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March 25, 2007

HMT discussion for Sunday 25-March-07

Our main storm is still on track with timing as we have been discussing and the latest model precipitation totals generally in the 1.25-1.5 inch range (somewhat more on the 00z GFS), but forecasters going more in the 1.5 to 1.8 inch range. The tropical plume we have been watching is currently into northern California and will sink over the HMT area by late today and tonight. Max PW from SSMI around an inch and this agrees with the model analyses. The operational models don't predict much of anything today and forecasters in the area have extremely low pops today gradually upping to scattered overnight. The moisture looks pretty high-based right now on the Oakland sounding so apparently will take a while to moisten things up. Your timing is pretty safe for not missing anything in terms of the collection for rainwater, not sure how much will happen overnight but some showers are possible as this plume moves over the area and gradual moistening occurs. The GFS 12z run does get more precip going tomorrow morning in the 6-h up to 18z, somewhat more to the north, and our local models also gradually shift a band of precip southward. But for the most part agreement remains excellent on the main period from 18z/Monday through 03z or so Tue (Monday night), when trough passage occurs through 06z. Thereafter some variation on the amount of cold advection precip with lowering snow levels (4000 feet or so?, maybe lower), with the end to precip by 00z-03z/Wed. Our local WRFNMM downscaled to 3 km has 1-1.5 inches of precip by 12z/Tue, then shows a nice burst of convection Tuesday afternoon in the cold air especially over the higher terrain with up to another half an inch before things end by 03z. This seems reasonable with good orographic flow and the cold air.

So all in all should be a good dynamic event, apparently relatively null for anything significant with this moisture plume until it wraps into and just ahead of the main system tomorrow.

ed szoke NOAA/GSD

March 24, 2007

HMT discussion for Saturday 24-March-07

Summary: Plume is still present and currently directed into the Pacific Northwest where flood watches and warnings prevail in Washington. Main change appears to be a slower southward movement to the plume, so that it is not really over the HMT area until later Sunday. Operational models still not showing precip with this except hinting at maybe scattered late Sunday activity, but some get decent precip close by 12z Monday. Maybe a later start to the soundes? Otherwise overall system still looks good with strong dynamics and at least brief entrainment of tropical moisture ahead of trough passage monday night.

Discussion: Our system is still on track with a well-defined plume present in the water vapor and SSMI imagery, the later showing up to 35 mm or so of total PW, which agrees with the analysis on the 12z NAM and GFS models. There continues to be some run to run variation on the overall total precipitation but overall things still look good for the main event with the strong dynamics passing in good position for the HMT area and the heaviest period probably from 18z/Mon through 03 to 06z/Tue, which is a good consensus time for trough passage near 700 mb. Decent cold advection precip thereafter until about 00z/Wed. Totals from the 12z GFS are 1.5-1.75 in a solid area centered on the HMT area, 1 to up to 1.5 to the south a tad on the 12z NAM. The 00z GFS had more, up to 2.5 centered on SAC, and the other 00z models all looked favorable and moist. HPC in their discussion felt the 00z GFS was too moist, and also were unimpressed with any tropical connection. Perhaps they are thinking more of a direct Hawaii type connection and not such a stretched out one like we now have. The HPC has a total of 1.75 for the event. Finally, the 09z SREF run has similar timing to the GFS and other models, with mean precip totalling 1-1.25 and some members over 2 inches.

The tricky part remains whether the moisture increasing as the tropical plume sinks south over the region will produce precip. Pretty much nothing but some very light activity suggested by the models until perhaps 12z/Mon, with the GFS getting closer by this point and then having a decent max, separate from the main event, just to the nw of the HMT for the 12z-18z/Monday period. The NCAR/RAP weather site has a nice total PW presentation from the GFS and NAM and this indicates the plume is slower to sink south than yesterday, but does show some increase in PW by 00z/Mon then the plume sinking over the area. Our local models at this point only go out to 00z/Monday, but by then there are echoes and a band of accumulated precip about 70 miles to the nw of BLU, all with max values under 0.50 inches, and also lighter activity into SFO. The WRF-NAM at 3 km out to 72 h (12/Tue) has total precip values of around an inch, which seems low, and some maxima up to 1.5. Precip in this model does develop Sunday afternoon but to the nw of the HMT, moving into the higher terrain of the HMT area by 12z/Mon, then increasing after 18z through Monday evening. The 700 mb flow becomes more southerly in the cross-section for this run on Monday and increases to at least 40 kts, consistent with the GFS and NAM, then trough passage 06-09z Tue. So overall it looks like the plume may be slower to set up but still does so, with the advent of precip holding off in the models despite its presence. Thus we still have a potential null case perhpas, or maybe there will be more precip beginning earlier. The main issue it seems would be whether to delay the start of the sondes 6h or so.

Looking ahead, overall the ridge actually starts to build with anything for Thu-Sat becoming much less likely at this point, and uncertainty beyond that but no system of note.

ed szoke NOAA/GSD

March 23, 2007

HMT discussion for Fri/23 Mar

Still looking good for Monday's IOP with the 00z GFS upping the precip totals (though down in the 06z run, however not sure how much to trust the off-hours runs). Uncertainty remains for the amount of precipitation on Sunday with the 00z GFS and 00z NOGAPS both showing some while other models including the 12z NAM don't have much until the main event on Monday. The SSMI imagery nicely shows the plume with a good core over 3 mm and up to 3.5 or so max. Likewise water vapor imagery is very impressive with a plume from the northern Pacific Northwest way back to the southwest Pacific source. Associated upper level jet remains quite strong, with a 180-190 kt core at 250 mb around 36N and from 167-177W. This jet gradually sags south over the next couple of days as the associated upper level trough sharpens and approaches the West Coast. This then does allow the moisture plume to sag southward enough to be over the HMT area by Sunday. Currently SAC WFO is not very impressed with this feature with low POPS and forecasts of perhaps a tenth of an inch at BLU Sun afternoon. Will take a closer look at this for the telcon but right now might be worth considering sampling given the amount of uncertainty with this type of feature.

As for the main wave, here are some numbers from the 00z GFS, which is consistent for the main wave with the other models.
Precip falling on Sunday: 0.16 at BLU by 00z/Mon but just 25 miles to the nw up to 0.50 inches, flow at 700 mb wsw/25kts
Sunday night: More scattered type precip thru 18z Mon with total at BLU 0.35 by 18z/Mon, 0.62 to the nw. 700 mb at 18z Mon: sw at 35 kts
18z/Mon-00z/Tue: things really pick up with 1.0 by 00z/Tue total at BLU (700 mb: sw-ssw 40 w/peak 55 kts)
00-06z/Tue: Strongest period with 1.2 inches new at BLU for total of 2.2 by 06z
09z or so Tue is trough passage at 700 mb, by 12z Tue another 0.40 new
12z/Tue - 00z/Wed quite a lot of precip in the cold air with orographics and instability, by the time it ends overnight Tuesday night BLU has 3.42 inches and is basically near the overall max of precip.

The 12z run of the GFS overall is down on the total precip for the event but still fairly impressive. For the Sun/Sun night timeframe it keeps the precip farther to the north with nothing at BLU. The 12z GFS is also about 6h slower on Monday with the main precip, with only 0.04 inches at BLU by 00z/Tue, but then the heavy stuff begins, with 1-1.25 in the HMT area in the next 6 h period in strong sw flow ahead of the upper level system. Another 0.5 to 1.3 in 6 h within the area by 12z, at which point specifically BLU has totaled 1.63 for the storm and SAC 1.2. More precip falls in the cold air following trough passage with 2.21 total by 12z/Wed at BLU. Other 12z model runs are consistent for the most part with the 12z GFS in timing, holding off the most intense precip until beginning near 00z/Tue rather than 18z/Mon, and generally having 1-2 inches or so in amounts (bit hard to pinpoint on some of the models) but with the max nicely placed in the HMT area. The most intrigueing part of this system and difficult to predict is how much of the current atmospheric river will be entrained into it, and when. As the jet sags south over the next couple of days the narrowing but still present river positions itself over the HMT area by Sunday afternoon, yet, except for the 00z GFS and NOGAPS, precip is hard to get in the HMT area with this. Some of the models do show it farther downstream but despite decent ~30 kt flow at 700 mb from the wsw to sw and PW values forecast to rise to 0.52 inches on the 00z/Mon GFS model sounding for BLU, nothing in terms of precip reaching the ground. It was deemed this would be an interesting enough period to launch soundings beginning on Sunday, even if to try and document an environment with at least some plume but no precip. The other aspect of the storm as it makes it into California on Monday is how it then entrains some of the atmospheric river, as shown nicely at the U of Hawaii site with the PW forecast from the GFS. This moisture gets squeezed out in a 6 h or so period monday evening (ending ~06z) just ahead of the trough passage, with the model sounding for BLU at 00z/Wed having 40 kt straight southerly flow in a deep layer. So even if the plume amounts to little on Sunday, it appears it gets involved in an intense albeit short period of warm advection precipitation later on Monday with the main trough.

Beyond this system differences for potential wed night into Friday with the Canadian most robust while nothing on the farther north GFS. Likewise with something by day 10 (31 March) with the GFS deterministic run from 00z weakening the approaching wave before ever reaching the coast but other models and some ensemble members more optimistic.

ed szoke NOAA/GSD


March 22, 2007

HMT discussion for Thu/22 Mar

Summary: strong and cold system still on track for monday-tuesday with potential for 1-2 inches and locally more. Wild card is possible tap into atmospheric river now in place, with potential for precip as early as Sunday that will have to be watched, as well as more warm advection precip on Monday ahead of the trough.

