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February 10, 2021 – Quiet and mild today with a pattern change to unsettled and cooler late week. Widespread precipitation returns Thursday, with precipitation chances into the weekend. Mountain travel impacts are expected.

Discussion

Radar imagery early this morning shows a few lingering isolated showers over the mountains, generally south of I-80, as the upper level shortwave trough continues moving east. Satellite imagery shows high clouds are beginning to clear over interior Northern California, as well. If skies continue to clear, there is the chance of patchy fog formation in the Central Valley this morning. HREF continues to indicate a chance of visibilities less than 1 to 3 miles expanding from the Sacramento area into the northern San Joaquin Valley from around 5 am to 10 am. Dense fog is not expected to be widespread, but more patchy in nature.

Shortwave upper level ridging is expected today over California which will bring warm and dry conditions to the area. Models have not quite been able to capture high temperature forecast the past couple days, which seems to be the same with today. Have bumped up high temperatures across the CWA a few degrees from guidance to around the 75th percentile NBM temperature forecast. Generally expecting Valley highs in the upper 60s and mountains in the 40s to mid 50s.

Widespread wet weather is forecast to return Thursday into the weekend. Ensemble guidance shows a quick hitting upper level trough moving through northern California Thursday into Friday. Latest GEFS and Euro ENS are more consistent with timing. Precipitation is forecast to slowly spread north to south Thursday, becoming widespread by Thursday night. Heaviest precipitation is expected Thursday evening, becoming showers by the early morning hours on Friday.

At this point precipitation is expected to completely move out of the area by Friday morning, though exact timing is still uncertain. Although it will move through the area quickly, the low is expected to tap into some tropical moisture. The Valley is forecast to receive around 0.3 to 0.75 inch of rain with around 0.75 to 1.5 inch in the foothills.

This will be a relatively warm storm with snow levels above 5500 to 6000 feet through Thursday night. Snow levels are forecast to fall to 4000 to 4500 feet early Friday morning, though by this time most of the precipitation will be moving out of the forecast area and only light accumulation is expected. Around 7 to 12 inches of snow is forecast for Sierra locations above 5500 to 6000 feet with less than an inch possible down to lower elevations.

A mostly dry but dreary day is forecast Friday before the next weather system is expected to move in early Saturday. Differences still remain in latest GEFS and Euro ENS runs for timing and strength of this system. Overall, an upper level trough is expected to move south through NorCal and into the desert SW area Saturday. Precipitation is forecast to move into the area early Saturday morning, bringing a nice shot of precipitation through the day before dissipating overnight. At this point forecast amounts are slightly lower than the Thursday night system, though the spread between models is decent. Current forecast is around 0.1 to 0.4 inch of rain in the Valley and up to 0.75 inch in the foothills. Snow levels will be a little lower with this storm with around 4 to 10 inches of snow forecast above 4500 to 5000 feet.

Mountain travel impacts are likely with both storms this week, especially on Saturday as the majority of snowfall is expected during the daytime. Travelers are advised to prepare for extended travel delays up to hours as well as chain controls and slick roads. Travel is not encouraged, and if you must travel, bring plenty of warm clothes and extra food and water.

Extended discussion (Sunday through Wednesday)

Ensembles continue to indicate little change in the pattern early next week with another weak to moderate system forecast to break through the eastern Pacific ridge as it approaches on Sunday, then moves into NorCal on Monday (Washington’s Birthday Holiday). IVT charts indicate a wide swath of deeper moisture across the Pacific between 35N-45N that pushes into NorCal, however forcing remains weak which will limit overall QPF (but enough to potentially result in travel impacts across the higher mountain passes).

Following Monday’s system, “dirty” ridging is forecast through mid-week (similar to the current pattern) with generally drier conditions (perhaps some sprinkles or light showers at times, especially over the mountains) and slightly milder temperatures.