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HMT forecast discussion for Saturday 26 January 2008 (entered 22z/Sat)

There was some precip across the ARB last night as the persistent band made inland progress but now the upper low has sunk south and elongated, resulting in a retrogression of the band to mostly off the coast and near the coast north of SFO. So this morning things are dry in the ARB; BLU at 15z is 35 with a SE wind at only 5 kts and 24-h precip of 0.40", while SAC is 52 with a light SE wind and 24-h precip of 0.46". It looks like any digging should be over as the strong jet aloft has rotated now around the upper low. There appears to be wave forming se of the upper low near 30N/130W with associated cloud enhancement. Another area of enhanced moisture is associated with the subtropical jet and is approaching LAX/SAN where Flash Flood Watches have been hoisted for tonight and Sunday. The models this morning are consistent with last night's runs overall and generally with the picture from yesterday, except perhaps drier today with the precip not beginning until late tonight. In brief summary (details below), the upper low gradually weakens and ejects to the ene, passing the ARB on Sunday at 700 mb between 18-21z with the remnants of the band of precip now off the coast hitting the ARB. Ahead of this though could get into warmer temps aloft and the northern fringe of the southern moisture. Then the next wave quickly follows and enhances precip late Sunday into Monday with falling temps and a transition to cold advection precip. Still uncertainties of course, but I think Sunday should prove to be interesting.

As discussed in the telcon, there was agreement that the start of the main precip in the ARB will be later than thought yesterday, about 09z/Sun at BLU, but there were a few optimistic signs in regards to the ARB getting into the subtropical southern moisture plume tomorrow. These included a more favorable looking 12z ECMWF run that pulled the moisture northward in a more concentrated fashion than the GFS, and the movement observed on satellite of the moisture itself. Here is an update as of 22z/Sat; the 18z run of the NAM is a little slower now in bringing the offshore upper low past the ARB on Sunday, although still has trouble bringing a lot of the southern plume moisture into the ARB. It has about the same timing as the 12z run with the northern wave, passing the ARB about 09z/Mon. Precip begins near BLU about 07-08z/Sun, and increases after 15z, with ~0.5" at SAC and almost an inch at BLU by 00z/Mon, then by 12z/Mon 2.25" at BLU and ~0.9 at SAC. It gradually diminishes but never completely stops in the higher elevations of the ARB, with a total near 3" at BLU by 00z/Tue, then almost ending but picking up by 15-18z Tue with an inch or so new by 00z/Wed with the next system (which would be still ongoing at that time). The 18z GFS has similar timing to the 18z NAM, but more precip (~1.25 at both BLU and SAC) by 00z/Mon, even though it looks like most of the southern plume moisture stays to the south and east. About 2.25 at BLU by 12z/Mon and 1.4 at SAC, then only light precip Mon with a break until about 18z/Tue. The next system comes in as 2 shortwaves, with precip ending by 00z/Thu. Totals for the Tue-Wed wave are about 0.5 at SAC and 1.0 at BLU, so somewhat marginal, and a colder event. The latest satellite loops suggest more moisture from the southern plume is heading straighter to the north than perhaps suggested by the models, as noted on the telcon by WFO RNO. Finally, the timing in the latest 18z runs is supported by our local 18z runs as well, with precip forecasts of over 4" by 12z/Mon within the ARB. The freezing level in the 3 km run hovers around 800 mb or 2 km until a sharp drop about 02z/Mon.

Some details from the 12z NAM. SSW 700 mb flow increases dramatically ahead of the remnants of the upper low wave it looks like some of the southern plume of moisture (the one hitting LAX tonight) briefly makes it into the ARB on Sunday ahead of the wave, which itself will carry what is left of the band of precip now off the coast. Together this should yield an intense period of precip (unless Dave Reynold's 40 N rule holds, but since it is an open wave when it comes through think we are okay) from ~15z Sun through ~06z Mon. Kind of complicated as the first part could be delayed IF the southern moisture never makes it far enough north. Two parts to the moisture after this, one with the remnants of the upper low, the other with the next wave which quickly passes after this. The first trof passage at 700 mb as noted would be ~20z in the ARB, with ssw flow at 700 mb forecast to increase to 60 kts with the temp rising to about -6C (~+2C at 850 mb) at 18z Sun. Not much cooling and only a slight wind shift (more a decrease) with the first wave passage (remnants of the upper low), but then more cooling begins by 00z/Mon with the next wave approaching, and this passes the ARB at 700 mb at roughly 09-12z Monday. In the NAM precip begins at BLU ~09z/Sun, increasing by ~15z, peaking 21z/Sun-06z Mon, but with a secondary max at BLU in the colder air on Monday morning, and precip does not really end with the next system picking things up again by 18z Tue and continuing through the end of the NAM on 00z/Wed (with good orographic flow but colder, -12 or so at 700 mb). Precip totals: by 00z/Mon 1.1 at BLU (2.5 just to the nw) and 0.45 at SAC, then by 12z Mon total precip 2.7 at BLU and .77 at SAC, but 3.25 at BLU by 18z Mon and then it just slowly piles up, 4.2 by 12z/Tue (but over 5" nearby), and then increases on Tue, with nearly 5" by 00z/Wed. This run would suggest no real solid break between the events, a definite snow event for Tue into Wed, and perhaps a brief changeover to rain on Sunday before going to snow Sunday night at the radar site.

In the 00z ECMWF run there is a break between the 2 systems with about 12h of now precip from ~06z-18z Tue, but then it starts up again for about 18-24 h but with only about 0.5" or so in the EC model for the Tue-Wed wave. Then the EC has another system Fri-early Sat with 0.5-1", then another fast mover Sunday night-Mon (3-4 Feb) with about the same amount. The 12z EC is similar, with yet another system lined up for about Wed/6 Feb. The 12z GFS is similar with the break, with not much from 21z/Mon through 18z Tue, then similar in structure and amount to the EC Tue-Wed, so very marginal in both these global runs. The Friday event is juicier in the GFS with over an inch at BLU but is still a shortwave trough that is relatively fast moving so too early to tell.

In terms of the immediate storm, the GFS is similar in timing to the NAM, but most of the precip is done by 12z Mon. The GFS does not get much of the southern plume into the ARB, but still has the heavier precip on Sunday and sunday night. Totals are not quite as high as the NAM but still over 2" at BLU by 12z Mon, 3" to the nw of the ARB. Our local 3 km runs have precip beginning near BLU by 09z, then a nice burst of heavy echoes moving up from the south by 15z Sun, with 3" max amounts in the ARB by the end of Sunday. So a lot depends on the amount of the southern plume getting into the system, but all in all still looks like a worthy day to sample on Sunday.

ed szoke NOAA/CIRA/GSD