Showers possible through Thursday before widespread rain on Friday

December 11, 2019 – Light showers possible on Wednesday and Thursday. Widespread precipitation expected later in the week and into the weekend. Drier weather Sunday and early next week.

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A weak weather system moved through NorCal last night spreading precipitation across the area. The upper level trough has moved east into Nevada with only lingering showers remaining early this morning. Areas of light fog have developed behind the frontal passage. Fog could become denser in areas this morning, though it will be dependent on high cloud cover.

Upper level flow expected to become more zonal over NorCal today. A shortwave along a deep trough centered around the Gulf of Alaska pushes into the West Coast today, bringing an increased chance of light precipitation to areas generally north of I-80 this afternoon. Increased precipitation is expected across most of the area overnight Wednesday into Thursday, tapering off significantly south of the Sacramento area. Heaviest precip amounts forecast in northern Shasta County and mountains north of I-80 with amounts up to 0.75 to 1 inch possible. Lighter amounts expected in the rest of the Valley with only a few hundredths south of Sacramento up to around a quarter of an inch in the northern Sacramento Valley. Snow levels will mainly be above 7000 to 8000 feet with minimal accumulation expected.

Ensembles continue to suggest precipitation chances continue Friday through Saturday as impulses move along the zonal flow. The majority of precipitation will remain in the foothills and mountains with precipitation amounts of half an inch to an inch and a quarter forecast. The Valley could see anywhere from a few hundredths of an inch to a quarter of an inch. Snow levels will begin to drop Friday to around 6000 to 7000 feet before dropping below pass level overnight Friday with light accumulation down to 4000 to 4500 feet possible. Snow accumulation around 5 to 10 inches is possible over the Sierra which could cause hazardous travel conditions over the Sierra passes.

Extended Discussion (Sunday through Wednesday)

To conclude the weekend, broad troughing will have shifted into the Four Corners which allows for heights to build over the region from Sunday into early next week. This supports a period of dry weather accompanied by mostly sunny skies and near average temperatures. From Tuesday onward, there is a greater amount of uncertainty in the offshore (upstream) pattern. Progressive flow will carry a longwave trough across the central Pacific this weekend with its fate dictating impacts over northern California. While 06Z GEFS/00Z ECMWF ensemble means show a trough entering the picture next Tuesday/Wednesday (December 17/18), deterministic guidance show the potential for a closed low to separate itself from the prevailing westerlies. Such solutions are wildly variable so confidence is low in the eventual outcome. However, did raise the chances for precipitation in the forecast due to the presence of the trough.