Our strong upper level jet of 150-160 kts plus at 250 mb has crossed the Dateline and stretches from 170W to 140E between 30-40N with an upper level system centered near 47N/178W at 12z this morning. Water vapor and SSMI imagery clearly show the jet has entrained moisture from the convection in the southwest Pacific ala the discussions over the last week or so by Ed Berry and Klaus. Much of this moisture will pound the Pacific Northwest over the next few days with inches of rain. By late Saturday into Sunday (24-25 March) the jet sags south enough to push some of this moisture very close to the HMT area but the models continue to be rather sparse with any precipitation in this warm air ahead of the main system, presumably because of limited moisture left in the plume. Model soundings for Blue Canyon (BLU) from the GFS indicate precipitable water rises to 0.58 inches by 18z/Sun (0.62 in the latest GFS run) but the lapse rate is quite stable (better though by 00z/Mon in the 12z GFS). Then PW decreases again until Monday (hitting a peak of .51 inches at 00z/Tue on the 06z GFS run; note that on the 12z GFS this is up to 0.57 inches). This initial moisture will have to be watched in the event the plume is more robust, and in fact the latest 12z GFS brings up to a third of an inch of rain just to the nw of the HMT area on Sunday afternoon, then weakens the band as it drifts south, so stand by. If we assume that the initial moisture is not significant, our main event remains the aforementioned trough noted above that amplifies as it approaches the West Coast on Mon/26 Mar and then hits the HMT area on Mon-Tue. Everything still on track with this system to be a strong one with very cold air in the upper level low that should result in quite a bit of convective cold advection precipitation following trough passage near or just before 12z/Tue 27 Mar. The slower ECMWF model has come around more to the other model solutions in the 00z run. Total precipitation amounts vary some between the 6-h GFS runs, but the latest tendency has been to have the maximum right over the HMT area, which is encouraging. Overall the consensus among the various 00z models is for a starting time for significant precipitation of 18-21z/Mon/26 Mar, with heavy precipitation by 00z/Tue (or in the case of the 06z GFS, about 3-6 h earlier) that begins to taper off some after 06-09z/Tue with the trough passage. But as noted quite a bit of precip after trough passage, almost .7 inches inthe 06z GFS run, through Tue night before ending around 12z Wed/28 Mar. The total precip in the 06z run was 2.17-2.25 near BLU, with the 00z run around 1.35 inches. Both runs peak the wsw to sw 700 mb flow ahead of the trough around 50 kts. Snow levels will lower way down behind the system with 850 temperatures near -5C by 12z/Tue/27 Mar and so could drop to 3000 feet or so on Tuesday.

The latest 12z GFS is just coming in and as noted it brings more precipitation closer to the HMT area on Sunday afternoon. Expanding to a bigger view clearly shows that this precip is in the warm sector and has a direct connection to the current atmospheric river from the tropical Pacific into the jet now cruising across the central Pacific. Furthermore, on Mon/26 Mar this moisture consolidates and pushes into the HMT area ahead of the main trough. This suggests the potential for a greater amount of warm sector precip than the models may be forecasting at this point. The 12z GFS puts the heaviest of this moisture ~70 mi to the nw of BLU 18z/Mon-00z/Tue with over an inch in this period, with 0.35 inches at BLU. 700 mb SW winds are 50-60 kts at 00z/Tue. In the next 6 h ending at 06z/Tue 1.23-1.40 inches fall in the BLU area. Then the winds shift to west with the trough passage, with a secondary (more than a third of an inch in 6 h ending 00z/Wed) max in the cold air advection the following afternoon, with precip ending by 06z/Wed. Total precipitation from the event for the 12z GFS then is 2.7 inches at BLU to 3.5 to the northwest. So it still looks like a system worth watching with strong dynamics and good potential for over an inch of precipitation, eventual low snow levels, and locally higher precip amounts given strong orgraphic flow just ahead and behind the trough in an unstable atmosphere. Some remaining atmospheric river is possible, and there are signs the river now in place may be a factor in this storm if it hangs together, with the wild card whether the precip starts as early as Sunday. Other 12z runs just coming in include the Canadian Global model with a strong trough that is about 12 h slower than the GFS but moist (not showing much though on Sunday) and with over 2 inches of total precip in the HMT area. On the other hand the 12z NOGAPS does appear to sag the tropical plume far enough south to begin good precip near the HMT area before 12z/Mon, with heavy rains during the day and continuing into Monday night. Total precip is in the multiple inch category though kind of hard to pinpoint on the web graphics.

Beyond this system models vary between a close call and a quick shot of over an inch with the next wave approaching out of the wnw for around Thu into Fri (29-30 Mar). The 12z GFS keeps pretty much all the precip to the north of the HMT with this wave. It appears that how much this system digs is related to the progression eastward of a high amplitude system traveling across the Pacific (this is the closed low feature trailing the major jet and moving slowly eastwards noted in previous discussions that has been in a number of the model runs). In the latest GFS this system produces downstream ridging that forces the Thu/Fri wave to the north of the HMT area. In the 12z GFS the Pacific wave is a rather large closed low near 30N/137W with a substantial tap into tropical moisture by 240 h (12z Sun/1 Apr), which could be tempting if there is not a hard cutoff to the program at the end of the month, although in the current GFS beyond 240 h the system weakens when it does reach the coast late Mon-Tue/2-3 April. Of course there is a lot of uncertainty in all this, with the EC and Canadian runs having this wave come in sooner. The GFS ensembles have not been available since a partial run from 12z yesterday on most sites but tracking down some spaghetti plots generally find there is quite a bit of spread showing up by the end of the month but still a potentially active pattern in place. The 12z GFS ensembles are now coming in as usual and for the Mon-Tue event are in good agreement with the 12z GFS deterministic run, with about 40 percent of the members showing at least some precip Sunday afternoon in the HMT area. Otherwise, most members have the heavy precip falling Monday night. Spread increases considerably by the end of the week storm and even moreso for anything in April, but generally supports a trough in the West or near the West Coast.

ed szoke NOAA/GSD

March 21, 2007

HMT discussion for Wed/21 Mar

While yesterday's system missed the HMT area with only 0.03 inches at Blue Canyon, scattered thunderstorms did develop in the afternoon as predicted by WFO SAC, with 0.20 inches at SAC. The bulk of the precipitation though was just to the south where the southern portion of the trough slowed and a circulation developed. Max amounts seen on the ALERT gages were 1.99 east of Fresno, and 0.93 inches at Milo. So close but the split of the system likely cut the HMT area off from significant precipitation.

A look at the current hemispheric view shows our strong jet now crossing the Dateline with the center of the associated trough near 45N/175E. There has been a nice flareup of convection near 5-10N/130E and the water vapor loop shows a connection of moisture from this area into the system. As noted yesterday this moisture plume will continue to be stretched out as the jet and trough progress into the eastern Pacific with the plume first hitting more the Pacific Northwest this weekend and then ill-defined when the main trough comes through Mon-Tue of next week. Most of the models are progressive with the wave for early next week except for the ECMWF, which over the last two runs has been slower by 12-24 hours and deeper. Assuming all the precipitation this weekend remains to the north of the HMT, a potentially close call late Sunday, then the main precipitation via the GFS and most of the other models begins around 12-18z on Mon/26 Mar. The 12z GFS starts the rains about 18z on Monday. Heavy precipitation then falls late Monday/Monday night (especially 00-06z/Tue on the latest GFS runs with over an inch at Blue Canyon (BLU) in this period on the 00z GFS) before trough passage around 12-15z/Tue and then precipitation ending around 00z/Wed. Again, the ECMWF model would have a slower evolution. Total precip in the 00z and 06z GFS runs was 1.8 inches at BLU, with more to the nw (especially so with the 06z run). The12z GFS is a little bit faster with the trough passage and down some on the total precip at BLU, with 1.07 total but up to 1.75 to the northwest. Appears there would be some warm advection moisture initially and wsw to sw winds at 700 mb reach about 40 kts before trough passage in the 00z GFS, and actually peaking near 60 kts in the 12z GFS. The 12z Canadian model run was a little faster than the GFS by about 6 hours, and ends up with around 1.5 to 2 inches total precip in the area. The NOGAPS 12z run is also very progressive with a decent amount of precip. The new 12z ECMWF run is just coming in and remains a little slower and stronger than the other runs. As with the GFS, it is still a progressive system. All models then have a strong wave and a cold system, so probably good cold advection convective precipitation would follow whatever falls ahead of the trough. Additionally, in terms of any potential remaining atmospheric river, there does appear to be what remains of the extended band of higher precipitable water hitting the Pacific Northwest this weekend that shifts southward to near the HMT area ahead of the wave, but, as we noted in the call today, the models do not seem to produce precip with this. Perhaps if this moisture is a little better then we might expect an earlier start to the precip via warm sector precipitation ahead of the trough, so will watch for signs of that over the next couple of days in the model forecasts.

