January 30, 2020 – Dry with above normal temperatures through Saturday. Locally gusty north to east wind possible at times. Weak Pacific storm brings a threat of light precipitation Sunday into Sunday night, mainly over the foothills and mountains. Dry early next week with some breezy wind.
Discussion
Upper ridging setup along the West Coast. Embedded vorticity maxima moving through it spreading associated mid to high clouds over Interior NorCal. Some mist, creating MVFR visibilities, is being reported by surface observing stations in our Northern San Joaquin Valley attm. With varying amounts of cloudiness moving through overnight, visibilities where fog is trying to develop are likely to fluctuate significantly. This in response to longwave radiation variations through boundary layer. Low level northerly flow yesterday was strong enough to result in downslope warming off the Vaca mountains in the Fairfield area, setting an unofficial new record high of 73. Pressure gradients look lighter today. Above normal temperatures forecast over the CWA this afternoon with mid 60s for the Central Valley and upper 40s to around 60 in the mountains and foothills.
Upper ridging continues to slowly build over the area tonight and Friday. AMS further warms under increased subsidence. With less high level cloudiness tonight into Friday, overnight valley fog development looks more favorable. Max temps Friday forecast to be several degrees warmer in the Central Valley. More significant warming will be observed in portions of the foothills and mountains.
Upstream Pacific short wave trough approaches Saturday as upper ridging weakens and progresses. Clouds will be on the increase Saturday into Saturday night. Some precipitation possible late in the evening over the Coastal and Shasta mountains. Bulk of light precipitation is modeled over the foothills and mountains Sunday into Sunday night, with a few hundredths possible in northern and eastern portions of the Sacramento Valley. Snow levels lower Sunday as trough ushers in colder air. A few inches of snow are possible over higher elevations of Western Plumas mountains and Sierra Nevada which could slow mountain travel.
Extended Discussion (Monday through Thursday)
On Monday, the trough that brought some wet weather to NorCal will be located across the southwest US, continuing to dig into the southern Rockies through the day. An increased pressure gradient sets up behind this feature allowing for some gusty north to northeast winds Monday into Tuesday morning. Winds in the Valley may gust up to 30-40 mph, with higher gusts in the Sierra- Cascade mountains. Winds decrease on Tuesday, with primarily dry and benign weather expected the rest of the week. A system does skim the far northern portions of the CWA Wednesday, although most precipitation with this wave is expected to remain in the PacNW.
The start of the week does start out a bit cooler than normal with Valley highs in the low to mid 50s. Lows Monday night into Tuesday will be the coldest of the week with some Valley and foothill locations nearing freezing temperatures. A warming trend is forecast through the week with highs rising back to above normal by mid- week.