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December 10, 2019 – Quiet weather today with areas of morning valley fog and near average temperatures. Rain chances return to the region tonight with more widespread precipitation expected late in the week.

Discussion

The upper ridge that brought the dry weather to the region on Monday is moving east as the next upstream short-wave trough approaches. Satellite imagery shows high clouds out ahead of the system already moving over NorCal.

Fog has been developing in the southern half of the Sacramento Valley southward into the northern San Joaquin Valley since late Monday evening. So far, the most extensive dense fog has been in the Sacramento area with patches of dense fog in around Modesto and across portions of the Motherlode. The extensive high clouds may limit further spread of dense fog, and forecast soundings indicate it may lift to a low stratus deck this morning.

Dry weather will persist today, but a brief period of light precipitation is expected across NorCal tonight as the short-wave and a shot of warm-advection move through interacting with TPW of around 3/4 to 1 inch over the area. QPF of less than a tenth of an inch is likely in the valley, with less than a third of an inch over the mountains. An inch or two of snowfall accumulation will be possible above 5500-6000 feet.

Ridging in the wake of tonight’s wave gradually flattens the remainder of the week as strong mid/upper level jet across the eastern Pacific reaches the West Coast. Most of the forecast area is expected to see dry weather Wednesday, except across the mountains north of Redding where warm-advection may allow for some lingering light precipitation.

Light precipitation is then forecast to increase again along a stalled out front by Thursday as warm-advection ramps up and deeper moisture spreads into the region. The front may sag far enough south later Thursday into Friday to allow light precipitation to spread across the Sacramento region and into the I-80 corridor over the northern Sierra. Snow levels will be above 8k ft limiting winter travel impacts.

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Extended Discussion (Saturday through Tuesday)

Fairly zonal upper level flow is expected to remain over NorCal Saturday. Weak shortwave along the flow will bring a chance of light showers to the area Friday night through Saturday. Another 3 to 6 inches of snow accumulation is possible Saturday mainly for elevations above 5000 feet, though dropping snow levels early Saturday could bring light accumulation down to 4000 to 4500 feet. This could cause some travel issues over mountain passes Saturday.

Zonal flow breaks down behind this weather system as upper level ridging builds over the area Sunday into Monday, bring dry weather back to interior NorCal. Ensembles hinting at wetter weather system impacting the West Coast beginning Tuesday. However confidence is low at this time, so keep an eye out for forecast updates.