Looking beyond this system the GFS and Canadian models bring another wave more out of the northwest but rather potent for Thu perhaps into Friday with a quick shot of well over an inch of precipitation, although the new run of the GFS is farther north with this second wave and most of the precipitation misses the HMT area. Again, the ECMWF model (00z run) is way slower with this system, trying in fact to phase it with the closed low that is moving across the Pacific in all the models behind our system for early next week. So in the EC solution the system has not reached the coast even in 10 days. Interestingly, the 00z GFS ensembles support both GFS waves and there are really no members in the 00z GFS ensemble supporting the slower ECMWF solution for the Mon-Tue storm, and not many supporting the slower EC solution for later in the week. On the other hand, the ECMWF has done a decent long range job of showing a diving wave with the system currently in the western CONUS. Looking to the first week in April, the GFS ensembles support a general trough along the West Coast or in the far western CONUS. The 12z GFS ensembles have stalled in coming in, so won't wait for those at this point.

ed szoke NOAA/GSD

March 20, 2007

HMT discussion for Tue/20 Mar

All indications this morning is that our decider guy made a good decision to not have an IOP as the system continues to split and as a result most of the precipitation thusfar has been to the northwest of the HMT area (over an inch in far nw California), and now the greatest amounts from here forward look to be farther south. Blue Canyon (BLU) has yet to report any precipitation through 18z and the winds have been pretty light. SAC radar indicates scattered precipitation to the north of the HMT area with a more general area from SFO southward. The 12z models are all down considerably in the amount of precipitation forecast with this event, the GFS with only 0.23 inches total at BLU and the NAM about the same. Both have more to the south now. For the NAM this is in contrast to last night's run that had a solid area of .75-1.0 inches, while the GFS did better in keeping amounts much lower, generally in the .25-.5 inch category. Our 3 km models continued to be on the high side last night, with max amounts in the HMT area of around an inch and 2 to even 3 inches in that spot 70 miles to the nw. This mornings 12z local model runs are down on the total amounts but still high versus other models, with generally over half an inch forecast in the HMT higher terrain for storm total. A loop of the radar imagery suggests the large area of echoes just to the south will just miss the HMT area today, hence the lower amounts expected. Had this been just a little farther to the north then this might have been an event approaching an inch in places.

Now looking ahead, the other good news is that the system for early next week continues to look good, with a much more significant event possible. Looking at the big picture, the jet that has moved off of Asia is very impressive, with the 12z GFS analysis having 215 kts at 250 mb in the core near 35N/155E. This jet progresses across the Pacific this week and by 00z/Sat/24 Mar stretches from the Pacific Northwest back to the Dateline, where a trough will be deepening in the entrance region of the jet. The lead part of the jet will bring a dump of moisture this weekend but this should stay to the north in the Pacific Northwest, although it could get close to the area late Sunday. Our system will be the trough in the entrance region that gradually strengthens as it approaches the West Coast. The 00z models were all in good agreement with a moist system in the Monday to Tue/26-27 March timeframe except that the GFS was 12 to 18 h faster. The 00z GFS ensembles showed quite good agreement with a system, as did the Canadian ensembles, which often have quite a bit more spread. The ensemble mean of NCEP and Canadian members has over an inch of precipitation for the system, which seems respectable for a mean amount. The 12z GFS just coming in is slower, in agreement with the 00z model consensus, although it is not quite as deep as the earlier run, and so has a 2.75 inch total precipitation maximum more to the nw of the HMT area. The 12z GFS ensembles also support a Mon-Tue timeframe and there is good agreement among the members for a good system. The bottom line right now is this looks to be a much stronger system that could slow down somewhat (and in fact the ECMWF model does slow it down as it closes off the system just west of SFO on 12z/Tue in the very latest 12z run just coming in). There is a tropical tap back to the western Pacific initially with this system that gets stretched out as the jet moves eastward so when it does hit the West Coast there is not any real atmospheric river present, but the system is strong enough and will apparently carry enough moisture to have a 1 to 3 inch type potential precipitation event.

Beyond this system things remain active with the models consistent in showing another trailing cutoff low but there is a lot of spread in when or if this low would ever make it onto the coast, and that would be no sooner than the end of March and possibly not until early April.

ed szoke NOAA/GSD

March 19, 2007

HMT discussion for Mon/19 Mar

Nothing very persuasive among the observations or operational models this morning to think this system will be more than what was discussed yesterday. A few things remain that make it possible there could be more than an inch in some areas, most notably that, 1) the operational models all show more precipitation a mere 70 miles to the now of the HMT area, some over an inch, and 2) our 3 km runs all generate more precipitation (both ~70 miles to the nw with over 2 inches, and over the HMT area, with locally over an inch), and 3) there is a period of pretty decent orographic sw flow on Tuesday. Not sure how well the local models have been verifying on these max amounts, but there is up to 40 knots of sw flow at 700 mb for awhile on Tue ahead of the trough, before the winds shift to nw and weaken by 00z/Wed. Otherwise, the trend in the models has been towards more southern energy which is slowing things down even somewhat more, so most indications are that good precipitation would not start in the area until closer to 12z/Tue, with most but not all done by 00z/Wed, as showers linger to about 06z/Wed. The OAK RAOB this morning does not have a lot of moisture even though it is now into the intial lead plume, so this plume certainly has weakened a lot. SSMI imagery earlier this morning shows a couple of narrow portions exceeding 2 cm lurking offshore, however. HPC and the Sacramento WFO have both lowered their precipitation forecasts to under .75 inches max in the Blue Canyon area. The 09z SREF is generally in the .25-.5 inch range in the mean, with most members not far from this. So all in all still marginal, but still some chance (based on the 2 points above and the period of good 700 mb flow) that still could generate interesting precipitation amounts in the higher terrain.

Looking ahead, the trend with more southern energy is reflected now in the models diving the southern portion of this system to Baja and then the question is how long before it ejects to the northeast. This could affect the next system that was scheduled for this weekend, riding the strong Pacific jet stream of this week. This jet stream that has come out of Asia is quite impressive, forecast to be around 200 kts at 250 mb by later today. In fact, by 18z Tue the 12z GFS has an elongated 200 plus kt core from ~145 to 165E near 35N. The jet is forecast to weaken by the end of the week as it reaches the eastern Pacific but will carry a nice wave with it that now is predicted to wait until after the weekend to potentially affect the HMT area, perhaps this delay owing to the slow movement of the closed low that develops over the Southwest this week. Certainly a lot of uncertainty in all this, but the 12z GFS now forecasts a pretty moist system for the Monday/Tue timeframe (26-27 Mar) with ~1.5 inches in the GFS in the HMT area. This is followed by another potential storm towards the end of the week (Thu-Fri/29-30 Mar) and then another the following week. The EMCWF 00z run is consistent with the 12z GFS solution through the end of next week, as is last night's 00z GFS. Additionally, a number of members of the 12z GFS ensemble are in agreement especially with the system early in the week but also for something later in the week.

ed szoke NOAA/GSD

Looking more promising

Tropical convective forcing is getting better organized with the centroid ~0/120E while extending from the central Indian Ocean into the South Pacific. SST forcing and interactions involving the tropics and extratropics have been assisting the slow development of this region of enhanced tropical rainfall.

Other faster time scales of tropical forcing are also occurring. Several weak-moderate flare-ups of convection occurred across the warm SSTs of the west central-South Pacific during the past couple of weeks. A response was for zonal mean anomalous westerly flow (~5 m/s) across the northern hemisphere subtropical atmosphere (~25N). Wave energy dispersing through the Eastern Hemisphere subtropics interacting with this added westerly flow has led to a local intensification of the of the jet coming off of east Asia, leading to the split flow pattern across the North Pacific. This was the type of west Pacific jet intensification we thought was probable a couple of weeks ago, and felt the models would not capture this until that signal was represented in their initial conditions.

We think several different time scales of forcing involving complex tropical-extratropical interactions have a possibility of phasing together during the next couple of weeks to allow a MJO to develop in the region of the Indian Ocean to Indonesia. The Wheeler phase plot already supports this notion. Zonal mean anomalous easterly flow remains quite strong throughout the tropical and subtropical atmospheres contributing to a low relative atmospheric angular momentum regime. We think our loosely GSDM Stage 1 situation will mature during the next couple of weeks. For ARB, this means the troughs currently moving off of east Asia will be forced onto the west coast as ridge amplification occurs ~140-160W. The synoptic details are unclear; however, we think the odds do favor a "few" strong troughs to impact ARB during at weeks 2-3. Many ensembles are trending toward this solution.

Ed Berry and Klaus Weickmann
NOAA NWS and NOAA/ESRL/PSD

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March 18, 2007

HMT discussion for Sunday 18-March-07

Our system remains marginal for an IOP with a trend generally in the models towards less precipitation but still possibilities of around an inch in under 24 h for the higher terrain in the HMT area. Timing is about as discussed yesterday with onset of showers possibly as early as 03z/Tue but more likely after 06z, and then most of the precipitation ending by 21z/Tue to 00z/Wed, perhaps some cold advection showers continuing until about 06z/Wed. The main trend in the models today is towards more energy in the southern portion of the trough which is reflected in downstream digging much farther south, but too far to the south to benefit the HMT area. Additionally, the two plumes of moisture we were watching in the Pacific continue to diminish, with the SSMI imagery indicating barely an inch of precipitable water in the lead plume and less with the main trough, the center of which is now near 45N/145W.

