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March 14, 2018 – Series of Pacific storms will move through this week bringing periods of rain and mountain snow, heavy at times, with gusty wind and thunderstorms. Winter Storm Watch from Thursday afternoon through late Friday night for West Slope Northern Sierra Nevada.

Discussion

The first in a series of Pacific storm systems is currently passing through the region. A shortwave trough is bringing a quick round of precipitation across Northern CA early this morning. Snow-level radars and webcams suggest that snow levels are currently between 3000-4000 ft along the northern mountains, and around 4500 ft along the west slopes of the Northern Sierra. We’ve seen precipitation totals of up to 0.75″ across the Valley thus far, and 1-3 inches of liquid equivalent precipitation over the mountains.

Isolated showers will continue across the Central Valley this morning, as most of the showery activity moves eastward over the mountains. The next vort max then moves across the region this afternoon, kicking off another round of showery activity. Short- range models suggest that there should be enough instability to generate isolated thunderstorms across the Sacramento Valley. The primary concern with these thunderstorms will likely be small hail and locally heavy downpours. It is likely that this showery activity will make its way up the Sierra and generate another round of heavy snow over the Sierra this afternoon.

Isolated to scattered showers will continue over the mountains tonight into Thursday. The next storm system then begins to impact the region: bringing precipitation across the north Thursday morning, and spreading southward toward I-80 by the afternoon. Similar to the last storm, light to locally moderate precipitation is expected across the Valley along with gusty south winds. With cold air already in place, snow levels will start lower and remain low for the duration of this storm. With high snowfall rates, likely high snow-liquid ratios (which creates light, fluffy snow) and strong and gusty winds, a period of very hazardous travel with near-whiteout conditions is likely over the Sierra Thursday into Thursday night.

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Showery weather will continue into Friday as the upper low passes over NorCal. Moderate to heavy snow will continue across NorCal mountains on Friday, and an isolated afternoon Valley thunderstorm or two can’t be ruled out. Coverage of showers will continue to decrease on Saturday.

Extended discussion (Sunday through Wednesday)

Isolated shower threat will continue over the northeast half of the forecast area Sunday as the upper low bringing showers at the end of this week departs eastward out of the region. Cool airmass remains over the area so snow levels will remain at only a few thousand feet and daytime highs will remain several degrees below normal. GFS and ECMWF models in agreement in moving short term high pressure ridging over the state on Monday so it should be a generally dry day with a little warming.

Models showing a return to wetter pattern Tuesday as a Pacific low approaches the coast. Initial frontal band forecast to moves on to the central coast Tuesday spreading rain across the forecast area. This Pacific storm will come with a little higher snow levels but still moderately low for this time of year. Wet pattern continues into at least mid week as the Pacific trough pushes inland. Temperatures warm a bit Wednesday but are still forecast to come in a little below normal for this time of year.