Another weather system will bring widespread precipitation and gusty wind to the region through Saturday. A short break is expected early next year, then precipitation chances increase again by the middle of next week.
Discussion
Light rain showers across the Northern Sierra and Northern Sacramento Valley are ongoing late this morning ahead of an incoming weather system expected to impact the Valley this afternoon and lasting through Saturday. This system will bring widespread moderate to heavy rainfall with mountain snow developing late tonight into Saturday.
As a result, a Winter Weather Advisory has been issued for Western Plumas County, Lassen Park, and West Slope Northern Sierra Nevada, including Interstate 80 over Donner Pass, Highway 50 over Echo Summit, and Highway 88 over Carson Pass from 10 PM tonight through 10 PM Saturday.
Travelers can plan slippery and snow covered road conditions and could make weekend mountain traffic difficult with delays and chain controls. This weather system is being induced by a trough moving into the Pacific NW, sweeping rain from west to east across the forecast area.
Heaviest rainfall will be observed this afternoon and evening. There should be several hours of moderate rainfall with highest rates from along I-5, north of Yuba City through the Northern Sacramento Valley and the foothills.
Latest probabilities show 70-100% chance of seeing at least an inch or more from Sacramento north with chances increasing as you move northward through the area this afternoon through Saturday. For 2 inches or greater, chances drop to less than 15% throughout much of the Valley, but remain elevated (45-90%) for the mountains and Northern Sacramento Valley, including Redding.
Forecast totals for the system 1.00-1.50″ through the Central Valley with 1.50-2.50″ across the mountains, up to 3 inches in the Coastal Range.
Rain will begin to taper off Saturday morning with lingering showers in the mountains through Saturday night. There is also in isolated chance (10-20%) of thunderstorm development from this afternoon through Saturday morning.
Additionally, with this storm, mountain snow will develop late this evening and continue through Saturday night. Heaviest snow rates from I-80 to Hwy. 50 will be early Saturday morning with a 30-50% chance of seeing hourly snowfall rates greater than inch before moderating to light snowfall by Saturday late morning. Snow levels will initially be 6000-7000 feet, lowering to 5000-6000 feet by Saturday morning. 4 to 8 inches are forecasted over the highway passes with up to 12 to 18 inches at areas of higher elevations. Current NBM probabilities show a 60-80% chance of 8 inches or greater along I-80, lowering to 40-75% chance along Hwy. 50.
Gusty south winds are ongoing, primarily in the mountains and along the west side of the Valley. Gusts 20 to 30 MPH are expected with gusts up to 40-45 MPH in the mountains. Winds in the Valley and foothills will taper off tonight with strong gusts continuing in the mountains through Saturday morning.
As rain moves in and additional cloud cover, afternoon highs will decrease through the early portions of next week with Valley highs in the 50s and 40s and 30s in the foothills and mountains respectively.
Extended Discussion (Tuesday through Friday)
Another wet system is set to arrive late Tuesday through early Thursday per ensemble guidance and cluster analysis which will bring chances for widespread rain and mountain snow.
Precipitation totals have increased slightly for this system with the NBM indicating a 50-70% chance of a half inch in the Valley with a 50-70% chance for 1.00″ or more of total rainfall accumulations over the foothills/mountains. Additionally, the NBM is showing a 50-80% chance of 4 inches or more of total snowfall accumulations over the northern Sierra and southern Cascades during the same period and a 40-60% chance over higher portions of the Coastal Range in Tehama County.
Snow levels may be between 5000 and 5500 feet Tuesday, lowering to 4300 to 5000 feet overnight Tuesday into Wednesday morning.
On the tail end of this system, snow levels may lower to 3900 feet while dropping only a few tenths of snow during early Thursday morning. Locally gusty southerly winds will accompany this system on Tuesday into Wednesday.
Winds will transition to an east- northeasterly flow over the mountains and northerly through the Valley Wednesday night through Thursday afternoon, gusts up to 20 mph may be possible with the strongest localized over the northern San Joaquin Valley and Sierra Nevada Range. Winds weaken by Friday but will retain a northerly component through the day.
Dry weather returns late Thursday and Friday as upper- level ridging sets in. During this period, temperatures are on track to be near- or slightly below- normal for the first week of the new year.