Dense Valley Fog this morning – rain, mountain snow, and travel impacts return mid-week

December 14, 2020 – Dry weather early this week with areas of morning valley fog. Unsettled weather returns by mid-week.

Discussion

Radar indicates a few blips of light snow lingering early this morning across the higher elevations of the northern Sierra. Traffic cameras indicate snow has ended across most of the main high-elevation passes across the northern Sierra. Elsewhere, IR difference imagery and surface observations indicate fairly widespread dense fog across the Sacramento Valley, especially across the southern two thirds of the valley, and a dense fog advisory has been issued for this morning. Light north wind has developed across the far north end of the Sacramento Valley helping to clear the fog out of Redding and Red Bluff.

Otherwise, building high pressure is expected to bring mainly dry weather into mid-week. A weak system moving through the ridge on Tuesday will bring some clouds and perhaps some light showers to the northern mountains.

A more substantial system is expected to move through the region on Wednesday and Thursday bringing widespread rain and mountain snow. The system will be tapping a decent moisture plume of around an inch TPW, and significant precipitation will be possible.

Extended discussion (Friday through Monday)

Dry weather expected on Friday as upper ridging moves through interior NorCal. Warm front then extends into far northern portions of CA over the weekend. This will bring a threat of precipitation, mainly to the coastal and Shasta mountains.

Models differ with timing of associated cold front tracking into NorCal early next week. Forecast leans towards deeper and more progressive GFS attm, with some precipitation advertised north of I-80 and mainly in the foothills and mountains.