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

Continue reading "Looking more promising" »

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 ) 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

Klaus Weickmann and Ed Berry

Continue reading "IOP on track?" »

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 7, 2007

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.

Klaus Weickmann and Ed Berry

Continue reading "Uncertainty rules" »

February 2, 2007

Break through update, then what?

Some models are continuing to advertise the break through of westerlies on the west coast with the PSD ensemble showing a distinct shift toward probability of above normal precipitation in the California region in week 2 (Feb 10-16). Tropical convection is now established over the west Pacific and continues to be active over the Indian Ocean. The tendency for convective forcing to be further west should continue to effect the North Pacific circulation with more retrogression of features. This could eventually (beyond week 2) lead to weakened westerlies in midlatitudes and a return to a trough along the US west coast, especially if a new MJO develops over the Indian Ocean.







Klaus Weickmann and Ed Berry

February 1, 2007

Waiting for a break through?

The following are a few annotated images to "prep us" for tomorrow's extended discussion. We will probabaly add a few more slides. Tropical forcing remains concentrated around 5-10S/160E (putting the USA in a cold regime) while SPCZ starts to "heat up" again. Meanwhile activity continues to slowly increase from South Africa into the South Indian Ocean. As the anticyclonic anomaly around Alaska continues northwest, increasing subtropical flow looks more probable to impact the ARB region during week 2. If we would have had the "classic El-Nino" response, the polar and subtropical jets would have combined across the east Pacific (see slide 2).

Ed Berry and Klaus Weickmann

Continue reading "Waiting for a break through?" »

January 30, 2007

Right on track, but not El-Nino!

The following is a discussion that was posted on the Ed Berry "Atmospheric Insights" Blog this afternoon (link below). This information may be useful for HMT concerns, particularly given today's conference call.

Tropical convective forcing has consolidated around 140-160E along the equator while only sporadic convection persists across the South Pacific. In addition, tropical forcing has also been increasing across northern South America and South Africa. The latter is a response to the remnant dynamical signal of the December-early January MJO interacting with the Southern Hemisphere (SH) extratropics.

A strong Rossby wave energy dispersion (RWD) linked to the west Pacific tropical forcing is exciting the positive phase of a western Pacific wave train as I type (with SH symmetry). This pattern resembles the positive phase of the Pacific-North American teleconnection (PNA) but shifted west by about 20 degrees of longitude. As has been expected for about 10 days, bitterly cold Arctic air from central Siberia is currently being transported across the North Pole and will plunge into the CONUS by the end of this work week. The initial surge of cold air should be into Montana, with the Northern Plains, Upper Mississippi Valley and Great Lakes states being impacted the most severely.

So, what is the most probable evolution of our weather-climate situation during the next few weeks, given uncertainty and inadequate model guidance? Specific diagnostic factors to consider include transports and tendencies of relative angular momentum (AAM), the evolving SST and SST anomalies, the subseasonal tropical convective forcing and the blocking becoming established at the northern polar latitudes (with a possible warming of the stratosphere).

Poleward propagation of zonal mean zonal wind anomalies continues, with moderate westerly anomalies throughout the subtropical atmospheres, easterly anomalies around 50N and westerlies again farther north. The global signal of relative angular momentum is quite impressive with positive anomalies of roughly 2 standard deviations as of January 27th based on the reanalysis climatology. However, as discussed in past postings, this westerly flow did not evolve into a strong combined North Pacific jet typical of a warm ENSO. Strong easterly anomalies in the subtropics preceded the convection increase near the date line and may have interferred. Only during the last few days has there been a weak reversal of AAM transports with westerly flow being fluxed equatorward from the midlatitudes.

Looking farther out, a renewed active phase of the MJO may develop from South Africa into the southwest Indian Ocean during roughly weeks 2-3. Coupling with the warm west Paciific SSTs looks probable centered ~10S/160E while at least diurnally intense convection occurs across much of Brasil. This would suggest a return to a GSDM Stage 4-1 (La-Nina like) response meaning zonal mean easterly flow anomalies should re-appear across the deep tropics while the tendency of relative AAM becomes negative. For PNA sector, the large ridge currently developing west of Canada may shift northwest to Kamchatka during week 2 while the above mentioned anomalous subtropical westerly flow ”undercuts” the east Pacific ridge. Going into the middle of February there may be a situation of a cold trough extending from central Canada to just off the USA west coast interacting with a moist subtropical jet.

