HMT forecast discussion for Thursday 17 January 2008 (entered 20 UTC/17 Jan)
A high-amplitude upper-level ridge continues to sit off the West Coast, keeping things dry across the ARB, with a deep trough downstream over the west-central CONUS. A strong shortwave trough will ride over the ridge and plunge south over the weekend along the West Coast, retrograding the trough and bringing a chance of some precip and very cold temperatures to the ARB. Overall though the ridge is expected to remain through most of next week, preventing significant moisture from reaching the ARB. Longer term, however, this pattern is forecast to break down some, at least temporarily, which could (with considerable uncertainty!) usher in a chance of significant systems as early as next weekend (26-27 Jan) and into the last week of January. But as noted below and on the telcon, there is a lot of uncertainty in the forecasts beginning next weekend (26-27 Jan) and through to 2 Feb.
The first issue is whether the weekend system is of interest for a potential IOP, and the consensus on the telcon is no. As noted above the main feature continues to be a high amplitude ridge positioned off the West Coast. Westward across the Pacific a strong jet is present from the Dateline westward, south of a broad trough centered near 160E. A cutoff low that had been lurking to the northeast of Hawaii is opening up now and will combine with a shortwave trough in the northern stream to become the system that rides over the ridge on Friday and then plunges southward into early next week. There is good model agreement on this, with the system bottoming out near west-central CA early next week (mon-tue/21-22 Jan). As this happens extremely cold air will plunge southward into much of the nation, and some of this will reach far west to the ARB. So this will be a very cold wave, with snow levels possible at or below 2000 ft, but moisture limited with no real tap to anything substantial in the Pacific. So it is difficult to imagine a lot of snow out of this, though perhaps it could pile up some if the system stalls awhile. The 00z ECMWF run has less than a tenth of an inch total through Tue/22 Jan with this system in the ARB, and the latest 12z EC is similar. The latest 12z GFS has a little more, but not much, even though the upper level system closes off south of the ARB by Tuesday. There is up to a third of an inch from Sun night through Tue east of the ARB, and up to 0.7 much farther south in the south-central CA mountains. These amounts are generally in agreement with the longer range forecasts from the HPC.
The models and ensembles generally agree that the upper level trough should shift eastward by midweek (Wed/23 Jan), with at least a temporary change in the pattern over the CONUS to less of a deep trough, and a retrogression to the upper level ridge, shifting it further to the west. In a number of the ensemble members and the deterministic forecast from the ECMWF and moreso the latest GFS, this allows the next wave to dive off the West Coast and, at least in some members, close off as a deep upper low next weekend (26-27 Jan). In a good number of the 00z ensemble members this trough/closed low then eventually makes its way to the coast by early to midweek of the last week in January, with a potentially good precip event for the ARB. This is seen nicely in the 00z NAEFS meteogram for Sacramento, with precip chances increasing from 26 Jan onwards. There is, however, a lot of uncertainty in this whole scenario, with another set of ensembles simply shifting the ridge back to off the West Coast late January into the first couple of days in February, so as to end up with a pattern similar to what is present now. In fact, this is precisely the forecast from the latest GFS 12z deterministic run, with the aforementioned closed low staying too far off the coast, and then the upper level ridge building up, so that the forecast by day 15 (Fri/1 Feb) has a pattern that looks a lot like what we have today, and it gets there without ever having much precip hit the ARB. Comparing this GFS run to the new 12z ECMWF shows this diversity in solutions off the West Coast; instead of a closed low peeling off the main flow and settling in near 35N/145W with an upper level ridge to the north into Alaska in the GFS by the last weekend in January, the ECWMF has zonal flow with a low-amplitude trough moving the Gulf of Alaska. The 12z GFS ensembles show a lot of spread at 240 h, and a number of members by Day 15 (2 Feb) are right back to a pattern looking much like what is present right now.
ed szoke NOAA/CIRA/GSD