HMT Forecast 1/11/2008 20:10 UTC
This morning's conditions in the ARB are basically clear and good radiational cooling has dropped temperatures at upper elevations overnight into the teens in the mountains. The last surface observation of precipitation occurred north of the ARB at 03utc 1/11. We can say with certainty that the latest precipitation event has ended.
In the Pacific we still have two major circulation centers apparent in the water vapor imagery, one located at 40N 143W with the greatest moisture as far east as 41N 135W. Today there may be a reestablished connection to the tropics as it appears that moist air is entrained in the flow. The second maximum is now at 38N 173W and as progressed since yesterday. It still has a very notable tropical connection.
The near term forecast has a moist plume reaching the CONUS coast by about 12ut Saturday 1/12 but the main moisture stays off shore. This corresponds to a weak system progged to move through the ARB starting early Saturday (~00utc) and continuing through 1/13 06utc. The NAM produces no precipitation over the ARB with the major amounts staying near the coast and to the north. The GFS 00UTC run optimistically generates a total of 0.03+ inches of liquid equivalent, while the 12UT run only delivers 0.01, most of this falling at higher elevations (if at all) in the 12-18UTC timeframe on Saturday. The SREF gives a 20% chance of this to be snow early on and then increases the likelihood of rain even at higher elevations to 35-40% later. Thus, the forecast time trend (dProg/dt) of the GFS is towards less precipitation. The GFS has the upper wave corresponding to this weak event departing the ARB by 06UTC on Sunday 1/13. This system is deemed to be too minimal in precip amount to warrant research activity.
The ARB remains dry until a second moist plume reaches the extreme northern coast of the CONUS near the Monday timeframe and works its way south. It reaches the ARB latitude on or about Tuesday 1/15 at 06UTC but a developing low pressure system well out in the pacific appears to circulate the moisture away from the CONUS back to sea and it never reaches the ARB. Thus the potential for precipitation in the 03-12utc 1/15 timeframe (associated with a weak upper level wave passage) appears nil in models at this time.
The long range prospects appear to maintain dry conditions for the next several days with the GFS ensembles now showing no activity over the ARB through 1/27 with the exception of one precipitation event mainly affecting the Reno side of the high terrain as a cold airmass descends from Canada associated with the strong ridge that forms off the coast in the Tuesday - Wednesday time frame next week. The EC model develops a very strong ridge off the coast at 136 to 130 W and a 558 low center at 34N 148W for most of next week. This ridge then appears to break down to more of a zonal pattern to perhaps allow moisture into the area starting Saturday 1/19 - Monday 1/21. This seems to be perhaps the earliest we will see any potential of moisture reaching the ARB and will watch this in the coming days.
Dan Birkenheuer ESRL/GSD