February 28, 2018 – A series of cold winter storms will bring periods of heavy snow and gusty winds to the foothills and mountains with heavy rain showers and isolated thunderstorms over the valleys Wednesday into the weekend. A brief drying out period Monday and Tuesday will be followed by another shot of precipitation mid week.
High and mid cloudiness are spilling over the north end of the state this morning as a weak shortwave rides over west coast ridging. This system is in advance of a cold Pacific frontal system expected to push into northwest California late this afternoon and evening.
Main cold frontal band is forecast to push through northern California late tonight through Thursday pushing east of the forecast area late Thursday night and Friday morning.
Moderately heavy precipitation is expected during this time with snow levels still expected to be low enough for significant impacts to mountain travel. Snow level forecasts may still be a bit high and will need to be monitored. Only made minor changes to the previous forecast package with just a slight slow down in timing of the front.
Heaviest snowfall over the Sierra Cascade range now looks to be Thursday afternoon and evening. Will keep winter storm warning now in place unchanged as impacts will still be enough even if front slows down a bit.
Surface gradients max out just before the front mid day Thursday and wind speeds will approach wind advisory criteria in the valley at least for a short period of time. With slight slowing of frontal band, may not see as high a likelihood of thunderstorms Thursday afternoon but still enough uncertainty to leave them in the forecast for areas behind and north of the expected frontal position.
Showers continue on Friday and Saturday as main upper low slowly slides across NorCal. Cold air pouring into the low will bring down snow levels still farther with snow possible into the northern Sacramento valley on Friday morning.
Snow levels climb a bit Friday afternoon and Saturday but still remain quite low. Precipitation threat finally drops off significantly on Saturday night as main trough shifts east of the forecast area and upper ridging begins to push in over the north state.
Extended discussion (Sunday through Wednesday)
Drier weather expected Sunday and Monday as upper level ridging moves through. Forecast uncertainties with timing of next Pacific frontal system as GFS20 more progressive, spreading precip over Interior NorCal beginning Tuesday night while EC starts precip Wednesday morning.
Given progressive nature of pattern, forecast leans more towards EC guidance attm. System looks warmer than recent storms with snow levels through Wednesday generally above 4000 feet.