Another Pacific weather system will bring periods of rain and mountain snow, this afternoon into Tuesday. More impactful storms will move through NorCal Wednesday through next weekend. Strongest storms are expected Wednesday-Thursday and again over the weekend. All this precipitation will keep flooding threats elevated and mountain travel impacts at the forefront for the next week into next weekend. Winter Weather Advisory from 10 AM this morning to 4 AM PST Tuesday for West Slope Northern Sierra Nevada-Western Plumas County/Lassen Park.
Discussion
Satellite is showing dense mid to upper clouds moving into NorCal early this morning in response to moisture being drawn up ahead of the shortwave that will dig into NorCal later today. Moisture associated with this shortwave is not very impressive, especially comparing it to the recent Friday-Saturday systems, so expecting minimal rainfall impacts. Though, snow levels will be low through today, where snowfall totals of 1-4 inches between 2000′-3000′ in the Coastal and Cascade range will lead to hazardous travel along the higher elevations of I-5 corridor.
Slightly higher snow levels are expected in the Sierra/Nevada, limiting snowfall in the foothills with the primary impacts expected above 3500 feet, where 3 to 8 inches of snow will be possible, with locally higher amounts near the peaks.
The Valley will mainly see light precipitation today, with storm total amounts generally 0.15-0.30″ with locally up to 0.50″ in the foothills. In addition to rain and snow with today’s system, expected southerly breezes with gusts to 20-30 mph to develop later this afternoon-tonight, strongest winds will be south of I-80.
Snowfall is expected to decrease early morning Tuesday, resulting in mostly dry conditions through much of Tuesday morning.
Though, another weak shortwave will brush NorCal Tuesday afternoon- evening, allowing for some slight chances of seeing light precipitation across portions of the Sacramento Valley, with the best chances (up to 50-60%) mainly confined to Shasta County.
A more potent weather system is forecast to impact NorCal Wednesday- Thursday, with a moderate to strong Atmospheric River signal. Therefore, another round of moderate to heavy precipitation is expected with potentially more flooding concerns.
Early precipitation forecasts for the midweek storm looks to be around 2 to 3 inches possible in the Central Valley with 3 to 6 inches or more of liquid precip in the foothills and mountains. Based on latest guidance, the heaviest precipitation looks to be on Wednesday.
Snow levels start out around 3-4k ft early Wednesday, quickly rising to around 6-7k ft by early Wednesday evening, then dropping back down to 5-4k ft Thursday. With heavier precipitation expected, more mountain travel impacts are likely.
In addition to moderate to heavy precipitation mid- week, a strong low-level jet will also create strong southerly winds across the region. Wind gusts are forecast 35-50 mph in the Valley and foothills, with gusts up to 60 mph in the mountains. Still some uncertainty in the timing and magnitude of the strongest winds, but the latest forecast has the peak winds occurring Wednesday afternoon- Wednesday night.
By Thursday, the AR moisture will lessen as it shifts southward, but an additional weak wave will keep precip chances in the forecast through Thursday.
Extended Discussion (Friday through Monday)
Wet weather pattern continues in the extended forecast period, however model differences lead to forecast uncertainty with timing of waves and QPF. Weak trough expected to move through Friday with a continued threat of light precip, mainly north of I-80.
Stronger system forecast for Saturday however GFS showing much higher QPF than EC. Weak short wave ridging looks to provide a brief break in the precip early Sunday. QPF uncertainties increase significantly beyond this time frame as GFS advertising another strong AR into NorCal Sunday afternoon into evening continuing into Monday. GEFS Ensemble mean has a much weaker AR and EC showing significantly less QPF towards the end of the extended forecast period.