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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


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