HMT Daily Wx Discussion & Forecast: Sun. 04-Mar-2007
The upper ridge is migrating east of CA, with southerly-component flow kicking in. Also, the weak subtropical shortwave trough over the eastern Pacific is now on the move, with enhanced cold cloud tops approaching CA from the SW. Cirrus will increase, thicken, and lower as the day and evening progresses, but surface conditions will remain dry. All models show this S/W ejecting across CA tonight into tomorrow, but with scant QPF at best. The lack of significant QPF associated with this trough passage seems quite reasonable, given the weak dynamics associated with this trough, the large dewpoint depressions currently observed in the lower/middle troposphere over CA, and the predicted minimal onshore flow. Following the S/W trough passage tomorrow, shortwave ridging aloft will dominate the wx through Tuesday with generally dry conditions at the surface.
For midweek, all models have now seemed to stabilize on a rather unimpressive storm scenario for the ARB. The various solutions have the ARB under the influence of midtrop SWerly flow starting later on Tuesday as the trough slowly approaches, but the main pre-cold-frontal LLJ region is forecast to remain well to the NW of the ARB thru Tuesday night. Consequently, the frontal precip band is only impacting NWern CA by 12Z Wed. The shortwave trough and its associated low-level front eventually cross the ARB on Wednesday, but as a fairly weak and progressive system with only moderate orographics at best. In addition, the moisture plume associated with the storm is forecast to pinch off from its mid-Pacific subtropical origins before impacting the ARB. Based on the present forecast models, we could expect perhaps a 0.5-1.0” of liquid falling on the ARB on Wed.
Midtrop height rises are presently forecast to occur across the ARB toward the end of next week, with most of the precip shunted off to the north.