A warm storm system will bring moderate to heavy rain with flooding impacts to rivers, creeks, streams, and roadways at elevations below 4,000 feet, along with higher elevation snow and strong winds. A few thunderstorms through the weekend as unsettled weather persists. Another strong storm will affect the region early next week.
Discussion
Warm conveyor belt continues to bring waves of moderate to heavy rain onshore. Radar shows an area of rain currently moving across NorCal with embedded thunderstorms. Gage data and radar estimates indicated rates around 0.25 to 0.50 inches/hour with the heavier showers.
Surface pressure gradient is currently peaking and strong south to southeast wind gusts of 40-50 mph are occurring in the Central Valley. Gusts of 45-60 mph have been observed across the northern Sierra.
Up north, strong warm-advection has finally lifted snow levels across Shasta County with traffic cameras showing wet road surfaces around the Redding area. A mix of rain and snow will be possible in the Sacramento River canyon north of Redding early this morning before precipitation ends once the cold front drops south around daybreak.
In the Sierra, SL radars indicate the snow level is around 7-8k ft with heavy rain occurring except near the higher pass levels where snow is hanging in.
Several CHP reports indicate significant drainage issues are ongoing along I-80 where runoff is channeling down the highway unable to drain due to snowbanks along the edge. If at all possible, avoid travel in this area this morning!
Satellite and radar imagery show the cold front is finally beginning to make progress to the south and east as it has entered northwest California. The front is forecast to make its way down to around the I-80 corridor by midday before it stalls out.
Behind the front, precipitation will rapidly end across the northern half of the forecast area during the morning while it hangs in from about the Sacramento area southward (though rates will be significantly lower as the bulk of the strong AR shifts to our south).
The window for surface-based deep convection doesn’t look too promising today across the northern San Joaquin Valley as clouds and precipitation will persist through the afternoon. The gusty southeast winds are currently peaking as the cold front approaches, and will likely begin to subside this morning.
Small rivers and streams are already cresting, or will crest this morning, across the north end of the Sacramento Valley. Further south, runoff from the overnight rain will lead to rises through the day and into tonight. If you live in or near a flood prone area, continue to closely monitor the weather.
A soggy weekend is expected over NorCal as moisture continues to stream in from the west. The front is forecast to linger providing a lifting mechanism for lighter precipitation Saturday and Sunday.
Snow levels will come down a bit from their current levels and significant snowfall accumulations are expected across the higher elevations.
The threat for thunderstorms over the weekend will be conditional on seeing some clearing. HREF indicates the best potential will be across the Central Valley to the south of Sacramento on Saturday. Potential may spread further north on Sunday. Forecast soundings indicate sufficient shear and instability for a few longer-lived storms and their associated threats (primarily hail, but a few funnel clouds cannot be ruled out).
Extended Discussion (Tuesday through Friday)
Another moderately strong AR forecast to impact the forecast area early next week. Heavy precipitation Tuesday will continue flooding concerns. Snow levels Tuesday look to be around 5000 to 5500 in the Shasta mountains, and around 6000 to 7500 feet over the Cascades and Northern Sierra Nevada.
Strong southerly wind will accompany the front as it moves through Tuesday.
Scattered showers will possible Wednesday, mainly in the mountains. Storm total liquid QPF looks to be around 1 to 2.5 inches for the Central Valley, 2 to 6 inches for the foothills and mountains with multiple feet of snow over the higher elevations.
NBM keeps POPs in Thursday, however this looks to be overdone, as deterministic models showing interior NorCal under upper ridging during that period. Potential for some light precipitation towards the end of the week, but forecast confidence is low at this time.