As for details of the precipitation forecasts, the 12z GFS has barely 0.50 inches in the HMT area but a little bigger 0.25-.5 inch area than in the 00z run. More in the NAM in general, although right over the HMT about 0.5 inches, there is a spot with an inch to the north. The 09z ensembles (SREF) have a general area of .25-0.5 with most members in this range except a few Eta members around an inch as outliers. The 12z Canadian run is in the .5-.6 inch range. The latest GFE grids from SAC WFO have precipitation beginning around 06z/Tue with near 0.7 inches at Blue Canyon by 00z/Wed. The HPC forecast was more optimistic with almost a one inch max at the higher terrain. They noted the strong orographic flow which the models do have at 700 mb for a short period (maybe 12 h) of around 30-40 kts out of the sw through 18z/Tue then a trough passage 21-00z. Not such great flow at 850 mb, however. The potential for the orgraphic enhancement and the fact that there at least is an extensive, albeit diminishing, plume approaching (as the two plumes combine) with possible convective activity gives a little reason for optimism that could lead to local amounts over an inch. But the overall trend in the model forecasts goes the other way, so in general this remains marginal but perhaps interesting for a short-duration event.

In the longer range the very strong Pacific jet will be the main feature this week with zonal flow over 180 kts in the jet max. The lead edge of this jet reaches the west coast next weekend with a potential system probably in the Sunday/25 March timeframe. However, most of the ensemble members take the bulk of the precipitation to the north with this as it then dives into the intermountain west. This suggests another fast-moving event if it does materialize. A nice tap to the western Pacific tropical moisture early in the week with this system but then the moisture plume is stretched out tremendously by the time it reaches the west coast, so no great tropical connection is likely. Still, with such a strong jet could be potential for a decent event. Beyond this period the flow in the Pacific buckles again into an active pattern of progressive waves which tease the HMT area into April. Nothing definitive but potential, and not a strong ridge over California to say nothing, so lots to sort through tomorrow.

ed szoke NOAA/GSD

March 17, 2007

HMT discussion for Saturday 17-March-07

As we get closer the approaching system is still on track but remains marginal for an IOP and a tough decision to probably make today either way. The trend in the 00z models and the 12z runs is for a slightly slower evolution (moreso now in the 12z runs) with precipitation holding off until about 00z/Tue/20 Mar and then lasting until around 21z or so (maybe a little later) on Tue before becoming more showery and ending. Consensus on the timing of trough axis passage at the HMT area would be 00z/Wed. Amounts are down a bit, max in the 00z GFS (using web graphics today) in the 0.75-1.0 inch range, less on the NAM. Other models with precipitation also generally down a bit, below an inch total. ECMWF and NOGAPS are even a little slower than the the GFS but the RH field from the ECMWF (no precip available) looks less than yesterday. The 12z GFS has a broad area in the 0.5-.75 inch range with a tiny spot above .75 just north of the HMT, so down a little more from 00z. The NAM is a little less, in the 0.5-0.7 inch range. Just saw the 12z Canadian run and it agrees with a slightly slower solution, so the bulk of the precip probably 09z Tue through 00z/Wed with a trough axis passage around 00z Wed, and it is a little more moist than the 00z run.

We are getting into the range of the short range ensembles, and a look at the 09z set shows a mean total precipitation in the 0.5 to 0.75 inch range, with most runs in this vicinity, and a couple in the inch or more category. The 12z GFS ensembles are trickling in. For the Tue storm they agree on the slightly slower trend seen in the 12z deterministic run, with a few runs having more southern energy in the system.

A look at our wildcard Pacific plumes noted yesterday shows they are still there, the closest near 140W aligned more north to south but in the satellite loop gradually weakening with time. The U of Hawaii site had a 6-h forecast total PW from the 06z GFS and it suggests up to 1.5 inches max in this plume which looks to be in agreement with the SSMI imagery, but by 24 h into the forecast really diminishes the moisture. It has not weakened to that extent in the last 24 h according to the SSMI imagery, so this is something we could recheck on Sunday to see if the model is weakening all this too much. The wave for Tue is also carrying a moisture plume but the PW analysis and forecast shows it as VERY narrow (though the water vapor imagery is more impressive) and also weakening with time. Still, they are forecast to combine offshore and this is the moisture hitting the area on Tuesday. Based on all this and the latest 12z ensembles showing some slowing potential, would think this is still worth waiting another day to decide on, especially as it does not seem soundings would be needed before 00z/Tue.

Looking beyond this system the Pacific does remain active and waves moving off Asia tap into the tropical moisture in the western Pacific. A very strong jet (greater than 180 kts in the forecast from the 12z GFS) is predicted to move out of Asia early next week and progress across the Pacific. One potential wave of interest for next weekend (24-25 March), though it may stay to the north. However, last night's ECMWF run was most favorable with a closed low sitting off the central California coast by 00z Tue/27 March. And the 12z set of GFS ensembles coming in now are trending to having more members with potential for something on Sunday 25 March. Other systems are then in both the 00z and 12z ensemble forecasts for the last week of March with a lot of spread and large timing differences, but certainly potential as a number of the members show closed lows forming at least within the area of interest. Given this and that massive jet, appears we will be tempted to hang on the way it looks now, but something for the long range crew to sort out on Monday's telcon.

ed szoke NOAA/GSD

March 16, 2007

HMT discussion for Fri/16 Mar

Summary: still a potential IOP, latest GFS a little faster. Marginal system, with total precip around an inch or so but some uncertainty still given moisture plume lurking in the eastern Pacific.

The upper level ridge this morning is firmly established in the far west with the moisture plume way to the north and then extending into the Pacific where it meets an extensive north-south band of moisture near 145W that goes all the way south to the tropics just east of Hawaii. Some of the moisture in this north-south plume gets caught up ahead of next weeks trough and is likely what brings the initial warm precipitation later on Monday and Monday night. Our wave of interest is currently approaching the Dateline with an associated jet near 150 kts. The U of Hawaii site PW presentation from the 12z GFS indicates a very narrow plume of moisture from this system back to the southwest towards where the tropical convection discussed earlier in the week by Ed Berry has been flaring up, but it is a narrow stream and with time tends to diminish as the wave moves eastwards. Behind this system is another storm with a large jet streak now moving out of Asia near 30N that is forecast to increase in magnitude and keep our system moving. The 00z GFS has this jet near reaching the Dateline by 12z Wed/21 Mar at 180 kts, with the associated trough centered around 35 to 40N. As this system moves across the Pacific it is tapping much more into the tropical moisture, and in fact a bit of this moisture may be in the secondary wave that follows our Tue system, even though right now (as noted below) that second wave does not produce much precipitation in the HMT area. At first this jet drives into Vancouver and the Pacific Northwest late next week, but eventually (by 240 h/00z Mon/26 Mar) both the 00z GFS, ECMWF and Canadian models have a pretty good shortwave trough position just north of the HMT, although in the deterministic GFS run beyond 240 h it stays too far north to produce precipitation of note. The latest 12z GFS has this system much farther south for a potential IOP. Of course it is a long way out, and the 00z GFS and Canadian ensembles actually have quite a bit of spread by this time. The 12z GFS ensemble just coming in shows even more spread by 240 h, with no single member quite matching the deep trough shown in the deterministic 12z run.

As for the details of Tuesday's system, things are not largely changed from what we discussed yesterday. An open wave, fairly fast-moving, with a limited tap into any tropical moisture but some, including the band of moisture now sitting near 145W. This all adds up to around an inch of precipitation (about 0.80 in the 00z GFS) or so for the HMT area, with potential for more locally but still enough uncertainty that it could be less overall. The 00z model runs generally had similar timing with the trough axis reaching the California coast about 12z Tue/20 Mar, then passing the HMT area in the GFS around 18z, a little slower in the ECMWF and even a bit slower in the UKMET as well as the NOGAPS. The Canadian run is pretty close to the GFS but with less precipitation in the HMT area. The 00z GFS ensemble members are pretty similar, maybe a slight tendency to have a few more that are a little slower. The 12z GFS is a little (maybe 6 h) faster than the 00z GFS with the trough. As with the 00z solution, warm sector precipitation begins Monday after 18z apparently tapping into the plume now in place near 145W. The GFS40 forecasts only a few hundreths of an inch of precipitation by 00z/Tue in the HMT area, but close to 0.30 inches a little farther to the north. The main precipitation falls in the next 12 h, with near 0.70 inches in the HMT region but closer to an 1.25 inches to the north, all by 12z/Tue/20 Mar, with very little precipitation then falling after 12z Tue, owing to the faster trough movement. The 12z GFS ensemble forecasts are in excellent agreement for Tuesday's storm, with a minority of members a tad slower.

This latest GFS solution keeps the IOP potential on the marginal side, so suspect we will have to just update over the weekend. The wild card may be just how much moisture is left in that band now near 145W and whether the models have a good handle on this moisture. The other wild card is the trailing shortwave for Wednesday that seems to be tapping into some of the tropical moisture much farther to the west, and whether this will sag far enough south to produce anything in the HMT. The 12z GFS says no to the second wave, with enough ridge building to keep any significant moisture to the north. The 12z Canadian run is pretty similar to the GFS forecast. The 12z NOGAPS is a little slower, with the trough deepening as it enters California. For the HMT area, though, the timing of the precipitation is not a lot different than the GFS and Canadian models. Finally, the 12z ECMWF is just coming in. It is about 6 h slower than the 12z GFS with a little deeper overall trough, and then slows some as the trough axis goes east of California by 12z Wed. Trough axis passage though across the HMT area could be up to 12 h or so slower than the GFS. I don't get to see any precip output from the ECMWF but would think it could be more than the GFS given a slower movement and deeper system.

