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March 2, 2018 – Rain and snow showers today through Saturday with low snow levels. Isolated valley thunderstorms this afternoon and Saturday. A brief drying out period early next week will be followed by another shot of precipitation mid week.
Discussion (Today through Sunday)
Cold frontal band currently bringing light to moderate snowfall to the northern Sierra and a few light showers to the northern San Joaquin valley will be shifting south and east of the forecast area during the morning hours. Most of the remainder of the forecast area is getting a brief break in precipitation as much of NorCal is currently in a dry slot between disturbances. Cold air behind this last front will bring snow levels over the northern CWA down to below 1000 feet. Parts of the Redding area in the northern Sacramento valley could see some snowfall. Fortunately, precipitation amounts this morning are expected to remain quite small so impacts there should be minimal.
The next in a series of disturbances to pivot out of a cold upper low over the northwest U.S will pivot through the north state this afternoon and evening bringing another round of increased shower activity. The instability and dynamics associated with this disturbance will bring a threat of thunderstorms this afternoon and evening especially to the central valley where there may be some breaks in the cloud cover and increase surface heating. Higher elevation snowfall should be significant enough to continue the winter storm warning over forecast area mountains. Cooler airmass will bring down daytime highs today down to as much as 10 to 15 degrees below normal for this time of year.
Yet another, still weaker shortwave disturbance is forecast to pivot through NorCal on Saturday keeping the shower threat and mountain snow going for another day. Snow levels will continue quite low for this time of year at between 1000 and 2000 feet. Stability progs show general instability across most of the forecast area Saturday afternoon so kept large area of thunderstorm threat in the forecast through much of the day. This next disturbance shifts east of the forecast area by Saturday evening bringing decreased instability so have not included thunderstorms in the Saturday evening forecast. Overall airmass does not change much between Friday and Saturday so going with little change in daytime highs.
Main upper low finally shifts inland Saturday night and Sunday so precipitation threat drops off quickly although shortwave disturbances dropping down the back side of the departing low could continue some northern mountain showers into Sunday afternoon. Daytime temperatures don’t change much but may nudge up a bit from Saturday thanks to a little more sunshine. Upper ridging builds Sunday night ending the shower threat. There may be enough clearing to allow for some sub freezing temperatures in the valley Sunday night but duration of these below freezing readings should be brief. Building upper ridge should bring fair skies and warmer temperatures on Monday.
Extended discussion (Tuesday through Friday)
Models similar in showing upper ridging over Interior NorCal Tue into Wed for dry weather. GFS20 continues a more progressive pattern bringing associated precip with next Pacific storm into the forecast area Wednesday afternoon into night. EC is slower with precip spreading into the area Thursday morning. Both models showing next system to be on the weaker side with limited precipitation. Unsettled weather looks to persist into Friday. Temperatures guidance showing values near to slightly below normal through the extended forecast period.