z Monday, with fairly dry (or maybe just light snow
showers at KBLU) continuing through 12z/Tue.

The GFS then has the next system picking up by late Tue then pretty much
over by ~15z/Wed, with 1.5-2" indicated at this time for the ARB area.
A minor system may follow by Thu but then ridging still looks to be the
case thereafter through the weekend of 12-13 Jan. The 00z/ECMWF model
is similar to the latest GFS with the current system. For the Tue/Wed
wave it is wetter, and blends it into the next system so that clearing
does not occur until Thu/10 Jan, followed by ridging by the weekend.
The latest Global Canadian model forecast has a break on Monday into
early Tue, then the next system which ends by ~18z/Wed/9 Jan. NOGAPS
12z run is also similar in the timing of these systems, with the end of
the midweek event on midday Wed and nothing further as the next wave
stays farther north than in the ECMWF model run. This midweek system
could be mostly a rain event at KBLU at least as it currently looks,
with the snow level a little above that elevation. The 12z ECMWF is
generally similar to the earlier run and in agreement on the midweek
system, beginning before 12z/Tue and ending by ~18z Wed. The EC run
does have a close call over the next weekend it is shifted slightly
farther west with the ridge so that the next wave brushes across the ARB
instead of being farther east.

Otherwise, there is general consensus in the ensemble forecasts of a dry
period beginning later next week and continuing possibly thru the
following week as an upper level ridge builds along/near the West Coast
and the mean trough shifts east over the CONUS. There is actually
pretty good agreement in most members of the Canadian and GFS ensemble
systems for this longer period. The 12z GFS ensembles paint a similar
picture, basically a dry pattern for the ARB beginning after the midweek
event next week and continuing through the week of 14-20 January.

Bottom line then is a wild time with very heavy rains and strong winds
at KBLU for the next 6-7 h (perhaps 4-5 inches of rain in this period
potentially, then the cold front goes by near 00z with precip changing
to snow and likely blizzard conditions overnight into Saturday. A
possible lull Sat afternoon but then heavier snows late Sat through most
of Sunday, then things calming down with a break on Monday, and some
uncertainty about the significance of the Tue-Wed (or Thu) system.