In the very long range beyond 240 h as noted earlier both the Canadian and GFS 00z ensembles and the latest 12z GFS ensembles have considerable spread, with potential for something but certainly no overwhelming agreement on any system. Hopefully Monday's long range discussion with Ed and Klaus will give us a better idea of the longer range.

ed szoke NOAA/GSD

March 15, 2007

HMT discussion for Thu/15 Mar

I'll give the bottom line first...potential IOP is still there for early next week. An open wave solution looks likely, so not a prolonged event, and this means potentially marginal precipitation totals. But the amount of tropical tap is uncertain, so greater than 1 inch totals seem quite possible at least. Timing could be as early as precipitation beginning around 21z Monday or certainly Monday night, with things wrapping up late Tue to around 00z Wed at the latest the way it looks now. There was in general more optimism on the possibility of an IOP for this time frame among the folks on today's telcon. The gory weather details are below....

Took a look at the jet stream analysis at 12z this morning near and west of the Dateline over the southern latitudes near that blowup of convection discussed in our recent telcons. Not much difference between the 24-h forecast from the 00z/14 Mar GFS run and the jet analysis, but there is some difference between the previous 00z run (from 13 Mar, before the blowup) with the 48-h forecast jet not as far south as analyzed, especially near 155E. This jet has pushed eastwards from the upper low that had moved off of Asia in the western Pacific and with it is a strong upper level low at 500 mb currently near 170E/40N. Looking at the GFS on the U of Hawaii site, there is a small plume of moisture (model precipitable water) extending northeastwards from this tropical convection into this system near 170E. The upper low downstream from this system continues to weaken as it moves to the northeast, evolving into the deep upper low that extends into the Gulf of Alaska. There is a nice plume of moisture with all of this extending from east of Hawaii northward along 150W and then eastward mainly into Vancouver, south into northern Washington, with the downstream ridge building just off the California coast and downstream from this a trough deepening east of the Rockies. The strong wave near 170E/40N is the one that progresses across the Pacifc and provides an opportunity for an IOP early next week. Behind this wave a strong jet is forecast to emerge off of Asia early this coming weekend and the U of Hawaii site shows a much more significant moisture plume forecast to get involved in that system beginning tomorrow. This jet and associated wave remain quite strong as they progress across the Pacific, and this builds the ridge behind next week's system. In response the GFS and other models are forecasting a deepening of next week's system but after it passes the HMT area, so more over the intermountain west, by Wed-Fri/21-23 March. In the 00z GFS deterministic run the result was another ridge along the West Coast and dry in the HMT area almost through the rest of the month, although at day 15 in the GFS run there is a large system with a tropical connection lurking well off the West Coast. Variations, however, are found in the ensembles, discussed later.

As for details on the potential IOP next week, a review of the 00z deterministic runs indicates the forecasts continue to be split as they have been over the last couple of days, although big differences are dissappearing. The GFS remains the most progressive solution, with an open trough about onto the West Coast by 06z/20 March (Monday night), then the trough axis passing the HMT area 12 h later, with the precip diminishing on Tue afternoon (21 March). Total precipitation from the GFS is barely above 0.50 inches by late Tue. A second wave does follow this Tue night into Wed (20-21 March), but additional amounts are not great, into the 0.10-0.25 inch range. There is a rather weak tie to some tropical moisture, some of which is left behind by the current plume of moisture noted earlier near 150W. Other 00z model runs are somewhat more favorable, in that they are slower and some are deeper with the wave. This ranges from the ECMWF run, which is trending farther south (maybe a bit too far south for the HMT) and is about 12 h or more slower than the GFS. The UKMET has more of an open wave reaching the West Coast near 00z/Wed/21 March, so slower than the GFS. The Canadian run is a bit faster than the UKMET, maybe 6-12 h slower than the GFS, and with a little more precip than the 00z GFS. The NOGAPS is missing a critical time period but overall is also progressive, close to the Canadian run. The summary of the 00z runs is that this is a system we will have to watch, but at this point is still on the marginal side, for a quality IOP.

The 12z models are more encouraging, especially the GFS, in that while still an open and progressive wave, more precipitation is forecast, perhaps owing to a better tropical tap. Precipitation actually commences around 21z on Monday/19 Mar, and ends around 18z on Tue/20 Mar. Maximum is right in the HMT area and around 1.25 inches. Given orographic enhancement one could expect local amounts to be higher if this model solution verified. There is no secondary system right behind the main one in the 12z run, as a very weak shortwave stays well to the north on Wed. The 12z Canadian run is pretty close to the 00z GFS, and not quite as favorable as the 12z GFS. The 12z NOGAPS continues to be missing a key time, but appears to be a tad slower than the 12z GFS. Finally, the 12z ECMWF run, not quite fully in, is more favorable than the 00z run in that it has an open trough, deeper than the GFS, but not trending to quite as deep a southern portion as the 00z ECMWF run that would be too far south of the HMT area. Timing is a little slower than the GFS, maybe by 6-12 h or so.

Environment Canada has an interesting ensemble presentation that includes precipitation (at http://weatheroffice.ec.gc.ca/ensemble/cartes_e.html# ) for both the set of 00z Canadian runs and the 00z NCEP runs. They show a precipitation rate presentation at 24 h (valid at 00z) intervals and most of the members have the heaviest rate in the vicinity of the HMT 00z/21 March. A few still have something going on 24 h later, but they are certainly in the minority. As might be expected given we are not so far off into the future, the GFS 00z ensembles do not show a lot of variation with the system early next week, although there are a few members a little deeper and slower than the 00z deterministic run. There is a lot of variability into the following week, however, and this is true of the set of Canadian runs as well, with a number of members having potential for something into the HMT area rather than having the closed low staying well off the west coast. So at this point it would be impossible, it seems, to say that next week's system will be the last thing for March. The 12z GFS ensembles just coming in actually seem to have a little more spread for next week's system, with a few runs even a tad faster, but more of the runs slower and a little stronger than the 12z GFS deterministic run. And in the longer range, begining later next week, there is quite a lot of spread, similar to the 00z ensemble set, so lots of things open to possibility.

ed szoke NOAA/GSD

March 14, 2007

HMT discussion for Wed/14 Mar

There remains a chance of an IOP next week but at the moment the GFS model is the least favorable for IOP quality precipitation, while other models continue to be more favorable. The current situation is not much changed from what we have been discussing this week with warm and dry weather for the HMT area. The strong wave we were tracking across the central Pacific is now lifting to the northeast as it crosses 160W and will help build the ridge along the West Coast over the next couple of days as well as the downstream trough. Meanwhile our eventual system of interest and associated jet are making their way towards the Dateline around a strong upper level low that moved off of Asia. The burst of tropical convection noted by Dave Reynolds in yesterday's discussion may be finding its way into the longer range forecasts as there was quite a change from the 00z/13 March GFS run to the more recent runs in the structure of the various waves that follow the system for midweek next week.

The basic picture for our potential system next week is that the GFS deterministic runs remain on the weaker and faster side, including the latest 12z run. The GFS runs basically bring a fast-moving open wave across the HMT area very late Monday and Tue/19-20 March, with 0.5 to about 1 inch or so of total precipitation in the area. This is followed quickly by a second wave with cold advection precipitation but most of this is more to the north as this wave dives more into the intermountain west. The ECMWF 00z run was far more favorable for a slower-moving and potentially (unless it is too far south) wetter system, commencing about 24 h later than the GFS and coming as one main wave into perhaps Thu/22 Mar. The Canadian and NOGAPS 00z runs were more progressive but generally stronger than the GFS, with slightly slower timing. The Canadian 00z ensembles support more of a progressive system, as do the GFS ensembles, although there are some members that are slower and more favorable than the GFS deterministic run. A look at the latest 12z GFS ensemble forecasts indicates a few more members with a bit slower and more favorable solution than the 12z GFS deterministic run, so a favorable sign. Also, the latest 12z NOGAPS is stronger than the 00z run and more like the 00z ECMWF forecast, though somewhat faster with most of the precipitation for the HMT on Tue/20 Mar. And the latest 12z runs of the ECMWF, Canadian, and UKMET models are all very similar out to 144 h (12z Tue, the end of the Canadian and UKMET runs) with a deep (deeper than the GFS) trough about to come onshore in California, at least 12 h slower than the GFS 12z run. These other models would support an IOP possibility beginning on Tue/20 Mar. The ECMWF run goes out farther, and by 12z Wed the axis of the deep trough (one system, not 2 as in the GFS) has pushed into western Nevada. So this would imply a wetter system than the GFS but still a relatively fast-moving event on Tue extending into Wed/20-21 March.

Beyond the system next week, the ensembles show more variability, including the latest 12z GFS ensemble, perhaps in relation to the burst of tropical convection noted earlier. There are a couple of potential systems for days 9 to 15 that could be of interest, which, as noted earlier, is more active than the 00z/13 March ensembles discussed yesterday.

ed szoke NOAA/GSD

IOP on track?

The weather disturbance talked about Monday and yesterday for next week continues to provide an opportunity for an IOP. Tropical convection has been reorganizing over the past few days as a weak dynamical signal from the recent MJO moves through the western hemisphere and convection increases again over Indonesia and the west Pacific. The increase is being aided by warm SSTs centered around 160E and favorable upper level wind anomalies. The possibility exists that the mid-latitude distrubance next week or the one following will interact with this convection and help produce an extension of the Pacific jet stream and drive disturbances into the west coast.

