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December 22, 2016 – Cool and dry weather today. A winter storm with rain, wind and heavy mountain snow is expected to start to move into the area by Friday disrupting holiday travel through Saturday. Clearing Sunday. A chance of showers northern mountains early next week, otherwise dry.

Discussion

Fair skies this morning between weather systems. Some patchy light fog has formed in the central valley this morning. Model guidance not forecasting dense fog this morning but will need to keep monitoring. Otherwise, only expecting minor changes today with temperatures continuing to run a little above normal and some high cloud cover spreading in over the north state.

This cloud cover will be the first indications of a significant Pacific frontal system expected to move into the region beginning tonight.

Models consistent in pushing the main frontal system into the northwest corner of the state tonight then sweeping it through NorCal and into the Sierra on Friday.

Models now pretty consistent on timing and even fairly consistent on QPF although GFS a bit drier over the Sierra than the NAM.

Colder air is ushered in behind the front Friday afternoon so max temperatures take a big drop to below normal.

Surface pressure gradients ahead of the front forecast at about 8 mb from MFR to SAC so winds will be breezy but not overly so.

Snow levels drop quickly during the day losing about 1000 feet from morning to afternoon. Snow amounts over the higher elevations of the northern mountains could top a foot but much less is expected at pass levels.

However, by Friday evening, snow levels will drop to pass levels with some snow possible down to interstate 5 which could have significant impact on holiday travel.

Precipitation mainly ends over the northern mountains on Saturday after the frontal passage but showers continue over the Sierra Saturday as the associated upper trough moves across the north state.

Total snowfall amounts over the Sierra between Friday and Saturday add up to significant numbers with snowfall totals well over two feet at most higher elevations.

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Except for Sierra, precipitation ends Saturday night. Colder airmass and partial clearing of skies will make for another cold night in the valley with near or below freezing temperatures again. Upper ridging over the coast on Sunday results in cold dry conditions all areas.

Extended discussion (Monday through Thursday)

The offshore trajectory of the filling upper level low moving southeast offshore gives us decent confidence in the dry Monday forecast. The GFS solution would maybe bring some weak threat of precipitation to the Coastal Range, while the ECMWF is 100 to 200 miles farther offshore.

By Tuesday, an upper level trough will move over the Pacific Northwest with the majority of the energy and moisture staying well to our north. Although, the northern and western mountains of Shasta and Tehama counties may get some showers from the tail end of this system. Beyond this, a ridge builds over the area with a decent threat of morning Valley fog.

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