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December 5, 2019 – Scattered light showers today before another winter storm will bring periods of rain and mountain snow and hazardous travel Friday and into the weekend.

Discussion

Just a brief break in the wet wx for today as short wave ridging builds over the CWA in the wake of the departing low pressure system to our east and the next mid latitude cyclone near 140W. Lingering showers noted in the Sierra and over the 80/50 corridor with the Colfax snow profiler showing the snow level around 7 kft. Although many areas will not see precip today, models suggest some light WAA precip may develop or spread Nwd in our CWA over areas mainly to the north of the I-80 corridor during the day. Topographic/orographic influences from the upslope Sly flow from the rising 5H heights may be enough to squeeze out a few hundredths. Any partial clearing early this morning will be short-lived, as the GFS simulated satellite indicates clouds just offshore will overspread Norcal during the day.

The next winter storm (mentioned above near 140W) is shaping up to be another impactful winter storm that will bring hazardous mountain travel Friday into the weekend. U.S. West Coast AR Landfall Tool shows a moderate TPW plume (AR) at 37-41 deg N latitude putting the 80/50 corridor right in its path. Heavy mountain snow with significant travel impacts is expected to begin late Friday for elevations above 6000-6500 ft. Models have been slowing down the onset of the precip in our CWA, e.g. BUFKIT for BLU begins precip around 05z Sat (or 9 pm PST Fri). Will continue the Winter Storm Watch as another model run or two could assist in locking in better timing of the precip.

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, with as much as 7 inches in Shasta Co. (The QPF follows fairly closely with the ECMWF ensemble spread for the Valley sites, but exceeds the mean of the ensemble for BlU by about an inch and a half.) Once again this storm has the potential for several feet of snow to accumulate over the high Sierra. Snow levels will be above 6 Kft at the onset, and then 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, we have introduced a chance of thunderstorms in the Valley and W Slope Sierra foothills. 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.

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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.

Extended Discussion (Monday through Thursday)

Ensemble and model guidance hold firm on ridge building over the Pacific Northwest early in the week. This will lead to at least a short stretch of dry weather, with the potential for some fog Tuesday morning. Looking towards to the mid to late portion of the work week, model and ensemble guidance continue to deviate from each other. Much like yesterday afternoon’s runs, there is uncertainty as to when and where the next trough sets up. Some ensemble members suggest we could see light precip over northern California, with the NBM being one of them. Ended up going a blend of the NBM to get some PoP north of I-80 for Wednesday and then over the Cascades and Coastal range for Thursday. Confidence is low on this occurring for the time being, thus we’ll be keeping a close eye on how the models and ensembles handle the situation.

Temperatures are forecast to be within +/-5 degrees or so of their seasonal average for both daytime highs and morning lows.