Subtropical westerly winds are relatively weak although we expect some strengthening near 25N as the opportunity for an IOP approaches. Overall the pattern still favors ridges over the east-central Pacific with downstream troughs over the western North America. This pattern may re-exert itself after week 2 if convection shifts westward or a new MJO develops over the Indian Ocean. In the meanwhile we feel the ECMWF ensemble and deterministic GFS and ECMWF provide the best guidance for the next week.

For additional discussion please see

http://weatherclimatelink.blogspot.com/

Klaus Weickmann and Ed Berry
NOAA/ESRL/PSD and NOAA/NWS

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March 13, 2007

HMT discussion for Tue/13 March

Quiet weather continues across the HMT, while prospects remain for a potential IOP next week. First the situation in the near-term; a flattening of the ridge along the West Coast today and tomorrow as a very strong jet in zonal flow moves eastward across the Pacific Northwest along the U.S./Canadian border. As it moves farther to the east by Thursday strong ridge building occurs right into the HMT area with record setting temperatures likely towards the end of the week. Looking at the current situation in the Pacific, an upper low remains over Alaska extending south into the Gulf of Alaska, with a strong jet extending into the Pacific Northwest, as noted above. The upper level low that was crossing the Dateline yesterday has progressed eastward to 35N/165W. This system will weaken and become absorbed into the Gulf of Alaska system as it moves to the northeast, building the ridge downstream over California. A deep upper level low has moved eastward off of Asia with a 150 kt jet on its southern end, and it is this jet and associated wave that makes its way across the Pacific and eventually is our system of interest for next week.

The models remain in good agreement through this weekend with the dry and warm pattern for the HMT area. For next week there is agreement that a trough will move into California, but spread among the models as to both timing and especially strength. The GFS continues to be the most variable of the models, with the last two main (00z and now the latest 12z) runs tending to have a weaker and faster moving open trough that quickly passes across the HMT region around Tue/20 Mar with limited (under an inch) precipitation. The latest GFS 12z run, however, follows the first system with a more moist and colder system late Wed into Thu (21 to 22 Mar), which was far drier and farther north in the 00z run. I don't usually pay much attention in the longer term to the off-hour GFS runs (18z and 06z), but both of these were consistent in showing a slower and far more moist event in the Wed-Thu timeframe. As for the other models, the 00z NOGAPS is similar to the 00z GFS with a minor system, although the 12z NOGAPS has a stronger and slower wave approaching the West Coast by the end of its run on 00z/Wed/21 Mar. The ECMWF and Canadian 00z runs are similar with a slower and wetter system, beginning to impact the HMT area perhaps late on Tue/20 Mar but certainly by Wed, and perhaps continuing through Thu/22 Mar. A look at the 12z Canadian GEM model (only available through 144 h or 12z/Mon/19 Mar) forecasts a slower system with more energy in the southern part still off the coast. The ECMWF 12z run is just coming in but too slowly to make it for this discussion.

Probably it is not surprising, given the somewhat fluctuating GFS runs, that the 00z GFS ensembles have more spread now for the system, and a majority of the runs are deeper and slower with an open trough system for next week when compared to the deterministic GFS 00z and 12z runs. No 12z GFS ensembles are yet available. The set of Canadian ensemble forecasts from 00z have even more spread and generally a split between the ECMWF/Canadian deterministic solutions and the GFS. Bottom line at this point is a potential system still on track for next week, the earliest beginning time would probably be late Monday, but more likely Tue/20 Mar. Beyond this system, the models and ensembles have a lot of spread and possibilities of something else but no strong indications at this point. During the conference call Dave Reynolds mentioned the blowup of convection near 140E and this may lead to some model impact for tomorrow's runs, so stayed tuned.

ed szoke NOAA/GSD

March 12, 2007

HMT discussion for Mon/12 Mar

A distinct plume of moisture extends from a shortwave moving through the Pacific Northwest back to a separate southern wave near 30N/140W. This southern wave will remain separate from the overall upper level low over Alaska. West of the plume in the Central Pacific is a strong wave that has just crossed the Dateline near 45N, and farther to the west a strong wave has moved off of Asia just north of Japan, so pretty active in the Pacific. Quiet though this week for the HMT with the building of an upper level ridge commencing later Wednesday after the passage of today's system and another shortwave tomorrow. Very toasty temperatures with dry conditions will prevail this week. The southern system currently near 30N/140W that was mentioned yesterday as drifting to the east in the GFS forecast to near the California coast does not seem to be in the more recent forecasts although some other even weaker wave is that apparently interacts with the southern wave, but all of no consequence to the HMT this week as there is no moisture with any of this.

So attention remains on what might come next week. The 00z GFS went to an open trough solution, reaching the West Coast on late Tue/20 Mar to early Wed/21 March, followed by another wave somewhat farther north and then an intermountain trough by the weekend into the following week. The timing for midweek and the open trough is similar to the forecast from the 00z ECMWF run and previous runs, and also similar to the open trough solution in the 00z Canadian run that has a moist system hitting the HMT area on Wednesday. The latest (12z) GFS is similar to last night's run, though a tad faster, with a potential IOP beginning on Tue/20 Mar. All of these runs, while supporting a potential IOP next week, do forecast a fairly fast-moving system. A look at the 12z GFS solution on the U of Hawaii site for precipitable water shows a nice plume as the system approaches towards the West Coast this weekend but it weakens with time considerably and is less well-defined by the time the trough makes it onshore. The 00z/GFS ensemble now has most members supporting the open trough solution and not the closed upper level low we saw on a couple of the GFS runs. The Canadian 00z ensemble solutions show more spread but generally at least through midweek next week agree with a potential system. The solutions beyond the first wave for midweek next week generally do not support any additional IOP possibilities at this time, with some spread and mostly a trough farther inland in the West, although certainly more variability leaving open some possibilities. The latest 12z/GFS ensembles just in have all but one member now with an open wave solution that is even faster, so more like a potential Tue/20 Mar IOP, and quite diffferent from the deterministic GFS runs we saw yesterday and the day before. And, hot off the presses, the 12z ECMWF run is in very close agreement with timing and the structure of the system through day 10 (12z/Thu/22 March).

The bottom line then at this point is a quiet and warm week this week through the weekend, then good agreement on a potential IOP next week, now favoring the Tue-Wed/20-21 March timeframe rather than later in the week.

ed szoke NOAA/GSD

March 11, 2007

HMT discussion for 11-March-07

The plume of moisture in the eastern Pacific continues from near Hawaii to Washington and Vancouver this morning associated with a trough that extends north to the upper low in the Gulf of Alaska. The southern piece of this system is what breaks off to form a weak closed low that drifts to the California Coast later this week in the GFS but without any precipitation in the model forecasts. Other models are not so distinct with this feature. The main portion of this trough moves across the Pacific Northwest through Monday and then dives into a developing mean eastern CONUS trough while ridge building commences along the West Coast and the HMT area.

To the west in the Pacific another strong system is found near the Dateline at about 35 N, and then farther to the west a strong wave is exiting Asia into the western Pacific. The one near the Dateline phases with the overall Alaskan upper low and helps to build the ridge downstream this week. This is all in decent agreement amongst the models, but then things diverge by next weekend (17-18 March). The 00z GFS splits the trough well offshore and slowly sends a closed low, this time with moisture, into California mid to late week (~22-23 March), similar to what we saw in yesterday's 12z run. The 00z ECMWF run has similar timing but more of a phased trough without a distinct southern closed low. The deterministic Canadian model run through 240 h from last night is similar to the ECMWF run. The GFS ensemble forecasts from 00z are in good agreement with a potential system (varying somewhat in timing from early to midweek of the 18-24 March week to later in the week) and generally split between the closed low solution in the GFS deterministic run and the more phased trough found in the ECMWF and Canadian runs. As usual more spread in the Canadian ensemble (the one from the Penn State e-wall) but still a number of members with a strong trough into the West Coast in this timeframe.

The 12z GFS run just in is similar to the 00z GFS with a southern closed low solution but is even slower in moving it onto the coast, waiting until 25 Mar to do so and then without a lot of precipitation. Probably an attempt to frustrate HMT planners...the 12z ensemble has a majority of members with a closed low solution but most have better timing (mid to late in the 18-24 Mar week) than the deterministic run and there are a few solutions with a faster open wave solution similar to the 00z runs of the Canadian and ECMWF models. No 12z ECMWF run in yet with the time change.

ed szoke NOAA/GSD

March 10, 2007

HMT discussion for 10-March-07

A nice plume of moisture in the Eastern Pacific stretching back to Hawaii but unfortunately directed even farther north than yesterday towards Vancouver and Washington. The plume will sink to the south by late Sunday into early Monday as the wave moves onshore in the Pacific Northwest but precipitation will remain well north of the HMT area. Slight ridge building in California behind this wave but then the ridge is flattened a bit as a strong jet and associated fast-moving shortwave moves around the Alaska upper level low in zonal flow across the Pacific Northwest on Tuesday. Precipitation with this feature remains well to the north of the HMT. This is the wave that then dives into the eastern CONUS to create the trough there by later in the week with the substantial upstream ridging that then takes place along the West Coast. Excellent model agreement through the week with this scenario. So, overall, not much different from what has been talked about the last couple of days, dry for the HMT area.

