Periods of showers and mountain snow expected today into Saturday. Expect hazardous mountain travel Friday afternoon into mid-day Saturday. Slight chance for thunderstorms across portions of the Valley and Sierra foothills Friday afternoon. Dry conditions and warmer temperatures return early next week.
Discussion
Satellite imagery reveals clouds streaming into the region ahead of an approaching trough. This system is forecast to split with the southern portion moving into southern California tonight. Models indicate shower chances will be mainly confined to the Coastal Range with generally light accumulations expected. Ensembles depict another wave dropping out of the northeastern Pacific and closing off over NorCal on Friday. Best chances for measurable precipitation with this system will be over the foothills and mountains.
Precipitation totals could range from 0.50-1.25″ over the mountains with less than a tenth of an inch to around 0.20″ in the Valley. Main impacts with this system will be moderate snow that could bring hazardous mountain and upper foothill travel.
Snow levels will start around 6000 to 6500 feet Friday afternoon, before falling to around 2000 feet Saturday morning, locally lower. Forecast snow totals range from about 4 to 8 inches with localized amounts up to a foot possible over higher peaks. HREF shows the heaviest snow beginning late Friday afternoon.
A Winter Weather Advisory remains in effect for the southern Cascades and Sierra beginning Friday afternoon through mid-day Saturday. Have delayed onset of advisory for the Sierra Foothills until Fri night based on timing of lowering snow levels.
Be prepared for slick roads, chain controls and reduced visibilities at times. Closed upper low moving southward over the area Friday could lead to afternoon convection across portions of interior NorCal. Therefore, have introduced a slight chance for thunderstorms across portions of the Sacramento and northern San Joaquin valleys, as well as the adjacent Sierra foothills Friday afternoon.
A few snow showers may linger over the Sierra Nevada Saturday evening, mainly south of US-50. Then, dry conditions are expected on Sunday as the upper low exits the region. Expect Valley highs generally in the mid to upper 50s on Saturday. By Sunday, daytime temperatures will run 5 to 8 degrees warmer than Saturday’s.
Extended Discussion (Monday through Thursday)
Weakening cyclonic flow forecast Mon as a secondary short wave from Wrn Canada drops Swd into/across the Great Basin behind the closed low affecting Norcal in the near term. Modification of the air mass will allow temps to recover into the upper 60s to low 70s in the Valley and most foothill locations, about 3 to 8 degrees above normal. High Sierra will continue to run below normal however.
Minor cooling expected Tue/Wed as another trof drops Swd from B.C. into the Pac NW and eventually carves out a long wave trof impacting the Rockies and Ern parts of NOAM late next week. The 5H Cluster Analysis suggests a little retrogression of the Ern Pac ridge will allow some cooler Canadian air to infiltrate Norcal during this time. However, the precip is expected to remain N and E of our CWA. The Ern Pac ridge re-establishes itself for a modification of temps on Thu into Fri of next week.