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March 13, 2020 – Dry and mild weather today. Cold weather system will bring widespread rain and mountain snow this weekend into next week with mountain travel impacts and low snow levels.

Discussion

Short-wave ridging, between the low over the Desert Southwest and the deep trough upstream off the coast of BC, continues to provide clear skies across the forecast area early this morning. North to east surface pressure gradients continue to result in locally breezy north to northeast wind gusts (20-30 mph) across the northern Sacramento Valley and surrounding terrain. Current temperatures range from the mid 20s to lower 30s in the colder mountain valleys, to the mid 40s to mid 50s (except lower 60s over the breezier north end of the Sacramento Valley) across the Central Valley.

Ridging breaks down today ahead of the upstream deep trough. Temperatures will cool around 5-10 degrees compared to today as flow reverses to onshore and synoptic cooling begins.

The trough closes off into a deep closed low by tonight and will drift slowly southward off the coast through the weekend. This anomalous cold upper low will bring the potential for widespread precipitation, heavy mountain snow, gusty winds, and significantly cooler temperatures this weekend into early next week. Showers will develop over the higher elevations this evening, then increase and spread across the region into Saturday morning as a cold front pushes south through the area. Steady light to moderate precipitation will continue Saturday into Sunday before becoming more showery on Sunday.

Pattern is favorable for significant snowfall accumulation across the west slopes of the northern Sierra where ensembles continue to favor several feet of snowfall accumulation at KBLU. Snowfall accumulation of this magnitude will lead to potentially major travel impacts across the Sierra over the weekend.

In addition to widespread precipitation, thunderstorm development will be possible Sunday afternoon as instability develops. Small accumulating hail is possible with any thunderstorms.

With the upper system over the West, temperatures will remain 5 to 15 degrees cooler than normal over the weekend.

Extended discussion (Tuesday through Friday)

A broad long wave trough will dominate our weather through midweek. This means unsettled and cooler weather will be the main features, with precip chances lingering throughout at least Wednesday. The mountains still have the best chances of seeing additional precipitation; however, some light accumulating rain isn`t out of the question for the Valley and foothills. By Thursday, deterministic model data split suggesting that we could see a brief period of ridging. NBM guidance leans to a wetter solution keeping some precip chances over the mountains. Overall, expect to see partly to mostly cloudy skies and cooler than average temperatures are expected next week, with most highs running about 5-15 degrees below average.