December 9, 2020 – Dry weather with increased off shore winds later today into tomorrow. Gradual cooling into the weekend. A pattern change late week into the weekend with rain and snow chances returning to the forecast.
Discussion
An upper level trough is currently pushing into the PacNW pushing a cold front into the far NW part of the state. We are seeing some high clouds along this front. We are also seeing some high clouds in southern areas associated with a closed off upper level low located off the Baja coast. The upper level low will become an open wave today and push east as the weakening front slowly push south through the area. This front will have little impact only bringing some high clouds to the region. As the trough pushes into the northern Rockies it will bring some increased northerly and easterly flow but currently winds look to remain overall weak with gusts generally staying below 20 mph.
Short wave riding will briefly build in for early Thursday before we see a short wave trough and cold front track over northern portions of the area. With little to no moisture this feature will bring mainly dry conditions but an isolated snow shower will be possible over the Sierra for the second half of Thursday.
Quiet weather is expected during the day on Friday but we look to be shifting into a more active pattern as we head into the weekend. A deeper trough over the Pacific is going to be able to pull moisture back into NorCal by Friday night into Saturday. As the trough pushes east toward the west coast we will see warm air advection develop over NorCal starting after 0z Saturday. This will bring light shower chances starting in the Coastal range Friday night and spreading to the rest of the area into Saturday morning. The best chances for showers will be over the higher elevations. Cold air initially will be in place and some areas down to 2500 feet could see some snow mix in with the showers Friday night, little to no snow accumulation expected though. Snow levels will rise quickly Saturday rising above 5500 feet by the afternoon as the warmer air builds into the region. Shower chances lessen during the afternoon as the warm air advection weakens but a few showers will continue to be possible over the northern Sac Valley and higher elevations.
We will see a gradual cool down with our highs for the remainder of the week with them returning to average by Friday. Overnight lows tomorrow night should be slightly warmer but we can expect chilly lows Thursday night with frost likely.
Extended discussion (Sunday through Wednesday)
Unsettled weather likely into next week, as moist west to northwest flow keeps an active pattern in place. It will be hard to nail down the details of the numerous quick moving systems expected in the extended, until they progress more in the short term.
It does appear confidence is increasing for an above normal precipitation period from mid-December progressing towards the Christmas holiday. Will stay in line with NBM guidance at this time, given timing challenges.