December 9, 2019 – High pressure builds over the region on today, which will lead to a period of dry conditions and near average temperatures early this week. Precipitation chances return to areas north of Sacramento on Wednesday with more widespread precipitation expected late in the week.
Discussion
Skies have cleared as the trough continues to south of the area. IR difference imagery shows an extensive area of dense fog along the eastern edge of the Central Valley extending from about Oroville southward through the San Joaquin Valley. Light northerly flow over the western half of the Sacramento Valley has so far prevented the fog from shifting further to the west. Current temperatures are considerably cooler across most of the area compared to 24 hours ago and range from the mid 20s in the colder mountain valleys to the 40s to around 50 in the Central Valley.
Heights will continue to build today as an offshore ridge moves through. This ensures a period of dry weather to start the week with near average temperatures. Valley fog is expected to mix out by mid to late morning.
The ridge axis crosses the state early Tuesday before a weak shortwave moves through Tuesday evening into early Wednesday. Moisture seems to be meager so precipitation will be light, mainly focused over the northern Sacramento Valley and into the higher terrain. Any snowfall will lead to little to no accumulations.
Ridge briefly rebounds Wednesday, then a wide swath of deep moisture will approach NorCal beginning late Wednesday. Overall lift appears to be rather weak, but increasing warm-advection will likely result in some light precipitation across the northern half of the forecast area and northern Sierra Nevada. Snow levels expected to be over 8K ft, so little to no winter travel impacts expected.
Extended Discussion (Friday through Monday)
The extended forecast calls for zonal flow over the area. A weak shortwave disturbance will slide over us on Thursday, dragging a cold front into the region. The chance for showers remains in the forecast at this time, keeping the bulk of any precip over the mountainous regions. A more vigorous trough will dig into the area on Friday. Ensembles remain in good agreement with pushing a cold front south through the area Friday night into Saturday morning. This will be the best chances for widespread precip activity for interior NorCal, with the chance for showers lingering throughout the day on Saturday. Snow levels start off fairly high, but once the front passes through we could see accumulating snow at pass level on Saturday. By Sunday, a shortwave ridge builds over the area which will bring drier weather through Monday.