Warm weather continues today. Cooler temperatures return for Saturday with light precipitation likely in the mountains. Dry, breezy winds develop on Sunday. Gusty winds may produce difficult driving conditions for high profile vehicles and cause impacts to unsecured items such as Halloween decorations.
GOES-West nighttime microphysics imagery shows a few passing mid/high clouds over interior northern California this early Friday morning. Expecting rather tranquil weather again this afternoon with widespread 80s forecast for the Sacramento and northern San Joaquin Valleys. For comparison, normal October 21 highs for Sacramento Downtown and Redding both stand at 77 deg F.
Upper level trough will quickly dig from the Pacific Northwest towards northern California tonight and into Saturday. This will bring 1) shot at some light precipitation, mainly in the mountains, 2) much cooler temperatures, and 3) wind.
Light precipitation will develop from north to south overnight Friday and into Saturday. Best chances will generally be confined to the Shasta County Mountains, southern Cascades, and the Sierra. Most mountain locations can expect upwards of a tenth or two of precipitation, locally near a third of an inch near the crest along/south of I-80.
Unfortunately, most of the Central Valley will likely miss out on measurable precipitation chances with this system. Thinking most of the precipitation will remain in the form of rain at pass level as the cold air will filter southward after the the best precipitation chances. That said, can’t rule out a some snow flakes at pass level Saturday by late afternoon/ evening. Any accumulating snow is expected to remain above 8000 ft where 1 to 3″ of wet snow is possible up the the crest.
For temperatures, expect a very noticeable tumble from Friday to Saturday with widespread 70s for the Valley. Some locations in the northern Sacramento Valley may even struggle to get out of the upper 60s.
If you have weekend outdoor plans to the mountains, be sure to dress accordingly as low temperatures will drop to the 30s (and lower) at/above around 3000 ft.
For wind, expect breezy west winds to develop through the day on Saturday. Strongest gusts will impact the northern Sierra ridgetops and peaks with gusts in the 35 to 50 mph range. Valley wind gusts will be much lower in the 15 to 25 mph range.
By Sunday and Sunday night, winds will shift out of the north and east (offshore). Areas most prone to these dry, north/east winds include the western side of the Sacramento Valley, southwest/northeast-oriented canyons/gaps (e.g., Jarbo Gap), as well as ridgetops/peaks.
Peak Valley gusts on Sunday will range from about 25 to 35 mph. This combined with minimum relativity humidity values in the 15-25% range will result in elevated fire weather conditions. Will continue to keep a close eye on wind/RH trends over the coming days.
By Monday, upper-level ridge builds back over northern California with less wind and warmer temperatures forecast.
Extended Discussion (Tuesday through Friday)
Models point to another short wave trough digging through interior NorCal midweek bringing some precipitation to the CWA. Bulk of precip Wednesday in the Central Valley looks light and mainly north of I-80. A few inches of snow possible over the higher elevations of Lassen Park and Sierra Nevada.
Drier weather expected Thursday as high pressure builds inland and some gusty northerly wind possible. NBM brings in another weather system Friday, which may be too soon given current model solutions.