December 23, 2016 – A winter storm with rain, wind and heavy mountain snow moves through the area today disrupting holiday travel into Saturday. Clearing Sunday. A chance of showers northern mountains early next week, otherwise dry. Winter Storm Warning until 10 AM Saturday for the West Slope Northern Sierra Nevada-Western Plumas County/Lassen Park.
A moderately strong Pacific frontal system has begun moving into the northwest corner of the state this morning. Precipitation is currently falling northwest of about Chico and radar estimates are showing pockets of heavy rain embedded within the initial rain band. Winds have been running somewhat weaker than predicted by the model guidance and based on this and current and predicted surface gradients, have dropped wind speeds a bit for today. Snow levels based on current surface observations are around 3500 feet over Shasta county and in line with current forecast package.
Other than windspeeds for today, made only minor adjustments to the forecast grids and will leave current winter storm warning as is.
Models remain fairly consistent on timing of main frontal passage moving it into the Sacramento area around early afternoon and then on into the Sierra late this afternoon and early evening. Cold air pushing in behind the front this afternoon will bring cooler temperatures today with daytime highs dropping to below normal for this time of year.
Cold air filtering in to the region will bring a continued drop in snow levels tonight with Shasta county possibly seeing snow down to the 2000 foot level by morning. This could impact interstate 5 but precipitation should be on the decline by Saturday morning as the frontal system moves out of the state.
Upper low will bring continued showers over the Sierra on Saturday. Snow levels will be low enough to bring snow down into the Sierra foothills but snow amounts will be declining there as well on Saturday.
By Saturday evening, snow amounts become insignificant. Partial clearing over the valley and cold airmass will make for a chilly night in the valley with lows down around freezing or a little lower.
Sunday looks dry at this time under upper ridging along the west coast but cool airmass in place will keep daytime highs below normal. Models now becoming fairly consistent in keeping an upper low offshore on Monday so early next week remains dry with daytime highs warming slightly.
Extended discussion (Tuesday through Friday)
An upper level trough will move into the Pacific Northwest on Tuesday, only impacting Shasta and Tehama counties at best with some chances of light rain. The models are in agreement of a building ridge over the region with warming trend Wednesday and beyond. Some offshore winds will setup during the extended, but not terribly strong as the jetstream remains to the north and east. Valley fog is a legitimate possibility under the ridge depending on wind speeds, especially within the San Joaquin and eastern Sacramento Valleys.