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January 5, 2021 – Areas of dense fog in the Valley today, otherwise dry. Rain and snow likely to return to NorCal Wednesday and Friday.

Discussion

Yesterday’s relatively fast-moving storm system has moved into UT early this morning, but will slow its Ewd progression as it amplifies over the Rockies today. This will allow ridging along the coast to move over Norcal today with the offshore moisture plume and moderate AR steering into the Pac NW. We will begin to see the higher level clouds spread over our CWA today, after initially dealing with Valley stratus/fog. The HREF “IFR WX” progs indicate there is a high probability that stratus/fog will develop from the SJV into the MYV/CIC areas and possibly into the RBL area as well. The HREF is several hrs fast with the forecast low CIGS and VSBYS, but given the forecast soundings and moisture trapped below the shallow radiative inversion, we should see stratus/fog forming rapidly by/after sunrise, and have decided to issue the DFA.

With the forecast cloud depth less than 2 kft we should see the low clouds erode/dissipate during the afternoon, lingering in the Srn Sac/Nrn SJV the longest. (If depth is >= 2 kft then stratus difficult to totally erode until very late in the afternoon this time of year.) Just before 11z, fog product beginning to detect rapid development of lower clouds along the Sac River/I-5 areas between SAC and N of RBL.

Energy rotating around the Gulf of AK low will erode the ridge allowing another trof to spread precip over on Wed/Wed nite, and again Thu nite and Fri. For Norcal, these will be modest systems (weak ARs) with much less QPF/Snow amounts than what we experienced yesterday, albeit the latter systems looks a little wetter. For the Wed system, QPF is looking to be less than 0.10″ in the San Joaquin Valley and south of the I-80/US-50 corridor, and roughly a quarter to three quarters of an inch elsewhere. The latter system is a little deeper with more QPF for the mountains, a half inch to an inch, with only a few hundredths over most of the Valley, except around a third of an inch for the N Sac Vly. Lowest snow levels dropping to 3500 ft Nrn mtns to 5000-5500 ft Sierra on Wed, and 3500 ft Nrn mtns to 4500 ft Sierra with the deeper latter system.

Extended discussion (Saturday through Tuesday)

A trough digs into the PacNW and northern California on Friday with widespread rain and mountain snow anticipated. Snow levels range from 3.5-5.0 kft right now which will impact mountain travel for the end of the week. For now, precipitation amounts do look to be relatively light in the Valley with the heaviest QPF in the foothills and mountains.

Drier weather returns this weekend with the potential for some night and morning valley fog development under moist stable conditions. Some light precipitation may be seen in the northern mountains Sunday, although the better chances look to be on Monday at this time.