October 24, 2021 – A high-impact weather system continues to bring widespread heavy precipitation to NorCal. This has lead to flooding and some debris flows. Strong southerly winds will diminish during the evening but will remain breezy overnight. Rain will switch to heavy snow over the Sierra tonight into early Tuesday. Quieter and drier weather expected by the middle of next week.
Discussion
We are in the middle of our strong Atmospheric River and we have been seeing flooding impacts across the area especially in and around the Dixie Fire. You can see the trough spinning off of the coast of the PacNW pushing a cold front into NorCal. Ahead of this front we are seeing deep moisture with PWATs 0.80 to 1.75″, the 12z OAK sounding had 1.62″.
Rain rates over the foothills and mountains have generally ranged from 0.30 to 0.80″ per hour and 0.15 to 0.60″ in much of the Valley with lesser amounts in the northern Sac Valley. Locally higher rain rates will be possible right along the front.
Heavy rain will continue with these rain rates into the evening slowly working their way to the south with the front.
The majority of the heavy rain will clear the area by 10 pm. We are going to see heavy precip linger over the mountains and some of the foothills into Monday morning mainly from I-80 south as the trough axis moves through.
Snow levels are currently above pass level but we are going to see them fall from north to south reaching 5500 to 6500 feet by 4 am. Mountain travel will be very difficult above 6000 feet by daybreak tomorrow morning and heavy snow will continue throughout the morning. It will become more showery in the afternoon but mountain travel will likely remain poor.
We will also see some instability build in tomorrow mid morning into the afternoon in the valley and foothills. With some forcing from the trough axis we can expect a few scattered showers and thunderstorms with the best chances along I-5 east into the foothills. PWATs will remain elevated tomorrow (0.40-0.75″) and brief heavy rain will be possible with any thunderstorms along with some small hail. Right now the threat for thunderstorms over the burn scars look to remain low with the better chances staying in the lower elevations.
We are seeing gusty winds across the area and they will gradually diminish this evening but will remain on the breezy side overnight. Less wind is expected by tomorrow afternoon.
A few short wave troughs will pass to our north on Tuesday and that will keep light showers in the forecast from I-80 north with the best chances over the higher elevations. Snow levels Tuesday will be between 5000 and 6000 feet, snow accumulation will remain light. Upper level ridge begins to build in on Wednesday brining quiet and dry weather. Cold below normal daytime highs will continue with some warming Wednesday.
Extended Discussion (Thursday through Sunday)
Ensembles in good agreement with upper level ridge over the CWA Thursday, shifts into the Great Basin Friday as upstream Pacific storm approaches. There is considerable uncertainty with the strength and progression of this next system, though. Some light precipitation may be possible during the Friday night to Saturday morning time frame over Shasta County as it moves through. The NBM is now picking up on this and so light rain has been added to the forecast for that period. Most of the forecast area will remain dry. Dry weather across the area Sunday as upper ridging looks to rebuild over the forecast area.