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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

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