March 9, 2021 – Winter storm to bring mountain and foothill snow, Valley rain, and thunderstorms through Wednesday. Dry and milder weather expected to return by the end of the week.
Discussion
Satellite imagery shows the deep, cold, closed low off the PacNW coast moving slowly south southeast. Radar indicates the region is in a relative break in precipitation early this morning with light snow resulting in chain requirements across many of the mountain highways, and only isolated showers occurring elsewhere. Current temperatures are cooler compared to 24 hours ago and range from the 20s in the mountains to the lower to mid 40s in the Central Valley.
Precipitation will increase across the region today, especially by this afternoon, as the low edges closer. Several bands of showers, with embedded thunderstorms, will move through the region this afternoon and this evening.
This is a cold system with rather meager available moisture (TPW generally under 1/2 inch), but snow amounts are still expected to be fairly high due to steep mid-level lapse rates and strong forcing. Around 1 to 2 feet of snow is forecast for the upper foothill elevations and higher terrain, with a few inches possibly occurring down to around 1500-2000 feet in the mountains north of Redding. Mountain travel impacts will become more extensive and significant later today and tonight as hourly snowfall rates increase into the 1-2 inch range.
Conditions continue to look good for thunderstorm development today and Wednesday. CAMS continue to favor linear mode for storms, but instability and shear parameters do indicate a non- zero chance for some stronger thunderstorms should discrete cells develop in between more organized lines (small hail and perhaps a brief weak tornado would be possible). Limited moisture and low snow levels will also greatly limit any potential debris flow issues with recent burn scars.
Showers expected to diminish from the north by Wednesday evening as the low shifts south. Some showers may redevelop over the northern Sierra on Thursday afternoon and evening, but dry conditions will predominate elsewhere as the low moves south into SoCal. Colder morning lows in the 30s in the Valley and 20s to 30s in the foothills expected Thursday and Friday mornings as skies clear, but highs will return to the 60s across the Valley with increased sunshine returning.
Extended discussion (Saturday through Tuesday)
Upper ridge shifts inland over the area Saturday with above normal high temperatures. Pacific frontal system spreads light precipitation north of I-80, Sunday afternoon through Monday morning. EPAC upper ridging builds inland behind the wave Monday with ridge axis shifting inland Tuesday. This will return dry weather to the CWA early next week with warming temperatures.