Light showers today, dry Thursday before the next winter storm

December 4, 2019 – Low pressure moving south of NorCal will bring some showers into this evening. A break in the wet weather on Thursday before another winter storm will bring periods of rain and mountain snow and hazardous travel Friday and into the weekend.

Discussion

Upper low off the central CA coast will be tracking Ewd across the central portion of CA, south of our CWA. Radar shows echoes making their way Nwd to the I-80 corridor at 0200, and by 0900 is forecast by the NAM Model REF to spread Nwd towards Susanville to RBL to Paskenta. Thus we have spread the PoPs/QPF much farther Nwd in our CWA for this morning. The precip should then gradually end from W to E during the afternoon, with precip lingering over the Sierra into evening. Have upped the QPF over the Srn portion of the CWA in line with the ECMWF ensemble means just under a half inch (around .40″) at many of our Srn locations, with about a half inch to three quarters of an inch south of Hwy 50. With forecast snow levels 6500 to 7000 ft and up to some 3 to 7 inches of snow forecast on Hwys 50/88 today, will issue a snow advisory for those travelers going over the passes, i.e. Echo Summit and Carson Pass. A little less of an impact for I-80 where 2 to 5 inches are forecast.

Just a brief break in the wet wx on Thu as short wave ridging builds over the CWA in the wake of the departing low pressure system and the next mid latitude cyclone near 145W. Although many areas will not see precip, models suggest some light WAA precip may spread into our CWA mainly along the coast, but also N of I-80 during the day.

The next winter storm (mentioned above near 145W) 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 by late Friday afternoon for elevations above 5500 ft, and a Winter Storm Watch has been issued from Friday afternoon into Sunday afternoon.

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. 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.

Extended Discussion (Sunday through Wednesday)

Lingering rain and snow shower activity will continue through Sunday. Additional snow accumulations of 6 to 12 inches are possible. With the late week system exiting the region Sunday night, an upper level ridge will build along the West Coast early next week. This should result in dry weather across the region, as well as favorable conditions for fog. High temperatures will be near average through the period, with Valley highs in the mid to upper 50s. Latest ensemble guidance suggests some weak energy pushing through around midweek that could bring light precipitation, but confidence is low given model differences so have followed the NBM.