Next weather system moves in this morning and brings showers, possible t-storms, and mountain snow through Thursday evening. A Winter Weather Advisory is in effect from 10 AM to 10 PM Thursday, December 7 for the Central and Northern Sierra. Drier conditions return on Friday with locally breezy northerly winds.
Discussion
An upper level weather system will move into the area this morning and bring showers, possible t-storms, and mountain snow to the area today through Thursday night. For Wednesday, precipitation looks to get started around 10 AM in the Coastal Range before spreading throughout the area as we progress through the day.
The National Blend of Models (NBM) has a 55-95% chance of 1″ inch or more of rainfall for the Coastal Range, Redding and areas north, and the Southern Cascades and Northern Sierra for Wednesday. Rainfall totals are expected to be less as you move south through the area, with the NBM suggesting 50-60% chance of exceeding 0.50″ inches in Sacramento, and lower probabilities of 10-20% for the San Joaquin Valley. Generally, areas north of I-80 stand the best chance to see the most precipitation.
In addition to showers, there is also a 10-20% chance of thunderstorms developing, mainly for the Northern Sacramento Valley and the Southern Cascades and Sierra Nevada. Gusty southerly winds of 15-30 mph may develop for Valley locations today as well as the front pushed through the area. High elevation snow showers are also anticipated with this system.
Pass levels are forecast to receive 4-8 inches of snowfall from Wednesday’s system. The NBM has a 30-60% chance of snowfall totals of at least 6 inches at Donner Pass, and a general 20% chance at Echo Summit. Snow levels are currently forecast to be around 6000 – 6500 feet Wednesday night.
A second shortwave weather system will move across the area on Thursday. Rainfall amounts will be less than Wednesday’s system, but snow levels are forecast to fall (3500 feet) as cooler air overtakes the area, so we may see more impactful snowfall at pass levels Thursday. Shasta Dam and areas north in Shasta County are forecast to receive 0.50″ inches, while Valley locations are forecast to receive 0.10″ or less. Highest amounts are likely in the higher elevations.
Snowfall on Thursday the NBM has higher probabilities for 6″ inches or more for pass levels, with Donner Pass having a 60-70% chance and Echo Summit having 30-40% chance of exceeding 6″ inches. Hazardous travel conditions are likely, so make sure to prepare if traveling through pass levels both Wednesday and Thursday.
For Friday and Saturday, conditions will improve precipitation wise as upper-level ridging builds offshore and the weather system is pushed further east out of the area. Gusty northerly winds of 15-25 mph may develop Friday afternoon, mainly for the western Sacramento Valley.
Cooler overnight temperatures in the mid 30s are forecast for both Friday and Saturday nights, with a potential for areas of frost to develop in the Valley Saturday morning. High temperatures on both days will struggle to break 60 for Valley locations, and cooler 40s for higher elevations.
Extended Discussion (Sunday through Wednesday)
Drier weather expected through the extended forecast period under upper level ridging. Weak upper troughing moves through on Monday, but available moisture looks limited. Troughing moves into the Great Basin Tuesday as upstream ridging pushes back into NorCal. Models then diverge with progression of next upstream short wave trough midweek. For now, forecast Sunday through Wednesday is mostly dry with high temperatures slightly above normal.