Wet weather will continue across the region into early next week. A weather system will move through today followed by a stronger and wetter storm later Saturday and Sunday that will be accompanied by heavy precipitation and strong winds. Heavy snow will be possible in the Sierra Sunday night into early Tuesday. Flood Watch from Saturday evening through Monday afternoon, Flood Watch from Saturday evening through Monday afternoon in effect.
Radar shows the band of moderate to brief heavy rain is finally moving inland across the northern half of the forecast area as the upstream vort nearing the coast appears on satellite imagery to have stopped digging. Hourly rainfall rates have so far topped out in the 1/4 to 1/2 inch range across Shasta County. Gusty southerly winds of 30-40 mph have persisted across the northern Sacramento Valley since late Thursday evening. These winds will subside early this morning once the front moves through.
Rain will continue to spread southeast early this morning as the front moves inland. Rain will likely reach the Sacramento region around the morning commute. Rain will also increase over eastern Butte and western Plumas county early this morning as the latest HREF indicated high likelihood of rates exceeding 1/2 an inch/hour. A Flash Flood Watch remains in effect in that area for potential ash and debris flows.
Snow levels remain very high ahead of the front (above 8-9k ft), but will lower to around 6500 ft later this morning. A brief shot of snow (around 1-3 inches) will be possible over the higher northern Sierra passes which could lead to some travel impacts during the afternoon into early this evening.
Otherwise, if enough post-frontal clearing develops in the Sacramento Valley this afternoon, it looks favorable for a few thunderstorms to develop by mid-afternoon over the northern half of the valley ahead of the next approaching mid-level speed max.
A relative break in precipitation is expected tonight into Saturday ahead of the next stronger system. Warm-advection and southerly upslope flow will likely continue mostly light rain over Shasta County overnight tonight before light rain begins to spread south on Saturday.
The main event begins Saturday night and continues into early next week as the AR traverses NorCal. A prolonged period of moderate to heavy rain is likely over much of the region from late Saturday night through Sunday (IVT is off the charts) as a wide swath of deep moisture interacts with a frontal system slowly sliding south.
2-4 inches of rain will be possible in the valley, with 8 inches or more over the northern Sierra (supported by strong orographic enhancement). In addition, strong south/southwest winds are likely over much of the region.
The system begins to shift east Sunday night and snow levels will drop below pass level with heavy snow expected across the Sierra passes with several feet of accumulation possible into early Tuesday.
Significant impacts are likely across several categories, flood, wind and winter, and severe impacts are possible. Stay tuned.
Extended Discussion (Tuesday through Friday)
AR weakens early Tuesday with decreasing precipitation into Tuesday night. Drier weather expected midweek under building upper level ridge with warming temperatures Tuesday through Thursday.
Forecast uncertainty by the end of the week due to model differences with progression of next Pacific storm into NorCal.