December 21, 2020 – Areas of dense morning fog with only partial afternoon clearing across the Central Valley. Dry and mild elsewhere. A few mountain showers possible tonight and early Tuesday, otherwise the dry weather pattern continues thru mid-week with breezy north winds at times. Rain chances increase by Christmas.
Discussion
Winter has officially begun and strong high pressure remains overhead NorCal. IR difference imagery shows extensive fog across the northern San Joaquin Valley spreading slowly northwestward into the Delta and southern Sacramento Valley. Visibilities in the fog are generally around 1/4 mile, with locally lower visibilities around the periphery of the main fog area (dense fog advisory remains in effect until noon from about Yuba City southward in the Central Valley). Strong temperature inversion present with valley readings currently in the mid 30s to lower 40s while foothill ridges are in the lower to mid 50s.
Strong subsidence inversion will limit mixing today. The fog will likely lift to stratus by midday. However, the fog layer is only around 1k ft deep, so most areas may break out into some sunshine in the afternoon. Areas in the valley from about Sacramento southward may only see some partial afternoon clearing.
Fast moving short-wave brushes the region tonight and early Tuesday and may bring a few showers to mainly the northern mountains. QPF will be very light with perhaps an inch or two of snow accumulation possible above about 5500 feet.
Ridging and breezy northerly flow quickly return in its wake on Tuesday. We may see some fog/stratus redevelopment in the Central Valley tonight, but strengthening northerly breezes, and better mixing, will likely push the clouds south of the region Tuesday afternoon with milder temperatures returning to the valley floor.
Dry pattern with lighter northerly flow continues Wednesday and Thursday resulting in mild days (highs around 5-10 degrees above average) and chilly nights.
Extended discussion (Friday through Monday)
Active weather will be returning to NorCal during the extended period. A trough will be dropping out of the Gulf of Alaska on Christmas Day and will be able to pull in Pacific moisture ahead of its cold front. The IVT plume forecast continues to increase with each run with it now pushing above 250 kg/ms in the 25-26 time frame. This will give us a good chance for widespread rain and snow for Friday night as the cold front sweeps through the region.
Showers will start to develop though during the morning and afternoon hours Christmas Day with the heavier precip holding off until the overnight. Snow levels with this system are looking to be between 4500 and 5500 feet with the majority of the snow accumulation above 5500 feet. Mountain travel impacts are looking likely starting sometime during the evening Christmas Day and continuing into Saturday morning. If you have travel plans on Christmas Day into Saturday you will want to keep up with the latest forecast.
Mainly dry weather settles in by Saturday afternoon as short wave ridging builds in but some light showers will likely linger over the mountains. Dry weather will be short lived as another trough approaches NorCal on Sunday bringing the return of rain and snow. Highs will be near average for the period and overnight lows will be warmer closer to average.