October 21, 2021 – Two weather systems will move through Northern California today into tomorrow and Saturday into Tuesday. Moderate to locally heavy rain, gusty winds, and mountain snow will be possible at times. High temperatures across Northern California are forecast to be 10 to 20 degrees below average.
Water vapor imagery indicates a deep tough pushing in to the Pacific Northwest this afternoon. Showers associated with this feature have started to reach the interior northern Coastal Mountain Range as of 1:45 PM this afternoon. Showers and gusty winds are forecast to continue to increase into the overnight hours.
High resolution models are in good agreement a strong band of precip will move through interior northern California starting around midnight and clearing out of the Valley by the early afternoon. HREF continues to indicate high potential for hourly rainfall rates exceeding 1/2 inch/hour over several area burn scars.
A Flash Flood Watch is in effect to account for the threat of debris and ash flows. After the main precip band moves out of the area, we could still see some scattered showers and possibly isolated thunderstorms with best chances mainly for the northern Sacramento Valley in the afternoon through the early evening.
After a brief lull in the stormy weather Friday night, increasing showers are expected by early Saturday morning as another trough edges closer to California.
This late weekend into early next week system has the dynamics of a moderate atmospheric river and ramps up by Sunday with widespread impacts anticipated. Urban and small creek flooding, gusty southerly winds and heavy rain will be widespread Sunday. Ash and debris flow will also be possible in recently burned areas.
Snow level start above mountain pass levels decreasing below mountain pass levels by Sunday night and further decreasing to 5500 – 6500 feet by Monday morning. The EFI has the Sierra Nevada highlighted for expected snowfall. Significant travel impacts are likely.
Extended Discussion (Monday through Thursday)
Monday morning snow levels are expected to be around 5500 – 6500 feet leading to potentially hazardous mountain driving conditions with chain controls possible. Heavy snow will likely taper off throughout the day with some showers lingering into Tuesday. Several feet of snow accumulation will be possible.
By mid week, ridging from the eastern Pacific moves into NorCal. Milder temperatures and clearing skies will lead to mild weather through the remainder of the workweek. By next weekend another system may reach northern California. Stay tuned!