As mentioned above, the cold regime is on track for most of the USA for week 1. While there is likely to remain a cold air source for especially the northern states weeks 2-3, moderation of temperatures is expected. Intense convective lake effect snow is a good bet for week 1, with only light snow events across the Northern Plains on east. Portions of the Deep South to the east coast will need to be monitored for possible significant wintery precipitation. This whole pattern should shift north and west weeks 2-3, with possibly an active southwest flow storm track across the Plains by the middle of February. Much of the USA west coast should also finally get some decent precipitation weeks 2-3.

Please note: These are probabilistic statements, which we will try to quantify in future posts.

Ed Berry and Klaus Weickmann

January 29, 2007

Hope is on the horizon....always on the horizon!

The active tropical convective forcing has shifted to ~140-160E while weakening has occurred across the South Pacific, especially along the South Pacific Convergence Zone (SPCZ). Circulation anomalies over the PNA sector are retrograding as a response. There is a possibility of renewed MJO activity across the southwest Indian Ocean by the end of week 2 while forcing remains centered ~10S/160E. This situation would be more favorable to moist west and southwest flow for ARB.



Ed Berry and Klaus Weickmann

January 26, 2007

El Nino weakening, west Pacific convection not the answer

An MJO excited deep convection near the date line in early January 2007 but a "combined" Pacific jet and eastward shifted storm track did not develop. A persistent pattern of subtropical easterly and mid-high latitude westerly flow anomalies may have intervened. Date line convection is now weakening; a portion is shifting southeast into the southern hemisphere and another is shifting west toward the west Pacific. Combined with eastward shifting convective activity over the Indian Ocean, a consolidation of positive convection anomalies around Indonesia has already occurred. This activity may eventually spread over the warmest SSTs currently in the west Pacific, south of the equator.

The current short term amplification and retrogression of the circulation anomalies over the Pacific Ocean will dominate the weather patterns during the next 1-2 weeks. Prospects do not look good for rain along the west coast during this time. The behavior of the circulation beyond week 2 is partially linked with the strength and location of tropical forcing. The Indian Ocean SSTs continue quite warm and we expect convection to redevelop there, possibly aided by a dynamical component from the recent MJO. Convection should also stay active over the west Pacific. If the west Pacific convection dominates, a very boring weather pattern may be in the offing for the USA west coast beyond week 2. Increased activity over the Indian Ocean would lead to a better chance for a trough along the west coast and more favorable prospects for the ARB.





Klaus Weickmann and Ed Berry

January 24, 2007

Weather-climate update

The satellite picture (below) is from the Japan Meteorological Agency (JMA) on 24 January 2007 @ 8 am MST (1500 UTC). Tropical convective forcing continues to slowly increase ~120E while weakening over the South Pacific. We are already seeing a response of the East Asian storm track that will allow for the western North American ridge to retrograde over the next week or so.


Ed Berry and Klaus Weickmann

January 23, 2007

No fooling around; severe cold regime for a large portion of USA more probable by week 2

The following was published on my (Ed Berry) "Atmospheric Insights" Blog this afternoon (23 January). I thought this might also be useful to the audience of these postings.

Tropical convective forcing has been persistent across the South Pacific along the SPCZ for roughly a couple of weeks. This response was expected given the December 2006-early January 2007 MJO and warm SSTs (~1-2C anomalies) associated with ENSO. The global circulation has been responding with the recent large tendency in global relative atmospheric angular momentum (~30 Hadleys) and the appearance of twin upper tropospheric subtropical anticyclones around 160W. Rossby wave energy dispersion linked to these twin anticyclones (with inter-hemispheric symmetry) may be contributing to the onset of North Atlantic blocking (negative phase of the NAO). There has been poleward propagation of zonal mean westerly wind anomalies resulting in 5-10m/s anomalies at 200mb in the subtropical atmosphere. GSDM Stage 3 best describes the current global circulation pattern. However, as discussed in our January 19th posting, this El-Nino-juiced forcing has been evolving in a more persistent regime that developed just prior to December 1st. The regime has been characterized by transport of westerly momentum out of the tropics into the higher latitudes. The forcing from the South Pacific has not changed this La-Nina like (GSDM Stage 1-2) pattern.

