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November 8, 2020 – Scattered showers and moderate to locally heavy mountain snow expected today. Breezy north to east winds today into Monday with lowering humidity. Below average temperatures expected to continue through the week, with near freezing Valley temperatures expected Monday morning. The chance for rain and snow returns late in the week and into the weekend.

Discussion

Minimal changes to the forecast this morning as the broad upper level low spins over the West. Satellite imagery shows some convective clouds along the west slope of the Sierra, with isolated thunderstorms occurring in the lower elevations of the foothills and heavy snow happening in the higher terrain. Expect these conditions to continue through the mid morning hours, with snow and rain shower chances tapering this afternoon and evening as the trough exits the region. Until then expect mountain travel delays, chain controls, and slippery road conditions for areas around I-80 and southward as an additional 2 to 7 inches of snow will fall. Snow levels are currently around 3500 to 4000 feet but will continue to fall to roughly 2500 to 3500 feet today.

As the system exits the region later today, colder and drier air will filter in and breezy northerly and easterly winds will develop. Gusts up to 30 mph are expected, with relative humidity values about 20 percent or higher this afternoon. Winds are forecast to weaken during the overnight hours, but may stay elevated on the west side of the Valley.

Temperatures should remain a touch warmer there, while areas not affected by the wind fall. Lows in the Valley and low elevation foothills are forecast to span the upper 20s to the upper 30s. If the wind remains light enough in these areas, patchy frost could develop.

As a result, we’ve issued a Frost Advisory from 3AM to 9AM Monday morning. If you’re the in the upper elevations of the foothills and mountains, it’ll be even colder. Monday morning lows are forecast to be in the 20s and 30s for the foothills while the high elevations vary from the teens to the 20s.

Looking at the middle of the week, cool and dry weather remains in place. The next chance for rain and mountain snow looks to be later in the week and into the weekend.

Extended discussion (Thursday through Sunday)

Cluster ensemble analysis is in good agreement long wave troughing will continue over the Western US into mid-week before a more zonal pattern starts to emerge. This will likely bring rounds of cloudy and possibly unsettled weather to Northern California late in the week.

The next best chance for unsettled weather is a low pressure system projected to dig south out of the Gulf of Alaska and impact NorCal by next weekend.

Integrated water vapor transport analysis indicates moisture levels hitting the criteria for at least a weak atmospheric river associated as it taps deeper moisture.