March 13, 2021 – A weather system will move into northern California late Sunday and into Monday, bringing with it the next opportunity for Valley rain and mountain snow showers. Dry weather will return by Tuesday.
Dry conditions and mild temperatures prevail across interior NorCal this afternoon as a ridging remains in control. Afternoon temperatures are still on track to reach the upper 50s to 60s in the Valley and foothills, with 40s and 50s over the mountains. Increasing cloud cover expected tonight ahead of an approaching cold weather system.
Rain and snow chances return to interior northern CA Sunday afternoon into Monday as a cold system digs out of the Gulf of Alaska and drops into NorCal. Precipitation will develop over the Coastal Range and northern Sacramento Valley Sunday afternoon, and will overspread the region during the overnight hours. Heaviest precipitation is anticipated Sunday night into Monday morning as frontal boundary pushes through. Rain totals will be between 0.20″-1.25″ in the Valley and lower foothills; highest amounts expected in the northern Sacramento Valley and foothills.
Snowfall accumulations of 6 to 12 inches, locally higher, are forecast down to 2500 feet in Shasta County. Around 6 to 18 inches of snow, locally up to 2 feet, are possible down to 3000 feet for the southern Cascades and northern Sierra Nevada. Additionally, snow levels will plummet overnight Sunday and into Monday morning as the center of the cold-core mid/upper-low passes directly over northern California.
There is the potential for snow as low as 1,000 to 1,500 ft in portions of Shasta County by early Monday morning, and as low as 1,500 to 2,000 ft in the northern Sierra.
Latest HREF guidance suggests hourly snowfall rates potentially exceeding one inch per hour Sunday evening into early Monday morning; mainly between midnight and early Monday morning. Therefore, a Winter Storm Warning is in effect for the Sierra, southern Cascades and Shasta County late Sunday through Monday given the potential for hazardous foothill and mountain travel.
A Winter Weather Advisory is in effect for the Coastal Range late Sunday through noon Monday. Motorists with Sunday travel plans in the Sierra should carefully monitor the forecast, and expect weather- related impacts developing as early as the late afternoon or early evening.
High resolution models indicate precipitation gradually diminishing during the day on Monday. With afternoon clearing and increasing instability in the afternoon, can’t rule out a slight chance of a thunderstorm in portions of the Central Valley. Small hail is expected to be the main threat. Low snow levels and expected low rain rates will limit any potential debris flow issues in recent burn scars.
Dry and milder conditions will return for Tuesday as an upper level ridge develops over the West Coast.
Extended discussion (Wednesday through Saturday)
Dry weather is forecast to continue Wednesday as shortwave ridging remains in place. Ensemble guidance continues to depict a deep closed low centered in the Gulf of Alaska with multiple shortwaves moving along the low, producing multiple rounds of precipitation for interior NorCal Thursday through the weekend.
The first round of precipitation Thursday into Friday is expected to be a warmer system with snow levels generally above 4500-5500 feet. Best chances of precipitation remain north of I-80 with generally less than 1 inch of liquid precipitation forecast for most areas.
The next system could potentially be slightly wetter, but it’s too far out to have much confidence at this time. Keep an eye out for forecast updates on these weather systems.