March 15, 2018 – Drier weather has temporarily settled into Northern California in between storm systems. Yesterday evening’s convection has moved into the higher Sierra early this morning, resulting in scattered snow showers. These snow showers may continue through the morning hours.
The next storm system has begun to push inland across the NW California coast, where light radar returns are already being detected. Short range models show this area of precipitation moving across the CWA starting mid to late this morning, sweeping across the region during the afternoon and overnight hours. Precipitation across the Valley should remain mostly light to moderate, with less than 0.75″ rain expected.
Orographics should provide significant enhancement over the Sierra however, with 1 to 3 inches of liquid equivalent precipitation expected by Saturday. Breezy winds are also expected to accompany this storm, mainly this afternoon and evening.
With cold air already in place, snow levels will start out low and stay low with this storm. We’re currently expecting snow levels of 2000-3000 ft across the northern mountains, and 3000-4000 ft along the west slopes of the Sierra.
With snow levels this low, we’re expecting snow liquid ratios (SLRs) of roughly 15 to 20:1 – light, fluffy snow that can pile up quickly.
The combination of high snowfall rates and gusty winds will likely lead to periods of very hazardous travel with near-whiteout conditions over the high Sierra, particularly late this afternoon through tonight.
All told, an additional 3 to 5 feet of snow accumulation is expected over the high Sierra. Even lower mountain passes are likely to be affected. Winter Weather Advisories and Warnings have been issued across much of interior NorCal’s mountains.
Showery weather will continue into Friday as the upper low passes over NorCal. Moderate to heavy snow will continue across NorCal mountains on Friday, and isolated afternoon Valley thunderstorms are possible. Coverage of showers will decrease on Saturday and Sunday.
Extended discussion (Monday through Thursday)
Dry conditions return over the CWA on Monday as shortwave upper ridge slides over the west coast. Daytime highs warm a little Monday but are still expected to come in a little below normal. Models diverge on Tuesday making forecast outcome on Tuesday a little more uncertain. GFS brings a band of Pacific moisture into norcal on Tuesday while ECMWF holds on to ridge a bit longer keeping the state dry Tuesday. Have kept light precipitation across the CWA Tuesday which is more in line with previous forecasts and surrounding offices. Better model coordination returns on Wednesday as both GFS and ECMWF bring deeper moisture with a tropical connection into northern California on Wednesday. Significant precipitation will be possible with this warmer storm but snow levels will still be low enough for mountain travel impacts.