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December 31, 2020 – A few showers will linger across the northern Sierra Nevada this morning, otherwise dry weather is expected across the region today and New Year’s. Another weak system will bring a chance of rain and mountain snow on Friday night into Saturday before the pattern becomes more active Sunday into next week.

Discussion

Radar and satellite show the weak system moving to the southeast with a band of showers remaining across the northern Sierra, mainly to the south of I-80 now. Precipitation amounts overnight have been light, ranging from mostly a few hundredths of an inch across the valley, up to a tenth to a quarter inch across the foothills and mountains. Several inches of snowfall occurred over the higher elevations prompting chain requirements across the mountains. The remaining showers are expected to move southeast out of the forecast area by shortly after sunrise.

Upper level ridging moves in today and will bring a brief period of dry weather through New Year’s. Locally breezy northerly flow will develop through the Central Valley this morning and is expected to clear out any remaining areas of low clouds by early afternoon resulting in another afternoon of sunshine and mild temperatures (highs expected to mainly be in the upper 50s and lower 60s). The northerly flow will weaken tonight, so fog will be possible along the eastern edge of the Sacramento Valley southward into the northern San Joaquin Valley by late tonight into Friday (New Year’s) morning.

Ensemble guidance shows the upper level flow flattening out over NorCal by late Friday. This will allow the wet pattern to return as a series of weather systems are forecast to impact the region beginning late Friday night and continuing into at least the middle of next week.

The first weather system will be another quick-hitting one Friday night into Saturday with only light QPF expected. Moisture flow continues Sunday, and warm-advection should be enough to continue squeezing out light QPF over mainly the mountains to the north of I-80, with perhaps some light orographic enhancement over the mountains to the north of Redding and across the Feather Basin.

Extended discussion (Monday through Thursday)

Zonal flow will give way to a trough sliding into the West Coast on Monday, leading a potentially wet start to the work week. Guidance has this trough arriving early Monday morning which will bring widespread rain and high elevation mountain snow to the region. As this AFD, snow elevations start out around 6000 to 6500 feet and fall to around 4500 to 5500 feet by Tuesday morning.

Precip will likely not be as widespread with the lower snow levels, but some showers will be possible late Monday into early Tuesday morning as the through departs. Behind that trough, models suggest another period of weak ridging for Tuesday with wet weather potentially returning on Wednesday. Snow levels do look a bit lower for this system than the one on Monday.

Being this far out, the main takeaway for the extended period should be that there will be periods of hazardous mountain travel early next week and possibly multiple feet of snow by mid- week.

In the Valley, expect periods of rain showers with some breaks in the precipitation. If traveling in the mountains this weekend or early next week, pay close attention to the forecast and adjust plans appropriately.