HMT Forecast 1/15/2008 19:40 UTC
Again there is no precipitation activity in the ARB this morning; temperatures have warmed to the upper 30s and 40s throughout the area. Today a short wave is progged to pass to the north skipping the area and denying it active weather, the best the SREF can do is 5% chance of flurries in the high terrain. The low pressure system at 31N 148W is very symmetrical on the water vapor imagery and has a dry band now extending the flow from the ARB into it across the Pacific. Thus the ARB is in sort of a negative atmospheric "river" flow regime. At 45N 158W there is a hint of a circulation associated with a moisture plume extending to the tropics, the one that we have watched for days. This moisture is still now entraining in active convection west of the large Pacific low circulation that was noted yesterday. Moisture supply from the Pacific is essentially turned off.
Once again, our focus remains on the long-term for hopes of precipitation. As of this morning it does not appear that we will see any possibility of significant precipitation in the region until possibly Sunday 1/20 at the earliest. We have been watching this scenario since this last weekend as there appears to be a potential of a cold air outbreak that might reach far enough west to affect the ARB. Starting with yesterday's 18ut GFS model run, the GFS is now looking more like the EC model did on Sunday where a strong moist surge out of Canada extends to the SW CONUS and actually cuts off some cold air and hangs over CA. Earlier EC model runs had the associated 1000-500 thickness down to 529, but today's (06ut) GFS run has this "cutoff" with a 534 thickness appearing on Sunday 1/20 at about 12ut and dropping about a quarter inch of moisture in the upper elevations of that ARB, likely in the form of snow. This morning's GFS 12ut now has much less precipitation but extended for a longer duration but only over the higher terrain (1/20 - 1/22) and a colder thickness (527) that reaches the CA coast on or about Tuesday of next week, on the other hand last night's EC agrees with a longer duration episode and with more total precip (~0.44 inches in the higher terrain). However, the GFS 12ut ensembles tend to put more of the emphasis on S CA and the ARB is only at the edge of the main precip area. Right now I would not get too excited about this potential event since it is still changing quite a bit in its overall nature run to run, but bears close watching.
Following this episode, there is a hint of something in the ensembles around 1/28-1/29. The mean ridge pattern appears to retrograde comparing the mean ridge position at the current time with the 384 hour ensemble positions. Therefore, I see no major breakdown evident in the current dominant ridge pattern but it might move in the anticipated direction.
Dan Birkenheuer ESRL/GSD