December 6, 2019 – Another strong winter storm will bring periods of rain, mountain snow, strong winds and hazardous travel Friday and into the weekend. Dry conditions return next week with near average temperatures.
Discussion
Deep closed low (about a -4 standardized height anomaly) vicinity of 40N/135W will bring another round of impactful winter wx to Norcal this weekend. Hazardous mountain travel due to periods of heavy snow, roadway flooding, strong winds, and thunderstorms containing small hail (graupel) are expected at times over the next 3 days.
The U.S. West Coast AR Landfall Tool shows a moderate TPW plume (AR) on our doorstep, and at 37-41 deg N latitude putting the Feather River basin and 80/50 corridor right in its path. Heavy mountain snow with significant travel impacts is expected to begin later this evening for elevations above 6000-6500 ft. NAM BUFKIT for BLU begins precip around 05z Sat (or 9 pm PST Fri), and likely a couple hrs earlier for Wrn Plumas Co given the storm`s trajectory.
Another soaking rain is expected with 1 to 3 inches of rain forecast for the Valley and 4 to over 6 inches of liquid precip for the Nrn mtns/W Slope Sierra. Several feet of snow is expected over the high Sierra. Snow levels should lower below 6 kft on Sun as the upper low drops Swd over our Norcal. Satellite indicates a well developed comma cloud (baroclinic leaf) with lots of cold, unstable air associated with it. As this colder air approaches our area Sat, and moves over CWA on Sun, thunderstorms are likely in our CWA, especially Sat afternoon and evening as the upper low moves into Norcal. On Sun, the storms could be more confined to just the east side of the Valley and West Slope Sierra given the trajectory and timing of the upper low. Small hail/graupel is likely with these storms as indicated by the “thin CAPE” on the forecast soundings. Precip forecast to wind down Sun nite, followed by dry wx to start the work week.
NAM model REF forecasts, along with the NBM QPFs, suggest a line of intense rainfall is possible Sat afternoon from the Lake Co area, across the Sac Vly and into the Feather River basin before becoming more scattered and generally less organized over the West Slopes early in the evening. This is when we could see some roadway flooding from a few hours of locally heavy rainfall rates of a quarter to a half inch per hr over the coastal range and valley, and over a half inch per hr in the Feather River Basin. Looks as if only an hr or two of these heavy rates over the coastal range before the line moves through, with about 3-4 hrs over the Feather Basin. Travel over the high Sierra could be impacted with near “whiteout” conditions from mid afternoon Sat and into early Sat evening.
Additionally, Sly pressure gradients should tighten dramatically with the approaching surface front leading to WAD criteria for the Valley as a 50+ kt LLJ (NAM 925 mbs winds) develops Fri evening. Breezy to windy conditions should last into Sat. Similar to the last event, the isothermal lapse rates caused by the WAA will preclude downward momentum transfer of all of the LLJ wind. As a rule-of- thumb, the peak wind gusts end up to be about 2/3 of the 925 mbs winds in these isothermal profile cases. The ECMWF ensemble wind gusts for RDD indicates the potential for a AWW (airport wx warning) for gusts in excess of 40 mph.
Drier conditions expected to spread from N to S over our CWA on Sun and Sun night as the upper low drops S of our area and heights begin to rise. Dry Nly flow prevails over Norcal on Mon.
Extended Discussion (Tuesday through Friday)
Ensemble guidance for the extended forecast shows the ridge that builds over on Monday us holds steady for Tuesday with a weak trough trying slide over NorCal for Wednesday. NBM guidance and even deterministic models are in agreement that the Coastals, Cascades, and perhaps even the northern Sierra could squeeze some light precip out of this disturbance. Once this slides through ensembles show ridging building back over California, with zonal flow looking likely for the weekend. NBM guidance carries a bit of PoP over the mountainous terrain, but overall expect the forecast to be mostly dry.