A slight wild card on the 12z deterministic run is that it takes a piece of the southern energy from a strong wave moving across the Pacific and separates a closed low that then drifts to the West Coast by next weekend, but confidence in this would be fairly low. Additionally, the 12z GFS has absolutely no precipitation with this particular feature. If that feature doesn't amount to anything than the hope remains on an overall change to a more western trough for the following week (period of about March 20 and beyond). Last night's deterministic run of the GFS was not very favorable for precipitation along the West Coast as it set up the trough too far inland, but this morning's 12z run is substantially different, with a full-latitude trough approaching the West Coast on Tue/20 March with the southern portion cutting off and plowing right into the West Coast by Thu/22 March with lots of precipitation. The eventual result is a far weaker western trough than earlier runs of the GFS and the GFS ensembles. It appears the differences in the runs are related to whether the associated system coming across the Pacific later this week closes off and stays well to the west in a position north of Hawaii (as in last night's GFS and ECMWF run) or phases at least for awhile with the Alaskan upper level low that opens up. Just looked at the latest 12z ECMWF run out through 240 h and it supports the 12z GFS solution.

The above model differences are reflected in a spread in the GFS and Canadian model ensemble forecasts from 00z, with spread increasing considerably after good agreement through next weekend (17-18 Mar). A number of the members support the 12z GFS solution for a potential event midweek (20-22 March). The 12z GFS ensembles are generally similar to the 00z set from the GFS, with greater spread by the last week of the program but several members supporting a potential IOP midweek (20-23 March or so).

ed szoke NOAA/GSD

March 9, 2007

HMT Daily Wx Discussion & Forecast: Fri. 09-Mar-2007

It remains pretty messy looking off the West Coast this morning in the water vapor imagery, but the models are consistent in keeping the deeper moisture north of the HMT area today and Saturday and then drying things out as an upper level ridge builds over California. This morning the main shortwave is moving into Oregon with good precipitation in the Pacific Northwest, extending southward to near Ukiah, California. It looks like any precipitation in the HMT area is either remaining aloft or quite light today, and none of our 3 km runs produce any precipitation of note in the HMT area. The overall plume of moisture stretches back well to the west-southwest to north of Hawaii, where the moisture is consolidating ahead of a sharp upper-level low near 160 W that is opening up after cruising across the Pacific. This system will move to the northeast around the southern portion of the deep upper level low in the Gulf of Alaska and flatten the ridge somewhat in California Sunday into Monday, but again the heavy (and probably all) of the precipitation will be aimed at the Pacific Northwest. After this system moves by serious ridge building develops along the West Coast ahead of another pretty good trough progressing across the Pacific and this then keeps things very dry in California next week (12-17 March) with a deepening downstream trough over the eastern CONUS. As was noted yesterday, there is good overall agreement with this scenario among the various models and ensembles.

In the longer range the deterministic and ensemble runs available are still indicating a shift to a more western trough for the week of 18-24 March. The change in the pattern appears to be tracked back to a strong jet emerging out of Asia into the western Pacific this weekend that then moves eastward, reaching the Dateline towards the end of next week as it shifts northward toward the Alaska upper level low by next weekend (17-18 Mar). In the GFS 00z deterministic run this jet then ends up diving down the backside of the Alaska upper low to create a fairly deep trough off the West Coast by midweek of the 18-23rd, then bringing potentially several days of rain to or at least near the HMT area for mid to late in the that week. The latest 12z GFS deterministic run is similar in bringing about a western trough during the later part of this week (18-24 March), but is farther inland and as a result never really brings much in the way of a rain event to the HMT area. There are a few members of the 12z GFS ensemble that have more of a trough off the West Coast and would be more favorable for precipitation in the HMT area. All this points out that even if we get such a pattern change there is no guarantee of an IOP out of it, but at least it brings a chance of something. Of course all this is a long way, but the ensembles do indicate at least this shift in the pattern so some hope after what should be a dry spell through at least about 18 March and perhaps even 21 March or so as it now looks.

ed szoke NOAA/GSD

March 8, 2007

HMT Daily Wx Discussion & Forecast: Thu. 08-Mar-2007

For awhile there I thought I would not be doing any more forecasting for the project as it looked like all the resources would be used up by now, but the spigot shut off and now the quiet period continues for the HMT project area. It's not like there isn't a lot of action in the eastern Pacific; a look at the water vapor loop shows a nice plume of moisture connecting back to the tropics and all the way to the West Coast just to the south of an extensive jet. Unfortunately, all this is aimed more at the Pacific Northwest and this looks like it will remain the case through the weekend with the southern extent of the moisture perhaps getting tantalizingly close at times early in the weekend before more ridge building pushes things farther to the north. At this point there is no model solution to be found that would bring enough moisture far enough south this weekend to make for an IOP.

I am not very optimistic for the next 10 day period based on current deterministic runs and the GFS ensembles. As mentioned above the ridge actually starts to build along the West Coast by Sunday as a trough now passing the Dateline phases with the persistent deep upper low now in the Gulf of Alaska, pumping up the ridge downstream. By early to mid next week a decent shortwave trough emerges out of the Gulf of Alaska system and may change the weather around here mid to late week, but this system is well east of the HMT area which remains in a dry upper level ridge. This is the case through the next weekend (17-18 Mar) on the latest GFS, which by 240 h (12z/Sunday/18 Mar) has a longwave ridge centered over California and a broad and deep trough in the eastern CONUS. This is in excellent agreement with last night's GFS deterministic run as well as the 00z ECMWF 240 h forecast and the just-in 12z ECMWF run. The GFS ensembles from 00z as well as the Canadian ensemble forecasts have pretty decent agreement on this scenario.

Speculating beyond day 10, the 00z GFS forecasts a return to a western trough with a strong wave dropping southeastward in the nw flow off the Pacific and a subsequent retrogression of the upper level ridge. This may or may not lead to a potential wet system late in the week of 19-24 March, but it is a change and there is surprisingly good agreement at least in the GFS 00z ensembles with this change, as well as the Canadian ensembles. The 12z GFS ensembles just coming in are pretty much in agreement with the above scenario, especially through day 10, and then maybe just a bit faster for the potential system days 11-15. Bottom line for now is wet in the Pacific Northwest but very low chance of anything far enough south in the next few days, then most likely a prolonged dry period for some time.

ed szoke NOAA/GSD

March 7, 2007

HMT Daily Wx Discussion & Forecast: Wed. 07-Mar-2007

IR satellite imagery shows only scant cloud-top enhancement with the landfall of the weak shortwave trough across N CA. There is a hint of a baroclinic cloud leaf near Cape Mendocino, but that’s pushing it. The approaching cold front is seen as a quasi-organized band of warmer clouds arcing SSW from Cape Mendocino to ~32N/130W. Shallow cellular Cu is evident on the cold side of the front. SSM/I imagery portrays a very weak plume of enhanced PW barely surpassing 2 cm. NEXRAD radar coverage shows some light precip starting to impact the far N end of the Sac Valley. Somewhat heavier precip is affecting the extreme NW part of the state with the landfall of the shortwave.

The operational models still show the passage of the weak shortwave trough and dying cold front across N CA later today into tonight, with only very light QPF (<~1/4 inch) in the ARB region. Midtrop heights will rebound on Thursday behind the shortwave trough in response to warm advection downstream of a developing Gulf-of-Alaska cyclone. A flat (then amplifying) ridge over most of CA at the end of the week and beyond will likely keep significant precip north of the ARB, although extreme northern CA may have some precip in the extended and the Pac NW will certainly see precip. The latest GFS is hinting at a disturbance dropping SEward across the northern half of CA the middle of next week.

Paul Neiman
NOAA/ESRL/PSD2

Uncertainty rules

There are more indications that the climate system is rapidly moving toward La NIna conditions. Sea surface temperature anomalies of -2C are now evident in the east-central Pacific. However, the west Pacific Ocean west of the dateline remains warm and there is an MJO signal moving eastward around 10S.

Satellite images show current regions of organized convection are centered around west Australia and the SPCZ. Daily SST anomalies and totals show the warmth west of the dateline where convection is currrently suppressed. Time-longitude diagrams of outgoing longwave radiation (proxy for deep convection) show the MJO signal in the left panel and considerable westward drift to convection once it develops in a region.

The 250 mb vector wind field for 6 March 2007 (Tuesday) illustrates a complicated Pacific-North American circulation pattern with two anticyclones split by a trough over the Gulf of Alaska. Best developments have been occurring with the ridge and associated anomalies over the west Pacific which appear to be responding to tropical forcing seen in the satellite images. The model ensemble forecasts are not favorable for rain in the northern California region within the next week and continue pessimistic into week 2. The following link is a posting on Ed Berry's from yesterday, for additional information.

http://weatherclimatelink.blogspot.com/


Klaus Weickmann and Ed Berry

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March 6, 2007

HMT Daily Wx Discussion & Forecast: Tue. 06-Mar-2007

An elongated N-S cloud band (stretching from BC to 20N) is slowly approaching CA. IR imagery shows that the cloud tops in this band are warming with time. High clouds may start streaming across the ARB later today as this band begins to move ashore and a midtrop shortwave ridge migrates eastward into the Great Basin.

