A pattern change this week will bring a significant cooling trend, breezy winds, and widespread rain and mountain snow mainly Tuesday and Wednesday. Cold morning lows, possible morning frost, and breezy offshore winds Thursday. Unsettled weather possible again this weekend. Winter Storm Warning from noon today to 8 PM Wednesday for West Slope Northern Sierra Nevada.
Forecast still on track so only minor changes to the current forecast. Rain and mountain snow will be moving into the region today. Precipitation will be heaviest this morning over the far north end of the valley and the coastal range but transition south and east for the afternoon and evening.
The heaviest snowfall is expected this afternoon through this evening and when conditions traveling over the Sierra will deteriorate. Lowering snow levels this afternoon and evening will likely mean chains will be required at some point. Snow levels will lower below the crest on today to around 6500 feet and continue to lower late this afternoon to around 5000 feet. Snow levels will then continue to lower down to 3500 to 4500 feet by Wednesday morning.
Post frontal instability is still indicated that may bring a few thunderstorms late this afternoon and evening mainly over the eastern foothills and mountains. Wednesday the models are not showing as much instability as before and has removed thunderstorm chances from the forecast. Still would not be surprised to see and isolated thunderstorm though.
The trough on Wednesday will still be over the region during the day and favors showers continuing over the region but mostly over the western slopes of the Sierra Nevada as the day goes on. Snow levels look to remain between 3500 to 4500 feet but amount of precipitation seems limited for Wednesday. Local 1 to 3 inches of snowfall could still occur.
Thursday and Friday the low moves east and breezy north winds develop for Thursday. Cold air over the region may bring some frost to parts of the valley and foothills both Thursday and Friday mornings with the best chances on Thursday and on the eastside of the valley. Daytime temperatures will warm some over the midweek temperatures but be colder at night.
Extended Discussion (Saturday through Tuesday)
Broad trough of low pressure will be over the region Saturday through Tuesday bringing precipitation chances each day. Temperatures will be cooler than normal during this time period by 5 to 10 degrees.