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December 30, 2016 – A weak system will bring a chance for showers on Saturday with light snow accumulations in the mountains. Colder system for later Sunday into Tuesday with snow levels possibly dropping down into the foothills and northern Sac Valley.

Discussion

Another day of dry weather and mild temperatures today as local area is sandwiched between closed upper low moving into SoCal and a upper level system dropping SE into the Pacific Northwest. Patchy fog has formed in the Valley from about Sacramento southward given increase in moisture/dewpoints into the mid 30s despite encroaching cloud clover. This may become locally dense in spots but should burn off after sunrise.

Afternoon highs will be similar to Thursday, although a few degrees cooler to the north as high clouds begin to stream in. Models continue to be in good agreement as system to the north drops south along the California coast on Saturday. System has little moisture to work with so showers in the Valley will be fairly high or miss with only light accumulations expected where it does rain. Best chance for precip remains over the mountains where a few inches of accumulation are possible down to around 4000 feet. Shouldn’t be a major impact but expect a few travel headaches due to slick roads and New Year’s Eve traffic. System quickly shifts south by Saturday evening with a break in weather expected for New Year’s Eve festivities.

Weak high pressure briefly builds in for most of Sunday before next wave drops south Sunday evening into Monday. Precipitation will spread south across the area overnight into Monday with highest chances across the mountains. This will be the system to watch as much colder air is entrenched in system.

Snow levels will start around 1500 feet in the north and around 3000 feet in the Sierra on Sunday but will quickly drop to 1000 feet or less by Monday. Latest model sounding profiles continue to indicate that snow may be possible down to the Redding area and Sierra foothill communities early Monday with more significant accumulations above 3000-4000 feet. Due to potential impacts, certainly will be something to keep an eye on moving forward as models straighten out the details. Regardless of accumulations, cooler temperatures will prevail.

Extended discussion (Tuesday through Friday)

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By Tuesday morning, the coldest air of the season will be overhead as the upper level low settles over our area. 850 mb temperatures look to be somewhere between -5 to -8 C with the coldest air in the northern half of our CWA. Both the GFS and ECMWF agree on bringing the vort max associated with this upper level low through our area sometime during the day on Tuesday which would provide the needed lift to squeeze some precipitation out of this unusually cold system.

As of right now, temperatures this cold could support snow levels around 500 ft in the Northern Sacramento Valley to 1000-1500 ft over the Sierra. Unfortunately, precipitation amounts are still very much uncertain but our confidence is growing that many low elevation communities not used to seeing snow may get at least a dusting on Tuesday.

The low will quickly move out of our area Tuesday night setting us up for several nights of sub-freezing temperatures. The typical north winds behind exiting troughs will begin on Wednesday bringing breezy and cold conditions to our area. If the boundary layer decouples Wednesday night, widespread mid 20s would certainly not be out of the question for the Central Valley. However, surface pressure gradients are expected to weaken further on Thursday so the coldest temperatures may actually occur Thursday night into Friday.

Beyond this models begin to diverge. The most interesting solution is the GFS which brings in another inside slider for Friday into Saturday which would set us up for another round of low elevation snow. However, this is just one of several solutions and will likely change as we get closer to this time period.

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