December 28, 2018 – Mainly dry through the middle of next week except very isolated mountain showers over the northern mountains on Sunday. Breezy north winds through tonight then again early next week.
NorCal currently under northerly upper level flow between a low pressure system over the southern Great Basin and high pressure ridging over the eastern Pacific. Cyclonic flow over the Sierra bringing a some cloud cover there but otherwise most of CWA is under fair skies. 8 mb surface gradient from MFR to SAC bringing continued breezy northerly winds to most of the CWA. Should see a slow decrease in winds today as the upper ridge pushes inland and surface gradients slowly relax.
Airmass warming and fair skies will bring a slight increase in daytime highs today and forecast to come in several degrees above normal. Fair skies and lighter winds tonight will allow for cool morning temperatures with some valley locations dropping to near freezing. Fair skies under the ridge on Saturday under the ridge will bring another warm day with daytime highs again several degrees above normal. A weak Pacific frontal system moving through the Pacific Northwest on Sunday will bring increased cloud cover and even a slight chance of isolated showers over the highest elevations north of about Red Bluff.
Clouds and cooler airmass will bring a little cooling on Sunday most locations but daytime highs are still likely to come in a little above normal for this time of year. Upper ridge amplifies back up over the eastern Pacific on Monday. This will bring clearing skies and an increase in northerly winds as increasing surface gradients and upper level northerly flow combine.
Extended Discussion (Tuesday through Friday)
The extended forecast continues to look mainly dry, with forecast high temperatures a little above normal and low temperatures a near to slightly below normal.
Models continue in agreement with a closed low developing near the 4-Corners and eastern Pacific ridging developing, bringing dry northerly flow over the area. Breezy northerly winds are again expected early in the week before they decrease later in the week as ridging moves inland.
A weakening cold front is forecast to begin to move into the Pacific Northwest Thursday/Friday. A slight chance of precipitation has been added to the northern Coastal Range, though confidence even in this remains low.