ed szoke NOAA/CIRA/GSD

January 3, 2008

HMT forecast discussion for Thursday 3 January 2008

The first part of our long-awaited 3-part event has begun. This morning rain has begun on the coast, with the first main band now extending inland but still west of the ARB. South winds gusting over 40 knots in buoy reports near the coast from SFO northwards. Dry air pervades the area at low levels, as air involved in part 1 of this event is mainly recirculated air from the North American interior. This should keep precip in the ARB today on the light side. However, this will change overnight, when the remnants of this first band combine with the frontal band to produce a period of heavy precipitation. The start of this period in the NAM is near 0300 UTC, with a little over an inch forecast for KBLU by 1200 UTC/Fri and close to an inch at KSAC. The first of the surface cyclone centers forecast by the progs is now off the OR - WA coast as about a 980mb center and moving nwd. Its frontal band is not too far off the coast of Nrn CA at present and will move inland later today and tonight, shifting the low level winds offshore more into the sw - w. There will be no real break between this event and part 2, as the next deep low center heads eastward and deepens, then turns more northward and heads for Vancouver Island. Part 2 promises to be a major storm over northern and central CA with strong winds and heavy pcpn, as has been indicated by the progs for several days now. Winds and melting-level evolution looks still consistent with yesterday's blog and telcon discussions. Heaviest pcpn is forecast for the daytime hours tomorrow into tomorrow evening. The NAM picks up the precip rates considerably by just after 1200 UTC/Fri, with a forecast of .75-1.00 inches per 3h thru 0000 UTC/Sat, then a burst of even heavier precip as the main wave pushes across the ARB, with ~1.7"/3h ending at 0300 UTC/Sat, then rates falling to "only" 0.3-0.5"/3h through 1200 UTC/Sat, by which time the total precip is ~6.3" at KBLU, but a max of near 8" just to the west, ranging down to 3.3" at KSAC. Cold advection orographic precip then continues for the next 24h at a pretty good clip, 0.25 to 0.5 or more inches per 3h period, so that by 1200 UTC/Sun/6 Jan the NAM has total precip values for the event of near 9" at KBLU, ~11" for a max value just to the west of KBLU, and 4" at KSAC. This appears to be the 3rd part of the system, which is a somewhat ill-defined wave moving out of the wnw around the big Alaska upper low that would then pass across the ARB near 1200 UTC/Sunday, after which the precip in the NAM then tapers off to light activity through the day on Sunday. These amounts are not far off the HPC forecasts: 5.2" near KBLU for the 24-h ending 0000 UTC/Sat, another 3-4" (max of 5+ is to the south in their forecast) for the next 24-h ending 0000 UTC/Sun, then more precip thereafter. Here is a comparison to some other models. The 1200 UTC GFS has similar evolution through Saturday to the NAM, but is not as tied to the topography, and so keeps its max amounts near the elevation of KBLU, and then shifts things south a bit more than by Friday night. Initial precip is somewhat less for part 1; by 1200 UTC/Fri ~0.7" at KBLU with 0.55" at KSAC. But precip cranks up on Friday after 1200 UTC, with ~4" at KBLU in the 12-h period ending 0000 UTC/Sat, then precip tapering off but continuing at 0.25-0.33"/6h through 1200 UTC Sun. Totals in the GFS then by 1200 UTC Sun/6 Jan are near 7" at KBLU and 3.8" at KSAC, so similar numbers to the NAM at lower elevations but less higher up, which could simply be a reflection of the better resolution in the NAM leading to greater orographically forced precip. This idea is consistent with the output from the hi-res 5 km NCEP window runs, with the 0600 UTC WRF/NAM having total precip by 0600 UTC/Sat of up to 8" in the ARB, and closer to 9" for the same time frame in the WRF/ARW. One difference that is present between the GFS and NAM is the precip does not diminish in the GFS at 1200 UTC/Sunday, but instead actually increases just ahead of the shortwave passage around 1800 UTC/Sunday, with a 6-h burst of ~0.8" ending at 1800 UTC/Sun before precip then diminishes. Some precip amounts for comparison from the ECMWF show less overall than the GFS but with some differences. For part 1, the EC is very similar to the NAM in total precip by 1200 UTC/Friday. Not quite as robust for the big show on Friday, with an additional total of ~3" in the ARB for the 24-h ending 1200 UTC/Sat, but a little heavier for the period later Sat into Sunday, with the precip then diminsihing by ~1800 UTC on Sunday in the ECMWF.

As for wind and snow levels, consensus on the telcon is for the snow level to initially be rather high, ~7000 ft or so, and remain above the KBLU elevation (5282 ft) thru the daytime hours on Friday when the very heavy precip is expected. Winds will be very strong, with 700 mb SW winds forecast to increase to a peak of 85 kts by 1800 UTC Fri (when 700 mb temps are predicted to be in the -2 to -3C range), then the 700 mb front passes around 0000 UTC in the ARB and temps drop, to -10C by 0600 UTC at KBLU with winds shifting more to WSW and decreasing to ~50 kts. The winds continue to decrease into the 30-40 kt range on Saturday with cold 700 mb temps around -10 to -12C. An increase in wind occurs as we near 1200 UTC/Sunday with the approach of the final wave in this series, with its passage then shifting the flow at 700 mb more to the west on Sunday, consistent with the decrease then in precip. Even though the heaviest precip falls as rain at KBLU on Friday, telecon consensus was for several feet of snow possible by the time all was said and done (the 9th or so) at KBLU, and good snowfall beginning Friday night and continuing into early Sunday.

Although precip tapers off in the models sometime on Sunday, there is another separate system that rapidly moves across the Pacific and reaches the ARB not long after Sunday. This is a fast-mover but potentially wet again, as it appears to have tapped into the long stretch of moisture that still would be in place across the Pacific. Precip with this one picks up in the ARB very late Sunday night (~0600 UTC/Mon) through Monday night, with another 1.5-2" in the GFS by 1200 UTC Tue/8 Jan. There are variations in the model solutions for this event, particularly with when the potentially heavier precip starts up. So it is quite possible that there could be a break in the heavier precip from ~1800 UTC/Sunday until sometime on Monday, the way it looks right now.

