HMT Forecast 1/13/2008 19:40 UTC
This morning the weather in the ARB is primarily clear with some fog in spots with early morning temperatures ranging from the 40s in the valley areas to teens in the mountains.
The prognosis for precipitation this coming week looks to be near zero with a slim potential on or about Tuesday 1/15 12utc when the models show a precipitation system passing to the north of the area. Ensembles show that this event should miss the ARB. There will be a cold arctic surge impacting the area on or about 1/17 00UTC (Thursday) but it now appears dry for the research area.
This morning's water vapor imagery over the Pacific shows a low has developed as forecast near 11N 129 W. A plume of moisture has reached British Columbia's coast, a small vorticity center at 32N 156W, another center at 50N 171E with a moisture plume with no tropical connection ridging between the two low centers. The water vapor imagery shows subsidence over the ARB at the current time.
For the intermediate forecast, the EC model has a short wave reaching the CONUS coast at about 00ut 1/15 (Tuesday) and the GFS moves any moisture associated with this wave north of the region and has it pass by around 15ut 1/15. So this is a very fast moving impulse but the models have the ARB remaining dry throughout with the SREF indicating less than 10% chance of snow near Reno at 21utc 1/15. At 18utc 1/15 the EC model builds the off-shore ridge and it reaches its maximum this week by 00utc Friday 1/18. It appears to be a stable persistent feature with the GFS showing a 585 ridge axis at 12utc 1/16 (Wednesday).
Looking to the long-range, the ensembles are now showing a fair chance of a cold air outbreak that brings precipitation (likely snow) to the ARB on or about 1/21 (Monday). This is manifest by a 500 low in N Canada at 55N 106W, a surface high pressure of 1034 in the great basin, and a 1044 high to its north in central to northern Alberta. The GFS 00ut run puts 0.25 inches of precipitation in the ARB in the first 6 hours of frontal impact, it appears that the 12utc run today is not as likely to bring precipitation into the area but the 12UT GFS ensembles have 7 of 12 frames showing precip over the ARB, we will have to watch this during the week.
Beyond this, the 00ut GFS ensemble now shows the potential for a moist plume to come up over Baja well to the south of the ARB in the 1/26 timeframe, something that showed up on ensembles a few days ago, then disappeared but now has returned. It looks to be associated with a closed upper low at the base of the high pressure ridge opening a moisture path to the tropics far to the south of the ARB. One can hope that if the ridge retrogrades this may put the ARB into a more favorable target zone for moisture plumes.
Dan Birkenheuer ESRL/GSD