Dry conditions with cold overnight temperatures are expected into Saturday. More significant rain and heavy mountain snow is forecast for the second half of the weekend and next week. Winter Storm Watch in effect from Saturday evening through Tuesday evening.
Dry weather will continue today, tonight and into early Saturday. Another night of cold temperatures is expected tonight with frost potential again across the Central Valley by early Saturday. Longwave trough will develop in the Gulf of Alaska, and drop south through the weekend and into early next week. This will reinforce the already-cold airmass, tap into some subtropical moisture, and result in periods of heavy rain and very heavy mountain snow across much of interior NorCal.
Winter Storm Watch remains posted for the mountains beginning late Saturday through Tuesday as there continues to be increasing confidence of a high-impact storm with strong winds and very heavy snow accumulations (measured in feet above 4k ft).
Extended Discussion (Tuesday through Friday)
Tuesday confidence remains high that a powerful winter storm will continue to impact the area as a cold trough from the Gulf of Alaska moves inland. Heavy mountain snow is expected to continue to disrupt travel.
The biggest change will be dropping snow levels as the cold air moves across the area. Shasta and Tehama County are expected to see snow levels dropping to around 2500 feet, with the northern Sierra around 3000-2500 feet.
The latest NBM suggest an additional 1 to 2 feet of snow are possible above 5000 feet in the Sierra, with the upper foothills potentially receiving some accumulation.
Gusty winds should continue early in the day and along with the snow will likely bring reduced visibility, especially in the morning hours. These should bring an extensive area of hazardous driving conditions.
The trough axis shifts eastward Tuesday night, with decreasing precipitation intensity. Snow levels drop further, though, as more cold air advects in.
Snow showers behind the trough and with another Gulf of Alaska trough could bring additional hazardous travel conditions through Wednesday. This could especially be the case for northern Shasta County, and could bring travel impacts for I-5. Some snow could possibly extend down to Redding.
The northern Coastal Range and the Sierra and Sierra foothills could also see inches of snow accumulation, but quite a bit of uncertainty remains in the exact details at this point.
Thursday could see additional precipitation across the area as the trough moves inland. Even Friday could remain unsettled before the system exits, though cluster analysis show some uncertainty in the pattern with some ridging potential for late in the week.