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December 13, 2019 – Light to moderate valley rain and moderate to heavy mountain snow Friday through Saturday night. Mountain travel delays are likely. Drier weather Sunday and early next week.

Discussion

Upper level flow remains mainly zonal flow over NorCal today. Multiple impulses along the flow will bring another weak Pacific moisture plume into NorCal today. Periods of light to moderate precipitation is expected to return to NorCal today through Saturday creating travel concerns over the Sierra. The first wave of precipitation today is expected to begin in the northern mountains & Valley early this morning slowly spreading south through the day. Hires models show heavier band of precipitation reaching the Central Valley by late afternoon, impacting the evening commute. Snow amounts of around 10 to 15 inches are forecast at pass level with the heaviest falling after 4pm this afternoon through tonight. Snow levels will be high today generally above 6500-7000 feet, beginning to fall overnight to around 4500 to 5500 feet by early Saturday. Liquid precipitation totals of a few hundredths up to half an inch are expected in the Valley with a half an inch to 2 inches forecast in the mountains. Winds will become breezy Friday over the mountains with gusts up to 30 to 40 mph possible at higher elevations. This combined with falling snow could create low visibilities over mountain passes.

Models continue to indicate a brief break in precipitation Saturday late morning and afternoon before a trough originating from the Gulf of Alaska brings another round of precipitation Saturday evening through the night, mainly limited to the mountains and foothills. As this is a colder system, snow levels will be around 4000 to 4500 feet. However, this will also be not as wet with additional liquid precipitation amounts of a few hundredths in the Valley to up to half an inch in the mountains. Additional snow amounts of 4 to 8 inches are possible above 5000 feet with a few inches possible down to 4000 feet. Confidence is still fairly low as higher resolution models are not bringing in as much precipitation with this wave as GFS & Euro.

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Upper level ridging returns Sunday, bringing dry conditions by Sunday morning through Monday. Overnight temperatures will drop Saturday night around 5 to 10 degrees after passage of surface front, then will drop another few degrees Sunday night as lack of cloud cover will allow radiational cooling effects. Valley temperatures expected to fall into the 30s with widespread teens and 20s in the mountains through Monday night. Daytime temperatures will remain fairly steady with a slight cooling Sunday.

Extended Discussion (Tuesday through Friday)

The brief dry trend from the weekend will continue into early next week. On Tuesday, the transitory ridge to likely shift over toward the Central Great Basin ahead of a compact upper low nearing the California coast. Given the complex of nature of this system separating from the westerlies and eventually closing off, uncertainty still plagues the forecast. While the GFS/ECMWF ensembles agree on the existence of this system, they continue to exhibit spatio-temporal differences. The former model suite remains the wetter solution with widespread precipitation entering the picture by mid-week. On the contrary, The ECMWF ensemble system takes a more southern track favoring impacts down in southern California. There is enough ensemble spread to keep either scenario in play so will continue maintaining precipitation in the forecast. Expect increasing cloud cover given the influx of Pacific moisture with near average temperatures for mid- December.

Looking toward the following weekend, there are still fairly strong signals of wet weather in response to a full-latitude trough approaching the Pacific coast. While some of the 24-hour probabilities for 0.50 and 1.00 inches of precipitation have come down a bit, this will continue to be watched. The Climate Prediction Center experimental Day 8-14 hazards supports this notion with Dec 20-22 being highlighted for heavy precipitation which includes snow in the mountains.