HMT Daily Wx Discussion & Forecast: Thu. 01-Mar-2007
The northern half of California is presently under the influence of cold, unsettled NWerly flow. Satellite loops show a disorganized shortwave trough making landfall this morning, with a hint of another S/W trough approaching CA from the NW. Consequently, spotty precip is falling in N CA based on surface and 88-D observations. Accumulations have been on the light side in the northern Sierra (<~0.1 to 0.2 in 6 h), given the unfavorable orographic wind flow (from the NW) and the very cold conditions in the lower/middle troposphere (-11C at 700mb at OAK and brightband height of ~1km at BBY).
The operational models resolve S/W energy persisting in the cold NWerly flow through ~08Z Friday am, during which a touch of light snow may fall from time to time in the ARB. However, total precip amounts will be light, not exceeding ~0.1-0.25 inch liquid equivalent. Thereafter, strong drying subsidence will move over CA in response to rapidly rising midtrop heights behind the final, trailing S/W.
Ridge building will be pronounced across the Great Basin over the weekend, resulting in warming temperatures aloft (>=0C at 700mb across the ARB by Sat. aftrn) and dry, offshore-directed flow descending the western slopes of the Sierras. Hence, surface temperatures across the ARB will also warm noticeably (perhaps to above normal by Sat.) with no chance of significant precipitation.
An ejecting southern-branch S/W trough will likely lift NEward across CA/OR on Sunday, but the low/mid levels will be dry due to the offshore flow during the period Fri. pm to Sun. am. Hence, we should see little more than thickening cirrus/altostratus across the ARB on Sunday. Decreasing clouds will follow the passage of the S/W Sunday night into Monday.
A much more substantial storm may be on the horizon for the late Mon. to Wed. timeframe. The GFS has been consistent in bringing a significant atmospheric river event ashore in northern CA with high snow levels during this period. Given the new snow that has accumulated to quite low altitudes in the ARB recently, this storm could pose hydro problems. Interestingly, the EC model has consistently kept most of the wave energy and moisture with this pending storm to the north of the ARB, as does the new Canadian spectral solution. All long-range models are suggesting a prolonged juicy atmospheric-river event somewhere in the northern half of the U.S. West Coast during the lattertwo-thirds of next week. This potentially significant West Coast storm should be interesting to watch unfold.