January 23, 2020 – Night and morning valley fog will be possible into Saturday. Some light precipitation possible, mainly over the mountains and northern portions of the Sacramento Valley, into Friday. Wetter Pacific storm moves through this weekend. Unsettled weather continues early next week.
Discussion
Upper level ridge axis moving through attm, will shift into the Great Basin today as upstream Pacific frontal system approaches. With less high level cloudiness overnight, potential for more widespread valley fog increases this morning. Already seeing some patchy dense fog beginning to develop within larger areas of lighter fog in the Sacramento and Northern San Joaquin Valleys. HREF showing increasing probability of visibilities below 3SM from KOVE southward early this morning, so will continue to monitor for more widespread dense fog development. Visibilities expected to improve by mid to late morning as boundary layer mixing increases with approaching short wave trough. Models spread some associated light precip into the mountains and possibly northern and central portions of the Sacramento Valley this afternoon. Highs today expected to be several degrees cooler than Wednesday.
Secondary trough follows through northern half of the CWA tonight into Friday keeping a threat of light precipitation. Overall precip amounts expected to be mostly below a tenth in the Central Valley through Friday, with around a quarter to third of an inch possible in portions of the Shasta and Western Plumas mountains. Snow levels will initially be above 7000 ft this afternoon through early Friday, then lower to around 5500 to 6500 feet Friday afternoon into evening. No significant mountain travel impacts expected during this period.
Upper ridging strengthens over Norcal Friday night into early Saturday, with some night and morning dense valley fog possible. Stronger Pacific frontal system approaches Saturday and looks to begin spreading precipitation into our Coastal Range by the afternoon. Baroclinic zone moves through Saturday night into Sunday morning with some moderate precipitation associated with it. Secondary wave keeps a threat of post-frontal showers over interior NorCal Sunday into Monday. Storm total QPF for the weekend system looks to range from a few tenths in the Northern San Joaquin Valley to upwards of an inch in the Northern Sacramento Valley. Up to around 2 inches of liquid precip possible in wetter portions of the foothills and mountains. Snow levels forecast to generally be above 7k feet ahead of the front on Saturday, lowering to around 5 to 6k feet Sunday and 4500 to 5500 feet Sunday night into Monday. Light to moderate snow amounts are expected with 6 to 12 inches possible at pass levels in the Sierra Nevada.
Extended Discussion (Monday through Thursday)
After the weekend system we will see the storm track shift to the north over the PacNW. They will see multiple waves of precip throughout the period. The majority of the precip will stay to our north but some light mountain showers are going to be possible especially out a head of any incoming waves. Right now there isn’t great agreement between ensembles on when the waves will move through the PacNW. That leaves uncertainty on when we will see the best chances for showers. Either way everything looks light and low impact for us. Towards the end of the week we will see more ridging and that will continue to push the storm track to the north and lessen the shower chances for mountain locations. Temperatures will be near average for much of the period but will warm to just above average by late in the week.