Rain and mountain snow return tonight, potential for localized flooding on Wednesday

February 12, 2019 – Generally light precipitation over the northern mountains today with dry weather continuing elsewhere. Widespread precipitation returns tonight with the potential for significant precipitation and local flooding on Wednesday. Active pattern may continue into the weekend.

Discussion

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Partly to mostly cloudy skies cover the region with the only area of light precipitation currently limited to far northern Shasta County. Satellite imagery shows the colder airmass dropping southward off the PacNW coast while moist subtropical air moves up from the southwest between Hawaii and the West Coast. Current temperatures are milder compared to 24 hours ago across most of the region and range from the single digit in the mountain valleys to the mid 30s to lower 40s across the Central Valley.

Dry weather continues today across most of interior NorCal, but increasing warm-advection will likely lead to the development of more widespread light precipitation across the far northern portion of the forecast area by this afternoon. Increased cloudiness is expected compared to Monday as mid and high clouds thicken overhead in advance of the approaching system.

The bigger event begins tonight and continues Wednesday and Thursday as repeated cyclogenesis forecast off the coast leads to several waves of moderate to heavy precipitation and strong winds. The trend continues toward higher snow levels and slightly higher overall QPF as the colder air associated with the trough remains backed off the coast until later Thursday allowing a prolonged period of milder air to move up from southwest over the region. Flood watch continues for much of the valley.

Significant mountain travel impacts are likely, and the winter storm watch remains in effect, but snow levels will need to be fine-tuned before warnings go out. A wind advisory is also in effect Wednesday, but a second round of strong winds may occur Thursday as another surface low moves up from the southwest into NorCal. This could bring down tree branches and cause local power outages.

Extended Discussion (Saturday through Tuesday)

Unsettled weather will likely continue through the weekend. While models still vary on exact details, the consensus is that a shortwave Saturday into Sunday will continue to bring precipitation chances to interior NorCal before the main trough moves through Sunday into Monday.

On Saturday, while there may be continued light Valley rain, the better chances for precipitation will continue in the mountains with low snow levels around 2500-3000 feet. The heaviest precipitation during this time may be overnight Saturday into Sunday which would lead to travel concerns through the weekend. Snow levels remain similar on Sunday to the previous day, with levels dropping to around 1500 Sunday night as this system is on its way out of the area. The main trough pushes into the SW CONUS by Monday afternoon. A break in precipitation is forecast early next week, although models are struggling with the overall pattern. Temperatures continue to be cooler than normal through early next week.