But all is not finished even then, as yet another wave in the series, this one likely to be even more moist than whatever happens on Monday, approaches the coast with the flow becoming more wsw and again tapping into some deep moisture. The result in the latest GFS is another burst of heavy precip, beginning right as the other one ends ~1200 UTC Tue, and continuing through ~1800 UTC Wed, with an additional 4-6" forecast for the KBLU vicinity in the GFS. Ridging follows this system and then builds along the West Coast through the weekend (12-13 Jan) with drier conditions in the GFS, finally. This is consistent with the ensembles and other model forecasts. In fact, the timing of these events from today through Wed is pretty close in the GFS to the latest ECMWF 1200 UTC run, with the EC being just a little quicker to end the final event (by ~1200 UTC/Wed). The 1200 UTC Canadian Global model is also relatively close to the GFS and ECMWF solutions, although one difference is it has more precip later Sunday night than some of the other models.

Looking much farther ahead, the 1200 UTC GFS ensembles support a drying period beginning after the Tue/Wed event and continuing not only through the weekend of 12-13 Jan, but through much of the following week as well, with a ridge along the West Coast.

ed szoke NOAA/CIRA/GSD

January 2, 2008

HMT Forecast Made 1930Z W 2 Jan 08

Big event still on tap for Th - Sunday 3-6 Jan. Progs remain consistent in showing a 3-part event, beginning Th and ending Sunday. GFS from 12z this mrng and ECMWF from last night both indicate 5" or more melted pcpn KBLU vicinity by 00z M 7 Jan. I see no reason to think that this will verify too high, although the trend in the progs has been to shift the heaviest pcpn a little south. This mrng's GFS actually has the heaviest total on the San Gabriels behind LA, and the ECMWF from last night has a whopping 10+" near Yosemite south to Kings Canyon. The NAM is indicating somewhat more than the GFS and may be excessive due to problems with nonconservation of their water-substance variable.

As is typical, the situation is evolving to be a bit more complex than earlier progs. The very strong jet noted in the central Pacific yesterday is now diving down the back side of the amplifying eastern Pacific trough. Subtropical moisture has been entrained into the trof's circulation and on IR satellite a feed of moisture is evident in the wsw flow aloft from near 28N/128W to near 20N/145W and then west to HI. The surface front offshore from CA is showing signs of wave development near 37N/152W. The vorticity max that constitutes part 1 of the event appears near 40N/148W. All this seems likely to coalesce at the surface into a low center that will track NEwd toward a point offshore from the CA - OR border sometime Th night. As it does so, the moist tongue noted in the IR above will likely track more NE to NNEwd toward the Bay Area.

All this has bearing on the time of onset of serious American-River-Basin (ARB) pcpn. It is noteworthy that there is more divergence in the GFS ensembles on start of pcpn over the ARB this mrng than a couple days ago. I think it is still likely that serious ARB pcpn will hold off till after sunset Th, though there may be periods of light pcpn at higher elevations, or at least low-hanging virga that will give radar returns during daylight hours as mid-level moisture comes in from the SW over the dry air at the surface. This first event seems likely to have a more S to SSW fetch at low levels; this and trajectories of the low level air feeding into the first system will favor the Coast Ranges over the Sierra for Part 1, especially early in the event. I expect that with the dry air entrained into this system, the melting layer at KBLU will start out at the surface, and that KBLU will remain in the melting layer overnight Th.

Part 2 of the event looks similar to what has been forecast for the past 3 days: deep low approaching the OR coast from the west on Friday, setting up a very strong fetch from the SW and bringing abundant pcpn to the northern 2/3 of CA. It was pointed out during the discussion that the moisture flux anomalies (don't recall what level) are 6 standard deviations above the mean (although the distribution of these things is highly non-normal). Progs continue to indicate a frontal passage late Friday over the ARB, with the heaviest pcpn occurring with this. Winds are stil aniticpated to be strong all areas Friday, with combined barrier jet and strong presssure gradient. Dave Reynolds is expecting in excess of 100mph at Squaw Peak. Pcpn rates are likely to be ~ 1"/6h the last half of Friday in the upper reaches of the basin, heavier in the hour or 2 right around frontal passage. Melting layer should rise on Friday, and I think KBLU may well be just below the melting layer much of the day F into F evening. However, melting layer will descend back thru KBLU Friday night and with high confidence I think it will be below KBLU from Saturday daybreak onward.

There is at present no clear indication of a break coming between parts 2 and 3 of the event. Likewise, the timing and intensity of part 3 is uncertain. The GFS high resolution run continues to give more emphasis to this than does the ECMWF, and has it coming in Saturday night, rather than the later Sunday timing of the ECMWF. I think what we can say with some confidence is that there will continue to be strong orographic forcing behind the frontal passage Friday night, continuing at least through midday Sunday. This will result in continued snow above 3,000ft or so, with pcpn rates of 1/4 to 1/2"/6h Saturday and at least part of Sunday.