The models depict a large cyclonic circulation setting up over the Gulf-of-Alaska during the next several days. As this occurs, a weak shortwave trough aloft and dying polar cold front will rotate eastward across N CA tomorrow, perhaps clipping the ARB. Because the dynamics and orographics with this progressive shortwave will be weak, and the available moisture content modest at best, expect little more than light precipitation Wednesday midmorning through late evening (totaling <0.5”) across the ARB.

Midtrop heights will rebound on Thursday behind the shortwave trough in response to warm advection downstream of the developing Gulf-of-Alaska cyclone. A flat ridge over most of CA at the end of the week and beyond will likely keep significant precip north of the ARB, although extreme northern CA may have some precip in the extended and the Pac NW will certainly see precip. All in all, the wx across the ARB will likely remain dominated by relatively dry ridge conditions for the foreseeable future.

Paul Neiman
NOAA/ESRL/PSD2

March 5, 2007

HMT Daily Wx Discussion & Forecast: Mon. 05-Mar-2007

An ill-defined comma-cloud shield associated with the ejecting weak subtropical shortwave trough is presently exiting CA. There is some remnant midlevel cloudiness across the Sierras that should depart in the post-wave subsidence as the day progresses. Only a couple of precip gauges in the Sierras recorded very light amounts during the passage of this wave. Shortwave ridging tonight and tomorrow will keep the area dry.

The suite of operational models all depict a fairly weak shortwave trough clipping the northern Sierra on Wednesday and Wednesday night. The dynamics and orographics with this system will be weak, and the available moisture content modest, during passage of this wave and its associated dying cold front. It may be tough to squeeze more than ~0.5 inch of liquid in the ARB with this “storm.” Snow levels will be moderately high – in the 6-8 kft range.

A flat ridge over most of CA toward the end of the week and beyond will likely keep significant precip north of the ARB, although extreme northern CA may have some precip in the extended.

Paul Neiman
NOAA/ESRL/PSD2

March 4, 2007

HMT Daily Wx Discussion & Forecast: Sun. 04-Mar-2007

The upper ridge is migrating east of CA, with southerly-component flow kicking in. Also, the weak subtropical shortwave trough over the eastern Pacific is now on the move, with enhanced cold cloud tops approaching CA from the SW. Cirrus will increase, thicken, and lower as the day and evening progresses, but surface conditions will remain dry. All models show this S/W ejecting across CA tonight into tomorrow, but with scant QPF at best. The lack of significant QPF associated with this trough passage seems quite reasonable, given the weak dynamics associated with this trough, the large dewpoint depressions currently observed in the lower/middle troposphere over CA, and the predicted minimal onshore flow. Following the S/W trough passage tomorrow, shortwave ridging aloft will dominate the wx through Tuesday with generally dry conditions at the surface.

For midweek, all models have now seemed to stabilize on a rather unimpressive storm scenario for the ARB. The various solutions have the ARB under the influence of midtrop SWerly flow starting later on Tuesday as the trough slowly approaches, but the main pre-cold-frontal LLJ region is forecast to remain well to the NW of the ARB thru Tuesday night. Consequently, the frontal precip band is only impacting NWern CA by 12Z Wed. The shortwave trough and its associated low-level front eventually cross the ARB on Wednesday, but as a fairly weak and progressive system with only moderate orographics at best. In addition, the moisture plume associated with the storm is forecast to pinch off from its mid-Pacific subtropical origins before impacting the ARB. Based on the present forecast models, we could expect perhaps a 0.5-1.0” of liquid falling on the ARB on Wed.

Midtrop height rises are presently forecast to occur across the ARB toward the end of next week, with most of the precip shunted off to the north.

Paul Neiman
NOAA/ESRL/PSD2

March 3, 2007

HMT Daily Wx Discussion & Forecast: Sat. 03-Mar-2007

Beneath an amplifying ridge aloft, California’s high country is in store for a beautiful early spring day today, with mostly clear skies and moderating temperatures. The latest IR satellite loop shows the leading edge of enhanced cold cloud tops approaching CA from the SW, so some wispy cirrus may float across the ARB later today. The subtropical shortwave trough creating the cloud shield offshore is presently centered near 30N/135W.

The current operational models still show the S/W ejecting NEward across CA tomorrow into Monday. Thickening clouds may be all we can muster out of this system, given the very dry conditions presently residing across CA in the lower and middle troposphere. Once the wave moves downstream of CA on Monday, shortwave ridging will result in decreasing clouds on Monday into Tuesday. Bottom line: no significant precip is expected across the ARB from now into at least Tuesday.

Thereafter, each successive model run shows a slower, weaker system approaching the U.S. West Coast for Tuesday and Wednesday. The 12Z NAM and GFS has the ARB under the influence of midtrop SWerly flow starting on Tuesday as the trough slowly approaches, but the main pre-cold-frontal LLJ region is forecast to remain well to the NW of the ARB on Tuesday. Consequently, the frontal precip band is only marginally clipping the extreme NW portion of CA by 00Z. The shortwave trough and its associated low-level front eventually cross the ARB on Wednesday, but as a fairly weak and progressive system with only modest orographics at best. In addition, the moisture plume associated with the storm is forecast to pinch off from its mid-Pacific subtropical origins before impacting the ARB. Hence, both the present models and d(model)/dt suggest that the midweek storm across the ARB will not be a big one. With that said, this potentially warm storm still deserves our attention, given the existing low-elevation snowpack laid down in the Sierras during IOP6 and 7.

Paul Neiman
NOAA/ESRL/PSD2

March 2, 2007

HMT Daily Wx Discussion & Forecast: Fri. 02-Mar-2007

Midtrop heights across CA are rebounding today in response to warming temperatures aloft. The latest IR satellite loop shows mostly clear skies across the ARB, although enhanced clouds/some precip are impacting the far north of CA with the passage of a shortwave trough over the top of the building ridge. Although a few clouds (but no precip) may drift SEward into the ARB today, it should be a very pleasant day with moderating temperatures. Tomorrow into Sunday will be dry and mild with offshore flow.

The IR loop shows a subtropical stream of high clouds moving across Baja. An ejecting southern-branch S/W trough will lift NEward across CA/OR on Sunday into Monday and advect this mid/high level moisture across CA. Since the lower levels will start out quite dry due to antecedent offshore flow, we should see little more than thickening cirrus/altostratus across the ARB on Sunday. The latest 12Z NAM and GFS solutions actually produces some QPF Sunday night into Monday across parts of the Sierras, although the amounts are slim and could partly signify the presence of virga. A period of decreasing clouds should follow the passage of the S/W on Monday.

The medium-range models are still suggesting a more substantial storm for the Tue. to Wed. timeframe with relatively high snow levels. However, the new 00Z and 12Z GFS solutions are a bit slower and farther north with this storm than earlier simulations… more in line with the EC. In addition, the newer GFS solutions also portray an overall weaker storm system. With that said, the error bars are quite large with this forecast, given the nonlinear wave interactions that will likely occur over the east-central Pacific in the next few days. Clearly, the evolution of this storm will need to be watched closely, especially since there is a plump low-elevation snowpack in the Sierras.

Paul Neiman
NOAA/ESRL/PSD2

March 1, 2007

HMT Daily Wx Discussion & Forecast: Thu. 01-Mar-2007

The northern half of California is presently under the influence of cold, unsettled NWerly flow. Satellite loops show a disorganized shortwave trough making landfall this morning, with a hint of another S/W trough approaching CA from the NW. Consequently, spotty precip is falling in N CA based on surface and 88-D observations. Accumulations have been on the light side in the northern Sierra (<~0.1 to 0.2 in 6 h), given the unfavorable orographic wind flow (from the NW) and the very cold conditions in the lower/middle troposphere (-11C at 700mb at OAK and brightband height of ~1km at BBY).

The operational models resolve S/W energy persisting in the cold NWerly flow through ~08Z Friday am, during which a touch of light snow may fall from time to time in the ARB. However, total precip amounts will be light, not exceeding ~0.1-0.25 inch liquid equivalent. Thereafter, strong drying subsidence will move over CA in response to rapidly rising midtrop heights behind the final, trailing S/W.

Ridge building will be pronounced across the Great Basin over the weekend, resulting in warming temperatures aloft (>=0C at 700mb across the ARB by Sat. aftrn) and dry, offshore-directed flow descending the western slopes of the Sierras. Hence, surface temperatures across the ARB will also warm noticeably (perhaps to above normal by Sat.) with no chance of significant precipitation.

An ejecting southern-branch S/W trough will likely lift NEward across CA/OR on Sunday, but the low/mid levels will be dry due to the offshore flow during the period Fri. pm to Sun. am. Hence, we should see little more than thickening cirrus/altostratus across the ARB on Sunday. Decreasing clouds will follow the passage of the S/W Sunday night into Monday.

A much more substantial storm may be on the horizon for the late Mon. to Wed. timeframe. The GFS has been consistent in bringing a significant atmospheric river event ashore in northern CA with high snow levels during this period. Given the new snow that has accumulated to quite low altitudes in the ARB recently, this storm could pose hydro problems. Interestingly, the EC model has consistently kept most of the wave energy and moisture with this pending storm to the north of the ARB, as does the new Canadian spectral solution. All long-range models are suggesting a prolonged juicy atmospheric-river event somewhere in the northern half of the U.S. West Coast during the lattertwo-thirds of next week. This potentially significant West Coast storm should be interesting to watch unfold.

Paul Neiman
NOAA/ESRL/PSD2