Widespread rain, mountain snow, and travel impacts later today into Thursday

December 16, 2020 – Partly to mostly cloudy skies with patchy valley fog this morning. A winter storm will impact Northern California later today into Thursday bringing widespread rain and mountain snow. Dry weather returns Friday into the weekend.

Discussion

Lots of high clouds passing overhead as short-wave ridge moves across NorCal early this morning. Despite the clouds, dew points remain high enough for some patches of thicker fog over the Central Valley, and a few observing sites in the southern Sacramento Valley are currently reporting visibility around 1/4 mile.

Dry weather will continue this morning as the next system has slowed a bit. Some light precipitation will be possible later this afternoon into this evening across the northern half of the forecast area as warm-advection increases ahead of the approaching cold front. Precipitation will increase by late evening and continue overnight as the front moves southeast. Post-frontal showers will continue Thursday, especially over the mountains as the upper trough moves through.

Across the northern Sierra, snow will pick up late this evening and persist into Thursday morning before precipitation becomes more showery by midday. The main travel impacts across the higher passes are expected after midnight into early Thursday morning when around 8-12 inches of snowfall is expected.

For the lower elevations, some light precipitation will be possible late this afternoon across the northern Sacramento Valley before rain increases this evening. Further south, rain will likely hold off until late this evening for the Sacramento region and northern San Joaquin Valley.

There could be some decent hourly rainfall rates (quarter to a third of an inch) as the front passes through later tonight, but rates are expected to remain below 1/2 inch/hour limiting potential issues with the recent burn scars.

Any remaining showers expected to quickly taper off Thursday evening, and dry weather is then forecast into the weekend. Some areas of valley fog will possible each morning.

Extended discussion (Sunday through Wednesday)

Upper ridge moves into the Great Basin Sunday as Pacific frontal system moves into the PacNW. Baroclinic zone sags into NorCal Sunday into Monday bringing a threat of some light precipitation north of I-80. Best chances will be over the mountains. Deterministic models differ significantly by Tuesday but both solutions point to drier weather on day 7.