Rossby wave energy dispersions within the regime have led to a revival of tropical convection across the East Indian Ocean into Indonesia. Fast baroclinic wave packets moving through South Asia are interacting with this forcing leading to storm development across the west and central North Pacific. A discontinuous retrogression and amplification of the North American ridge into the Arctic is expected within the next week. While a convective signal in the South Pacific is expected to persist (allowing a subtropical jet across the Deep South at times), tropical forcing may become quite robust near 120E by sometime week 2, in which case, the ridge may retrograde to 150W. The ESRL/PSD and other model ensembles lend support to this scenario. In fact, blocking may develop all across the polar latitudes as we go through February. Another MJO may also develop across the Eastern Hemisphere next month which could lead to a further demise of our warm ENSO event.

The screaming message for a good part of the country is a turn to much colder temperatures by about a week from now. Bitterly cold Arctic air that has been “bottled up” across much of Siberia is likely to plunge into the USA with a few surges, likely centered on the Northern and Central Plains/Upper Mississippi Valley. Strong winds and brutal wind chills will be probable. Best opportunities for significant wintery precipitation should be from the central-southern Rockies into the Tennessee Valley, Deep South into the mid Atlantic States. Lighter snowfall events may occur with the Arctic surges (along with intense convective lake effect snow). This whole precipitation pattern may shift northwest later weeks 2 and 3 (troughs across the Rockies/western states) while temperatures slowly moderate. By around week 3 the ridge should be far enough northwest to allow strong and moist westerly flow (including “undercutting”) to impact the west coast with significant precipitation especially California.

Please note: These are probabilistic statements, which we will try to quantify in future posts. The decay time scale for momentum transport anomalies is on the order of 1-2 days. Thus this is the time scale on which a reversal of the transports could occur

Ed Berry and Klaus Weickmann

January 10, 2007

Phantom El-Nino impacts, or are we dealing with reality?

Full disk satellite imagery and other monitoring tools suggest the recent eastward shift of the tropical convective forcing from the Indian Ocean into Indonesia has stalled. The centroid of the forcing is currently at ~0/140E while a faster component has excited convection downstream at around 10S, 180E, enhancing an eastward shifted South Pacific Convergence Zone. The overall convective pattern is losing its MJO characteristics. Previous thinking was for the MJO to intensify the convection in the region of the South Pacific leading to the “classic warm ENSO” global circulation response by about the middle of this month. We now believe this is not likely. Instead, coupling may occur west of the date line near 10S/160-170E while the Indian Ocean remains convectively active, roughly every 30 days. In fact, the South Indian Ocean may see intense rainfall in about 10-14 days since SSTs remain above average there. Convective forcing over the Indian and west Pacific Oceans may consolidate around Indonesia during weeks 3-4.

Zonal mean easterly wind anomalies have propagated into the subtropics and been replaced by equatorial westerly flow anomalies. The latter have been most robust across the East Pacific. Some of this anomalous westerly flow is coming back into the Eastern Hemisphere, and should lead to respectable intensification of the subtropical westerly jet during the next couple of weeks. However, considering the tropical forcing and other dynamical processes, this intensification does not appear to be enough to allow an extended combined North Pacific jet to reach the USA west coast. Instead, as most ensembles are showing and as is already happening, ridge amplification will occur across the north central Pacific into Alaska with a downstream western USA trough for week 1 and then shift slightly east week 2 (GSDM Stages 1-2). For weeks 3-4, the most probable scenario would a retrogression of the ridge-trough pattern while a subtropical jet “undercuts” the ridge and heads into California. This scenario is not consistent with El-Nino composites, possibly due to strong convective forcing from other regions such as the Indian Ocean.

The USA is in for a huge weather change. As Arctic initially plunges into western states, significant baroclinic development on the Plains is likely especially Sunday-Tuesday next week. The models are playing catch up. Impacts will include another blizzard from the Rockies into the Upper Mississippi Valley while heavy rain and severe thunderstorms occur across the Deep South and Ohio Valley. While week 2 should be generally cold and dry for much of the country, weeks 3-4 may once again become stormy with continued cold. Locations such as the California coast may not see decent chances for precipitation until after the 20 January.