John B.

January 1, 2008

HMT Forecast made 1930Z Tu 1 Jan 08

6.7 micron satellite imagery shows that a continuous band of upper tropospheric moisture now extends from the equatorial Pacific se of Guam to s of HI to near 20N/145W. The low-latitude upper potential vorticity anomaly that has been plunging sewd across the Dateline toward HI is becoming deformed and elongated ENE-WSW and is reinforcing the subtropical jet near the north edge of the above-noted band of upper-tropospheric moisture. Farther N, the eastward moving upper trough upon which our hopes rest for a major IOP Th - Sunday 3-6 Jan is near 145W and showing the anticipated indications of amplification. The leading edge of the tropical upper-tropospheric moisture band is becoming entrained into this trof's upper wind field. SSM/I imagery shows this moist band has a lower tropospheric component, with integrated water vapor of up to 5 cm indicated s of HI. At this time, the low-level winds in the Pacific E of the Dateline are primarily easterly S of about 25N. This is still far enough south to permit entraining of some of the deeper Pacific moisture into this upcoming event.
The major trough in the westerlies noted yesterday as being over the Japan - Kamchatka area is slowly shearing out to the e and ene, with the nose of a 170kt jet and associated s/w trough on its se flank just now crossing the Dateline. This upper-tropospheric long-wave feature will be feeding short-wave troughs eastward that are expected to traverse the central Pacific ridge into the currently evolving eastern Pacific long-wave trough position. The first of these s/w trofs is a little east of the Dateline at present and is indicated to be the first part of the 3 part event for Th-Sunday. Meanwhile, a deep low associated with the upper trough near 145W is moving into the Gulf of Alaska. The occluded front with this low is near the BC coast and approaching the Northwest coast. With the collapse of the Pacific High during the past 24h, the surface winds over CA, OR and WA are offshore, bringing dry air into the southerly flow offshore, ahead of the approaching front. Surface dew point at KBLU is near -15C at present, although foggy conditions prevail in the Valley. A weak upper trough in the subtropical branch of the upper flow is approaching northern Baja, bringing some upper moisture toward srn CA, but no pcpn.

This morning's progs differ from earlier ones in details, but not in their main theme of a major event in 3 parts, in regard to Th - Sunday. Overall, relative to earlier forecasts, the second part of the event on Friday is given more emphasis, the onset of pcpn Th is delayed by maybe 6h from what was indicated yesterday, and there remains some difference of opinion on the third part of the event, both timing and intensity. The first part of the event, as noted above, is now just e of the Dateline, and is forecast to approach the Northwest coast Th evening. Close on its heels is part 2, manifesting itself in the progs as a deep surface low (965mb) approaching the Northwest coast late Friday. I think this will bring a frontal passage through the ARB Friday evening with probably the heaviest pcpn, and a drop in the melting layer to below KBLU by Saturday morning. The melting layer should start out below KBLU on Th 3 Jan and then rise fairly rapidly so that KBLU will be in the melting layer (or perhaps slightly below it briefly) through at least a signficant portion of Friday's heavier pcpn. The melting layer will remain below KBLU for the third part of the event late Saturday/Sunday. The prospects for strong surface winds, particularly Friday, remains, with a possiblilty of gusts to 50kts at KSAC and KSCK.

The matter of a break in the pcpn between parts 2 and 3 is still uncertain. This feature is not yet apparent as a distinct entity in the data I have examined, and so we are at the mercy of the progs. Even in the GFS ensembles (which often tend to underestimate uncertainty), there is considerable disagreement on the timing and even the existence of this feature. Today's GFS indicates less of a break than was suggested yesterday. I am confident that pcpn will be less intense on Saturday than F, but whether it will be sufficiently light to permit plowing at KBLU is uncertain. It may be Th 3 Jan before we can say with confidence whether or not there will be a major break in pcpn on Saturday.

Monday and Tuesday, 7-8 Jan appear likely to have strong w-nw winds at ridgetop, and some instability showers, but significant (> 1"/24h) pcpn seems unlikely. For late next week the potential for IOP-candidate situations decreases to less than climatology.