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January 18, 2018 – Active weather pattern continues with a colder storm system arriving today thru Friday. A similarly cold storm system is expected early next week. Winter Storm Warning from noon today to 4 PM PST Friday for West Slope Northern Sierra Nevada-Western Plumas County/Lassen Park.

Discussion

Latest radar imagery shows the main frontal band precipitation moving across northwest California. Stratus covers the entire Central Valley up to Redding and this has limited dense fog formation overnight with all surface observations reporting visibility above 1 mile.

Little change to overall forecast timing or precipitation amounts. Precipitation is expected to extend southward to the Sacramento region, and Interstate 80 in the Sierra, by mid to late afternoon. Snow levels will be considerably lower with this system, lowering to around 2500-3500 feet by early Friday.

The main impactful weather from the storm will result in precipitation approaching an inch for the north valley and 12-18 inches of snow for the northern Sierra through Friday. Latest forecast plumes indicate high likelihood of 6-8 inches of snow accumulation at KBLU with the heaviest snowfall occurring this evening into early Friday morning as the front transits the area. Travel for the I-80 and US 50 corridors will become difficult. Gusty winds between 25 and 35 mph could make travel difficult for higher profile vehicles.

The base of the trough swings through Friday afternoon into early Friday evening with precipitation then tapering off. A short-wave ridge of high pressure will develop Saturday with the potential for clearing skies and patchy fog for the valley before the next weather making system arrives Sunday.

Extended discussion (Monday through Thursday)

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Pacific frontal system will continue to push south of the forecast area on Monday. The Valley is expected to see about a tenth to a quarter inch of additional QPF amounts, whereas the mountains could receive up to half an inch. Precipitation is forecast to end across most locations late Monday, although lingering activity could persist over higher terrain. Additional snow amounts could approach 4-6 inches during this timeframe.

Late Monday into early Tuesday should be a transition period before the next system moves in. Forecast models indicate a deep trough moving along the West Coast by midweek. This upper system will bring another round of widespread precipitation, mountain snow, and breezy winds. Guidance solutions differ regarding the overall progression of the system, with the latest ECMWF run being the fastest output. Nevertheless, precipitation could approach across the northern mountains as early as Tuesday afternoon and then spread southward across the entire CWA on Wednesday.