Ed Berry and Klaus Weickmann

January 6, 2007

Watch for a developing Pacific jet

An intense baroclinic system is in the process of developing near Japan. It will contribute to an anticipated pattern change as a ridge develops near ~145W and a downstream trough digs over the western USA. A few of the PSD ensemble members from today (5 of 15) have the trough slightly further west but the majority of the models are consistent with yesterday's forecasts. The trough should set up over the western USA. Cold temperatures can be expected over the west, including the California region. The next stage in the atmosphere's evolution would be for the west Pacific jet to continue to intensify and start to shift east across the basin. The MJO component of this process can be seen in this forecast from Matt Wheeler:


As these forecasts suggest, the timing for west coast impacts would be beyond 15 January. This is consistent with Ed's discussion from yesterday where he also speculated on the synoptic details of how west coast impacts may occur. Presently none of the models show a strong jet in place in the extended range. The EC and NCEP ensembles have a feeble 500 mb low anomaly east of the dateline by day 11. It's a good bet that the models are underpredicting the jet intensification. A more detailed analysis of the weather-climate situation after 15 January will be posted on Monday.

January 3, 2007

Week-two discussion in the conference call of 3 Jan 07

This was a follow-on to the discussion yesterday. Dave Reynolds feels even more confident today that we are setting up for a multi-day wet period in California. Near as I can tell, he bases this on 1) the strong MJO convection advancing to 120E the last couple days (See previous blogs by Klaus Weickmann, 2) the large retrogression and amplification predicted by the GFS (particularly the T382 "deterministiic" run) yesterday and today, and 3) the oft-observed tendency for a very strong ridge aloft and anomalously strong Pacific High to be followed by a collapse of 500mb heights and a radical shift in the West Coast weather pattern. Klaus Weickmann acknowledges this possibility of a week-2 wet period, but is more conservative in his assessment of the liklihood. He says the next thing to watch for if this big retrogression is to happen is a major cold surge off Asia. (I presume this is to provide the baroclinity to support deepening of a trough near the Dateline as a precursor to the amplification and retrogressin of the Eastern Pacific Ridge. My own personal view is a certain amount of skepticism about this particular event. As Klaus Weickmann points out, "the devil is in the details". I have seen large ampliifications of the sort we are seeing in the GFS forecasts many times: ridge bulids into the Gulf of Alaska, cold air builds up in western Canada, deep trof digs over the West and at least partially cuts off as the cold air scoots down the High Plains and filters into the West.

If the trough cuts back offshore, things can get quite intersting along the West coast as cyclogenesis occurs offshore followed by copious precipitation falling as snow down to well below 5,000 ft. A situation like this occurred in Pacjet in early Feb of whatever year that was (2001?), Paul Neiman's "Bomb in the Bight"). I have seen several others in my years of watching California Wx.

In my experience, most of these conclude with a collapse of the high latitude portion of the eastern Pacific ridge once the deep trough or cutoff low over the West or offshore ejects to the east or northeast. However, occasionally a high-latitude blocking pattern forms, allowing a connection underneath the block as the southern portion of the east Asian jet advances under the block to connect with the cut-back trough that has deepened into the west or off the west coast. I think it is fair to say that many of the memorable heavy rain periods in central and southern California have occurred in such a pattern. Partly this is because there is the serious flood potential that arises after the first cold storm, as the connection is established across the Pacific. The succeeding storms are warmer and create the potential for extra runoff fron the melting snow from the original cold storm.

Both yesterday and today a few of the GFS ensemble members were indicating this type of scenario during week 2. However, most of the members were indicating the usual benign evolution of such events. The divergence between these 2 states suggests that there may be a bifurcation involved--perhaps rather subtle details in the flow from a planetary-wave perspective determine which state obtains.

There was considerable discussion during the conference call about the importance of this type of event to HMT, primarily because of its hydrological significance. If it were possible to say with confidence that a week 2 heavy pcpn event of this sort were coming, then reservoir managers could be informed and they could take precaution to draw down reservoirs in anticipation of additional inflow. The problem is that right now, according to Klaus Weickmann, we have no way to assess the probability of such an event coming. Nevertheless, Dave Reynolds advocated putting some comments to the effect that there is potential for heavy pcpn during week 2 in the extended section of the AFDs. But, the consensus of the active participants in this discussion seemed to be that there was not justification for taking action beyond this, at least today.

