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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


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