Periods of widespread Valley rain, mountain snow, and gusty winds expected this weekend into next week. Holiday travel will be impacted, especially over mountain passes. Foothill snow possible at times Sunday through Wednesday. Cold and drier weather expected late next week.
Discussion
With long wave troughing set up over the west we will continue to see an active unsettled weather pattern for NorCal. We are seeing widespread showers over the northern portion of the area with widespread mountain snow. This activity is associated with weak warm air advection and up slope flow ahead of our next short wave trough.
Shower activity will continue to become more widespread during the morning hours as the associated cold front pushes into our area. This front should reach the Sacramento to Lassen Park area by the late morning into the early afternoon. The front then looks to stall out over the San Joaquin Valley east to around the I-80 corridor during the late afternoon into the evening. Higher rain and snow rates can be expected along and just ahead of this front with localized rain rates up to 0.50″ /hr. and localized snow rates 4″+ /hr.
The trough axis will push through during the mid to late evening pushing the cold front south. This will bring diminishing showers for the overnight Sunday into Sunday morning.
We do see some CAPE build in behind the cold front and ahead of the trough axis. This will bring a chance for a few afternoon thunderstorms mainly from around Sacramento north to the Chico area. Small hail and brief down pours will be the main concern with any thunderstorms that do develop but we do also see strong 0-1 km shear of 15-20 knots, we could maybe see an isolated funnel with any of the stronger cells. The thunderstorm threat will diminish shortly after sunset.
Snow levels will be 3000-4000 feet over the Sierra and 2000-3000 feet over Shasta County today. They will be falling behind the cold front falling as low as 500 feet over Shasta County Sunday morning and as low as 1500 feet in the Sierra.
The majority of the precip will be moving out when we see these low snow levels but still looks like a dusting of snow will be possible down to these elevations Sunday morning.
The mainly dry weather Sunday morning will be short lived as our next short wave digs down the coast. Showers will become more widespread once again over the mountains and in northern areas during the afternoon and evening Sunday continuing to work south Sunday night into Monday with the trough axis and cold front.
This short wave will have a push of very cold air with it and we will see already low snow levels of 1500-2500 feet fall to 500-1500 Monday afternoon. Snow levels will continue to fall Monday night reaching the northern Sacramento Valley and only a few 100 feet over the Sierra. When we see these very low snow levels the majority of the showers will be diminished and not expecting much if any accumulation.
Another short wave trough will push over the area Tuesday afternoon into the evening. Very low snow levels will be in place on Tuesday of only a few hundred feet. This system will have very little moisture with it and that will limit accumulation with this but some snow flakes will be possible with any of the showers across much of the area including most of the Sacramento Valley.
Breezy winds will also continue with this pattern with the strongest winds expected this morning into the afternoon and again Sunday afternoon into Sunday night.
Extended Discussion (Wednesday through Saturday)
Very cold wx pattern forecast to continue over NorCal for the remainder of 2021, before some relief after the New Year. However, this relief could be short-lived as generally 4 of the 5, Day 8-10 Clusters indicate low pressure returns to the NErn Pac with the magnitude (depth) of the trough in question. After Tuesday’s short wave drops Swd along the W Coast, a subsequent short wave slightly retrogrades off the CA coast Wed/Thu continuing the cold wx but generally pushing the bulk of the precip Swd.
However, some relatively light precip (generally < 0.25″) is forecast to skirt our CWA which may bring some light snow to the Valley Wed morning. Probabilistic guidance for the 24 hr period ending 4 am Wed shows an 80% probability of a dusting (<1″) of snow in RDD with with upper range 2″, and zeroes for SAC albeit the upper end of the range also indicates a dusting. Thus, there is much uncertainty for any significant snow, with the most likely outcome a dusting at RDD.
Meanwhile the Sierra and foothill locations should continue to see some light/moderate snow continuing to accumulate during the midweek period. A cold north wind is likely behind these waves which should drop min temps below freezing in the Valley as early as Tue, and continuing in the Nrn half of the Valley through the rest of the week, while some moderation occurs in the Srn portion of the Valley.
Very likely we will be hoisting a Hard Freeze watch/warning in the coming forecast shifts, just too much going on for today given the actual holiday and the mixed bag of impactful wx occurring this weekend.