January 21, 2018 – Pacific storm moves through today into Monday bringing light to moderate precipitation. A stronger system impacts the area midweek into Friday bringing moderate to heavy rain and mountain snow, and gusty wind.
Pacific frontal system approaching the northern CA coast this morning with lowering cloud layers out ahead of the baroclinic zone. 88-D showing main frontal precip is along the coast right now. Front will be slow to move inland as 5H flow nearly parallels surface boundary. Latest HRRR pushes frontal precip into our Coastal Range and Shasta mountains late morning to midday and then across the Sacramento Valley towards Western Plumas County this afternoon. Snow levels today expected around 2500 to 3500 feet with light to moderate accumulations.
Bulk of the precipitation forecast to occur this evening and overnight with lingering showers Monday morning. Storm total QPF with this system looks fairly light ranging from less than a tenth in southern portions of the CWA to around an inch in the Shasta and Coastal mountains.
Precip diminishes Monday afternoon as subsidence increases over Interior NorCal. Upper ridging brings drier weather Monday night into Tuesday. Some patchy morning fog may develop in the southern portions of the Sacramento Valley and Northern San Joaquin Valley. Temperatures trend up early next week to near or slightly above normal by Tuesday.
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Stronger Pacific frontal system spreads inland over far NW CA Tuesday night then across Interior NorCal Wednesday. Breezy to windy conditions develop ahead of the front Wednesday. Models currently showing more significant QPF with this system. Snow levels attm initially look to be around 5000 to 6000 feet Wednesday morning, but lower to 2500 to 3500 Wednesday night into Thursday in the post-frontal colder AMS. Showery weather follows Thursday.
Extended discussion (Thursday through Sunday)
Models shift Pacific frontal band south and east of the CWA after 12z Thursday with upper trough shifting through the Pacific northwest and northern California during the day on Thursday. Shortwaves pivoting through the trough will bring periods of showers through the day Thursday. Cold air filtering in behind the front will bring down snow levels down to some of the lowest elevations of the season. Snow levels over Shasta county will drop to around 2000 feet with snow levels around 3000 feet over the northern Sierra Nevada. Showers will continue over the CWA Thursday night before lifting northward on Friday as high pressure amplifies over the west coast. Daytime temperatures will come in below normal on Thursday warming to near normal on Friday as areas south of the northern mountains begin to clear. The shower threat continues to shift northward on Saturday with daytime temperatures warming to a little above normal. Upper ridge amplifies still further by next Sunday bringing dry conditions, clearing skies and above normal temperatures.