Keys to the storm door have been found, rain returns to NorCal

January 3, 2018 – Precipitation spreads across NorCal this afternoon with the precipitation threat continuing through Saturday. A brief break Sunday then more wet weather Monday through mid week. Moderate to high snow levels.

Discussion

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High cloud cover over most of northern California this morning ahead of a low pressure system now centered off the central California coast. Some light fog has formed in parts of the central valley this morning but cloud cover should keep it from becoming significantly dense. Upper ridge remains over the west coast one more day so airmass remains warm. Daytime highs today will remain well above normal despite cloud cover. Models are in fairly good agreement in bringing a band of light precipitation on to the central California coast late this afternoon then across NorCal over night.

Precipitation amounts are expected to be fairly light but most of the CWA should get at least a little precipitation. The upper shortwave shifts north of the state by Thursday afternoon leaving NorCal under scattered showers. Through this initial precipitation period, snow levels remain quite high giving light precipitation even less impact.

Overall airmass cools a bit on Thursday for slightly cooler temperatures but highs Thursday will still remain well above normal. Another shortwave trough tracks through NorCal on Friday. This will be a slightly stronger system but rainfall amounts are still expected to remain fairly light. Southerly surface gradient indicates breezy south winds Friday afternoon while cooler airmass brings slightly cooler temperatures.

Upper trough axis shifts east of the state on Saturday bringing a transition of a more steady precipitation to more showery. Cooler air filtering into the region will bring more cooling on Saturday but daytime highs will still come in a bit above normal.

Extended discussion (Sunday through Wednesday)

Latest model forecasts are in the best agreement compared to the past several nights regarding timing and progression of atmospheric flow aloft for the extended period. Shortwave ridge of high pressure Sunday will break down as a high amplitude trough develops and swings southward for the beginning of next week. Sunday will be mostly dry with some scattered precipitation possible for the mountains. More widespread chances for precipitation across northern California are expected as the disturbance approaches the forecast area through the middle of next week.