December 6, 2021 – Dry weather with areas of dense Valley fog this morning. A few showers will be possible this afternoon and overnight as a weak system moves through. A colder weather system could bring mountain snow, light Valley showers, and colder temperatures late week.
Discussion
IR satellite imagery shows high clouds streaming into Northern California early this morning ahead of the weather system that is slated to move through the area later today. Fog developed late last evening for portions of the Valley and Motherlode and will continue through the early morning hours. A weather system will drop north from the Gulf of Alaska into Northern California today. This will bring a quick round of light precipitation hitting Shasta County by late morning and moving south through interior Northern California through the afternoon and evening hours. QPF will be limited with this system with only a few hundredths for most locations and up to 0.25 inch for the Sierra south of Highway 50. Light snow showers are possible at the highest elevations with a couple of inches forecast south of Highway 50. Temperatures will also be much cooler yesterday for most locations (those that were not impacted by stratus). 50s are forecast across the Valley and foothills, a 10-20 degree drop from yesterday for many locations, with 40s and 50s in the mountains.
Dry weather returns Tuesday with clearing skies and slightly warmer temperatures. Then ensemble guidance continues to indicate another weather system pushing south from the Gulf of Alaska Wednesday into Thursday, bringing another round of precipitation.
At this point there looks to be more moisture associated with this system than the Monday system, though the track of the weather system means the majority of precipitation will fall over the Sierra. QPF did increase with the latest model guidance, mainly for the Sierra and foothills, with around 0.5-1.5 inch of liquid precipitation over the Sierra and only 0.3-0.25 inch in the Valley.
This is a colder system so snow levels will quickly fall from around 6500 feet Wednesday evening to 4000-4500 feet by Thursday morning, which corresponds with the period of heaviest precipitation. With the higher QPF, snow amounts have increased slightly as well. Anywhere from 6-14 inches is possible above 5000 feet, with lighter amounts down to 4000 feet. Uncertainty still remains in forecast timing and amounts with this system, so keep an eye out for forecast updates.
Extended Discussion (Friday through Monday)
Long wave trough that should bring NorCal’s first significant precip in about a month’s time (i.e. Nov 8-9) during the middle of this week is forecast to progress eastward across NOAM Fri and through the weekend.
A drier but cool NW flow pattern is likely to develop over NorCal on Fri before transitioning to cyclonic westerly flow Sat/Sun with the positioning of the GOA/Yukon upper low. The associated frontal precip band should move onshore into the Pac NW early in the weekend trailing to the SW and across the NW portion of CA and potentially into our NW mountain zones.
Many of ensemble members show an aggressive digging trough over the eastern Pacific into early next week which may temporarily lift the frontal band northward and slow its eastward progression on Sunday before it moves onshore early next week.
This looks as if it will be a multi-day precip event for NorCal with at least a weak AR, if not moderate AR intensity especially for coastal locations next week. With the origin of this low from the Bering Sea and Arctic influences as well, this will be a cold system with the potential for heavy Sierra snowfall as well.
Timing may vary given the amplitude of this digging system, but as of now the 13th-15th looks to be main impact period. Until this system impacts NorCal, the clearer skies Fri/Sat mornings could produce frosty/freezing temps which may warrant the initial frost/freeze headlines. Max temps will be slightly below normal as well.