December 15, 2020 – Dry weather early this week with areas of morning valley fog and frost. An impactful winter storm will move into Northern California Wednesday into Thursday bringing widespread rain and mountain snow. Dry weather returns Friday. Winter Storm Watch from Wednesday evening through Thursday morning above 5,500 ft.
Discussion
Skies are partly to mostly cloudy early this morning as the tail end of a weaker weather system moves into the region. Increasing warm-advection is spreading some thicker mid-level clouds into NorCal along with some light showers moving onto the northwest California coast. The increase in cloud cover will likely limit the extent of fog this morning compared to early Monday, and dew points are high enough to preclude much frost.
Most of the region will remain dry today, though a few light showers will be possible across mainly the northern mountains as the weak weather system passes.
A more substantial system is expected to move through the region on Wednesday and Thursday bringing widespread rain and mountain snow. Precipitation will move from the northeast to southwest with snow levels beginning close to the higher pass levels.
Snow levels drop into Thursday morning with heavy mountain snow anticipated over western Plumas County and the Sierra Nevada mainly during the evening and overnight hours.
By Thursday morning, snow levels may be as low as 4000 feet although only light snow is expected at elevations lower than 5500 feet. Above 5500 feet, a Winter Storm Watch has been issued with hazardous mountain travel anticipated Wednesday evening into Thursday morning.
In the Valley, the heaviest rainfall looks to be Wednesday night, becoming much lighter or ending Thursday morning.
Models are hinting at some instability behind this system, but the bulk of colder mid and upper level temperatures shifts east by the afternoon. By Thursday night, dry weather is expected to return.
Extended discussion (Saturday through Tuesday)
Models trending drier over the extended forecast period. Upper ridging moves through Saturday, pushing into the Great Basin Sunday. Pacific frontal system rides the ridge with associated precipitation remaining generally north of the CWA.
Baroclinic zone may slide a little southward Monday into early Tuesday bringing a threat of some light precip into Shasta county however confidence low at this time.