A fast-moving system will bring scattered rain and snow showers today with hazardous mountain travel, especially this morning. Another system moving in over the weekend will bring another round of widespread precipitation, mountain & potential foothill snow and travel impacts.
Discussion
The coldest upper level trough of the season so far is passing through NorCal this morning and will deepen into an upper level low over southern Nevada and then Arizona, as it drops southeast through Thursday. This has brought a period of heavy snow over the Sierra with initial snow levels around 5,000 feet as the snow began during the early evening, but falling down to 3,000 feet overnight with snow at Camino and Dutch Flat.
Liquid precipitation amounts at Blue Canyon of 1.21 inches so far would suggest 10-14 inches of snow estimated at 5,000 feet and above thus far. Mountain travel has been hazardous and difficult over Interstate 80 and Highway 50 with I-80 closed at times between Alta and Truckee.
The widespread snow has tapered off some, but scattered snow showers, moderate to heavy at times will continue into the afternoon and early evening with several more inches of snow possible.
Rainfall amounts over the Valley have ranged from 0.25 to 1.00, heaviest over Shasta county and over the foothills. Precipitation will end tonight with patchy fog possible, as skies clear, especially over the Southern Sacramento Valley and Northern San Joaquin Thursday morning.
Increasing clouds and some north winds should decrease fog threat Friday morning. Very weak system dropping south in the Northwest flow behind the departing current system may bring a few light, mainly mountain showers Friday morning. Mountain snow amounts are expected to be an inch or less.
Ensembles and clusters indicate another relatively cold trough will dig south from the Gulf of Alaska into NorCal Saturday and Sunday with more widespread Valley rain and mountain snow and even lower snow levels than the current system down to 2,500-3,500 feet.
Hazardous mountain travel is again likely with this system with preliminary snow amounts from 10-16 inches above 5,000 feet. With this system from I-80 through south of Hwy. 50, probabilities for 12 inches or more are 60-80% and 30-60% chance to see 18 inches or more.
The current path of the system looks to keep impacts for the Valley at a minimum with only 0.10-0.25″ expected through the Valley Saturday and Sunday. Probabilities show a 30-50% chance of seeing 0.25″ or greater in the Valley south of Red Bluff with a 50-80% chance across Redding and the Northern Sacramento Valley.
Extended Discussion (Sunday through Wednesday)
Lingering showers possible Sunday, mainly over the mountains, as channeled vort max moves through in northerly flow aloft. Locally gusty northerly wind possible Sunday in the Central Valley.
Drier weather Sunday night into Monday as short wave upper ridging moves through. Monday morning low temperatures look to be near to below freezing in the Central Valley as drier colder airmass settles over the area.
Models then point to additional channeled vorts moving through the CWA Tuesday/Wednesday in NW flow aloft. Bulk of associated precip looks to be over northern and eastern portions of interior NorCal.