Given the interest by the NWS in improving prediction of highly significant weather events at least 3-5 days in advance, there is a need for research to be done on prediction of these types of situations because of their importance to the West Coast, and California in particular.

John B.

January 2, 2007

Dave Reynolds scenario

The following is from John Brown's 2 Jan 07 HMT discussion:

Farther ahead, I see no IOP prospects arriving before
midweek next week. Beyond this, the "deterministic" (T382 resolution) GFS run from 12Z this mrng
has a downstream amplification producing a massive ridge building into the Arctic followed by amplification of the downstream trof. This trof cuts back off the west coast of North America to ofshr the Pac NW and northern CA by the middle of next week. A few of the ensembles are in agreement with this scenario. Dave Reynolds points out that this could dynamically phase with the current MJO wet phase that is advancing Ewd from the Indian Ocean and has reached near 120E (see Klaus Weickmann's posting of yda). Dave believes there is a possibility that this could develop into a prolonged wet period for Nrn CA, with a westerly flow connection developing underneath a high latitude block between the offshore-cutting-back North American trough to the east and an eastward extention of the east Asian jet from the western Pacific. Such scenarios can bring heavy low-elevation snows with the cold cut-back trough followed by warmer storms once the cross-Pacific jet gets established. Something of this sort would help salvage this rainy season from the potentially serious dry spell that has afflicted central and parts of northern and southern California.

Where will west coast trough set up?

The cold regime we have been expecting looms in the medium range. Most models now show this scenario out in the 9-10 day range, 11-12 January. As the pattern change approaches the details of where the ridge-trough pattern will amplify and set up over the east Pacific- west North American region are important. Some models are suggesting the trough could be far enough west to give the ARB a chance for significant precipitation as a cold system moves in from the northwest. The GFS 12Z deterministic run from today illustrates this possibility.


We don't know what probability to assign this outcome but the recent anticyclonic wave break over the subtropical east Pacific combined with the phase of the MJO and other slow atmospheric behavior suggest it should be taken seriously. In the future, an event probability could be determined from multi-model ensembles.

January 1, 2007

Subseasonal still on track

The scenario Ed and I discussed last time is still on track. The MJO convection is moving east as seen in this update of the observed equatorial OLR time-longitude plot.

An updated forecast for the MJO shows the beginnings of a strong jet over the North Pacific, but not until the last panel. This provides some timing information for when the jet stream may start increasing there.


The MJO, combined with another "slow atmospheric process", suggests there will be a cold regime across the USA during the medium range, possibly moving from west to east This regime willl not favor much precipitation over the central CA coast. A forecast of 850 mb temperature anomalies from the PSD ensemble seen below looks reasonable for what to expect in the next ~10 days. After this upcoming precipitation event, favorable conidtions for precipitation may not return to the west coast until after 15 Jan, at the earliest.


December 29, 2006

Changes are underway, but...

The next step in the El Nino cycle is typically a strong subtropical westerly flow, which is anticipated to dominate the Jan-Mar 2007 period. The transition to this atmospheric state normally coincides with the onset of the southern hemisphere monsoon and an eastward moving MJO-like feature. Over the North Pacific the jet stream extends to the west coast and split flow develops over the western US and Canada. However, some unusual convective patterns have been observed in the last six months with present El Nino. In particular, the Indian Ocean has been very active convectively but the dateline region shows primarily transient (< 30 day) activity. The MJO has also been episodic. An MJO in September-October 2006 helped amplify the EL NIno SST pattern by initiating an oceanic Kelvin wave. Currently there appears to be another MJO that developed over the Indian Ocean in mid-December 2006. In just the last 5-10 days convectiont shows clear signs of moving eastward. This MJO is likely to lead to the onset of persistent deep convection over the dateline, probably in weeks 2-3. In the interim as convection shifts farther east into the west Pacific and North Australia, a cold regime is anticipated for the USA Plains sometime also in weeks 2-3 (10-15 January). Some links to images in the extended entry illustrate some of these developments.

Continue reading "Changes are underway, but..." »