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Periods of widespread Valley rain, mountain snow, and gusty winds this morning becoming showery this afternoon. Mountain travel will continue to be impacted today. A weak system will bring light showers Tuesday and Wednesday with very low snow levels, some light snow possible in the northern and western portions of the Sacramento Valley, Foothills and Mountains. Cold temperatures Monday night into next weekend.
Water vapor is showing a short wave trough pushing into the far NW part of the state early this morning. This is what is bringing widespread valley showers along with moderate to heavy mountain snow. This trough will push over us this morning and push off to the south and east during the afternoon. Showers will diminish from north to south during the morning hours with light showers lingering over the Sierra into the evening.
Snow levels are around 2000 feet and they will hold there for much of the morning except over Shasta County where we will start to see them drop, dropping to 500 to 1500 feet by the late morning. Snow levels will be falling in the Sierra during the afternoon falling to 1000 to 1500 feet by the mid to late afternoon.
Several inches to a few feet of additional snow accumulation is expected over the Sierra mainly this morning and mainly above 2000 feet.
Very cold air will build in behind this system and we will see some clearing overnight. This will bring very cold conditions overnight as we see lows fall into the 20s and 30s in the valley, 20s in the foothills and single digits and teens in the mountains.
Freeze Watch remains in effect for tonight. We are going to remain on the western edge of long wave troughing into the middle part of the week. A short wave will dig down early Tuesday and a little bit stronger short wave Tuesday night into Wednesday. With both of these waves a few light showers will be possible with showers being a bit more widespread with the Tuesday night/Wednesday wave.
Very cold air will remain in place when these short waves bring some showers and snow levels will only be a few hundred feet in the mornings rising up to 1500 feet or so late morning into the afternoon. Sounding profiles support mainly snow with the Tuesday night/Wednesday wave for Redding and Red Bluff with a rain snow mix potentially as far south as northern Sacramento County, a mix will also be possible in far eastern portions of Sacramento County. Snow accumulation in lower elevations is looking very light at this point with maybe an inch to a few inches in the far northern and western Sacramento Valley along with the foothills.
The mountains could see a little bit more with 3 to 7 inches. There is quite a bit of uncertainty with this system when it comes to how much QPF it will produce and also how low the snow levels will be, a change in just 1 or 2 degree can be the difference between all snow or all rain in lower elevations. Hopefully things become a bit more clear today with later model runs.
Short wave riding builds in briefly later Wednesday into Thursday. This will bring some clearing along with dry conditions. Temperatures will remain cold with highs generally 10-20 below average and lows 5-15 below average.
Extended Discussion (Friday through Monday)
Drier, but continued chilly/cold weather pattern setting up for NorCal for the first 3 days of the EFP. On Fri, the highly amplified Pac ridge along 140-150W will continue the chilly/cold NNWly flow into Norcal. Short wave energy dropping SSEwd from the Yukon and across B.C could bring some light to modest amounts of snow to the Nrn Mtns/Sierra early Fri before it digs a longer wave trough across the Great Basin/Central and Srn Rockys on Sat.
However, the short wave should be far enough east to bring clearing later Fri, but continued chilly/cold wx for our CWA. ECMWF EFI for temps shows a trend toward the lower end of the climatological norms during the EFP.
Issues with the Day 8-10 Clusters this morning negated further discussion of the apparent magnitude (depth) of the next large GOA/NErn Pac low, initially forming over the Bering Sea later this week and then phasing with a low over Ern AK by the new year. The flow of air from this source region is the reason our temps are well below normal. While the ECMWF is more progressive with this low, the GFS prolongs the duration of precip and cold wx over Norcal during the first week of the new year.