November 18, 2016 – Dry weather Friday, rain and higher elevation snow this weekend and early next week. A brief break Tuesday then another round of precipitation possible just prior to the Thanksgiving Day holiday.
Discussion
High pressure aloft over the west coast for mainly fair skies this morning. Not expecting any significant valley fog formation this morning as temp/dew point spreads are too large. Temperatures are running a few degrees cooler than 24 hours ago but still expecting overnight minimums in the central valley to remain mainly above the freezing point based on current readings.
Overall, today looks like a mainly fair day with near normal high temperatures with increasing high and mid clouds across the northern CWA.
Attention then turns to a Pacific frontal system now offshore that is expected to move into northwest California early this evening. This slow moving front will begin impacting the Shasta county mountains tonight but likely not make it far enough eastward to have a significant impact on the southern Cascades until the day on Saturday.
This still looks like a fairly moist storm with PW values of greater than 1 inch. Snow levels across the north remain a little above 4000 feet so if current forecasts are accurate, then impacts will be minimal across the Shasta area. Moisture and orographics will however bring moderate rainfall to this area overnight and if snow levels drop lower than forecasted then snowfall may become an issue as well.
This frontal band is forecast to bring significant precipitation to the southern Cascades by Saturday morning and to the northern Sierra by mid day Saturday. Have issued a snow advisory for the Sierra Cascade range to match current snow level forecasts with good orographics and moisture likely bringing sufficient snowfall. Meanwhile, the lower elevations will see moderate rainfall and breezy south winds.
The front continues to edge southeast Saturday afternoon and Saturday night bringing continued moderate snowfall over the Sierra with the lower elevations likely seeing post frontal showers. Showers continue on Sunday as the main upper trough pushes into the Pacific northwest and northern California. A shortwave rotating through this trough will keep moderate precipitation going over the Sierra Cascade range so have kept the snow advisory up for this area through the day on Sunday. Upper trough axis shifts east of the state Sunday night so showers will be on the decrease but Norcal remains under cyclonic flow so shower threat will continue into the day on Monday. Upper ridging pushing in over the west coast should bring an end to any shower threat by Monday night.
Extended discussion (Tuesday through Friday)
Dry but cool Tuesday as short wave ridging passes through the area. A cool trough moves in Tuesday night through Wednesday. The ECMWF and GFS are now in better agreement for bringing the potential for precipitation across the forecast area, though this system looks weaker than the previous weekend system. Models disagree somewhat in timing, with the ECMWF a few hours faster in bringing precipitation through. Overall, it looks like a fairly quick shot of rain and mountain snow, focused mainly during the day Wednesday. Snow amounts currently are estimated at a few inches, with snow levels up north around 4500 feet, 5000 feet over the northern Sierra. The timing could cause travel difficulties, though, as it is expected to peak while holiday traffic level is increasing. If possible, travel over mountain passes on Tuesday is recommended.
Lingering showers continue Wednesday night into early Thursday(Thanksgiving) morning, mainly over the mountains. As this exits, another approaching system could bring some northern mountain showers during the day, with dry weather elsewhere. Thursday night and Friday could see another shot of precipitation, mainly north of I80.