December 21, 2019 – Rain and snow return to the forecast this weekend with travel impacts above 4000 feet. Unsettled conditions continue into the Holiday week.
Long wave troughing over the west will bring the return of rain and snow to NorCal. A cold front will slowly push into the western part of the area Saturday afternoon. This will spread showers into the Coastal Range during the afternoon but the better chances for showers will hold off until after 1-2z/5-6 PM. As the trough pushes east we will continue to see the cold front work through NorCal. This will continue to spread rain and snow across the area Saturday night into Sunday morning. Rain and snow will continue over the Sierra into Sunday afternoon as part of the upper level trough pushes through.
Snow levels will start out in the 4500 to 5500 foot level Saturday and will fall to 4000 to 5000 feet Saturday night. Generally snow levels will stay in the 4000 to 5000 foot range Sunday but will fall just below 4000 feet Sunday afternoon. Southerly flow will increase Sunday morning out a head of the cold front and this will bring breezy winds. Winds diminish some in the afternoon but it will still be on the breezy side.
With the front moving through Sunday morning and upper level energy continuing to work through Sunday afternoon we could see the chance for some isolated t-storms in the valley. Overall models are not building in a lot of instability but it is a common pattern for us to see some isolated afternoon convection. Best chances for t-storms will be from Sacramento north to Redding and then east to Chico/Marysville.
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Things dry out Sunday evening into the overnight minus a few isolated mountain showers. Showers will continue to linger over the area south of highway 50 in the higher elevations as the trough axis digs into SoCal on Monday. The trough pushes east on Tuesday and we will see short wave ridging. This will dry things out. This will bring cold overnight lows Monday night. Patchy frost is going to be likely Tuesday morning. Shower chances will increase Tuesday night as another short wave trough approaches NorCal.
Extended Discussion (Wednesday through Saturday)
Ensembles indicate a broad trough continuing across the region on Christmas with quite a bit of uncertainty remaining on finer details. Scattered showers along with cool temperatures expected. Better confidence later in the week as the trough shifts east and ridging over the eastern Pacific shifts over NorCal bringing a return of drier weather with northerly flow heading into next weekend.