January 3, 2021 – Light showers over far northern Sacramento Valley and the northern mountains with widespread rain and mountain snow Monday and again Wednesday. Additional rain and snow late week.
We are seeing some weak warm air advection over northern portions of the area this afternoon and that is leading to some light showers mainly from Chico north. These shower chances will be lifting to the north this evening with mainly dry weather into the start of the overnight.
Warm air advection and increased low level flow will begin overnight into tomorrow morning as a trough digs out of the Gulf of Alaska and approaches NorCal. This will bring increasing showers across the area after 1 am. Shower coverage will continue to increase after daybreak and will become widespread late morning into the evening as a cold front works south through the area.
A period of moderate rain will be likely along the cold front along with heavy snow. Snow levels will start out quite high tomorrow morning above 6500 feet but will be falling quickly behind the cold front and we should see them fall to 4500-5000 feet by 5 pm with some light accumulation down to at least 5000 feet during the evening Monday. This system will have quite a bit of dynamics with it along with a strong low level jet and we should see the gradient increase ahead of the cold front leading to breezy southerly winds.
Wind advisory speeds expected over portions of the Sacramento Valley, NE Foothills, and Coastal Range and a Wind Advisory has been issued. The breezy winds along with heavy snow will make mountain travel extremely difficult during the late morning into the evening and a Winter Storm Warning is in effect starting early tomorrow morning continuing into Monday night.
The HREF along with some other model guidance does build in some weak CAPE behind the cold front during the afternoon from Sacramento north into Shasta County. We do also see the vort max with the trough axis push through during this time and that may be enough to kick off a few isolated thunderstorms, although overall thunderstorm chances look low.
Heavy rain will be possible with any thunderstorms but they will be quick moving and should not be a flooding or burn scar issue. Some ash flow cant be ruled out over new burs scars though as we see the moderate rain with the front and will also be possible if any thunderstorms push over the burn scars. Showers diminish quickly during the evening into the early overnight as short wave riding builds in.
Storm total precipitation amounts look to range from up to an inch in the Northern Sacramento Valley to 0.20-0-75″ south into the San Joaquin Valley and 1 to 2.5 inches in the foothills and mountains. Snow totals are expected to be 5-16 inches above 5500 feet with higher peaks seeing up to 2 feet. A few inches of light snow will be possible down to 5000 feet Monday evening.
Short wave riding will be short lived as we see another trough and cold front push into the region on Wednesday. Models have slowed this system down some bringing the shower chances in more during the late morning and for the afternoon.
The trend has also been keeping it a bit further to the north. This will favor the shower chance and higher QPF mainly from I-80 north. QPF is looking to be less than 0.10″ in the San Joaquin Valley with 0.15-0.45″ in the Sacramento Valley and 0.25-1.00″ over the foothills and mountains from I-80 north. Upper level ridging builds back in Wednesday night.
Extended discussion (Thursday through Sunday)
A weakening cold front moves in on Friday. Models trending towards this system holding together a little better than previously. While not a very wet system, this could bring a decent shot of precipitation to the area, up to 3/4″ of liquid in the mountains, which could bring up to 4 to 7 inches of snow. Saturday looks dry as ridging rebuilds. A weak system riding over the ridge could bring some precipitation to the northern mountains on Sunday.