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Morning fog and low clouds in the Central Valley and foothills, otherwise another weather system approaches today bringing showers tonight into Thursday. Colder weather expected late this week with valley frost possible Friday and Saturday mornings. More significant valley rain and mountain snow expected next week.

Discussion

Satellite imagery shows the initial band of high clouds moving across NorCal ahead of the approaching trough. The layer of fog and low clouds that has been entrenched over the Central Valley has begun to deepen and spread further north as the surface pressure gradient flattens or becomes light southerly. IR difference imagery showed the stratus surging northward Tuesday evening as it moved over Redding, and it’s now up to the northern border of Shasta County. Visibilities have mostly been in the 1-3 mile range in the valley, but some areas have been reporting visibility down to around 1/4 to 1/2 mile at times.

Precipitation chances return this afternoon as a shortwave rapidly approaches the region bringing a few showers. Precipitation forecast to increase tonight as forcing increases.

Main impacts with this system will be realized in the Sierra Nevada where accumulating snow is expected to develop in the late afternoon. Snow levels will initially be around 7000-8000 ft, but will quickly lower to 3000-5000 ft overnight as cold air filters into the region.

The heaviest snow is expected from about midnight until noon on Thursday with some flurries lingering south of I-80 through the afternoon. The higher passes may see 8-12 inches of snow and a Winter Weather Advisory remains in effect.

Behind the system, drier and colder northerly flow will invade the region late in the week. Clearing skies Thursday night and again on Friday night will allow for areas of frost and patchy fog in the valley with coldest lows dipping into the upper 20s and lower 30s.

Next series of stronger systems approach this weekend with precipitation chances returning by Saturday night.

Extended Discussion (Sunday through Wednesday)

Ensemble and cluster guidance continue to project the arrival of a wet pattern late Saturday through early next week, so confidence in a potential winter storm event is improving.

Widespread rain, mountain snow, and southerly winds are forecast Sunday into Wednesday as a deep trough from the Gulf of Alaska approaches and then moves inland. NBM is showing the potential for significant moisture, especially over the northern Sierra.

This system is fairly cold, so snow levels should be on the low side. This could bring impacts to travel across a large stretch of the mountains. Exact snow amounts are uncertain, but could be significant. Stay tuned as more details become available.