Active pattern returns late week into next weekend bringing widespread precipitation, extremely heavy mountain snow, strong winds, and low elevation (1500-3000) snow accumulations. Following this system, expect near freezing Valley temps and AM frost.
Discussion – Today through Friday
Key points of late week storm:
- Extremely heavy snow is expected late this week, heaviest Friday and Friday night. Storm total snowfall from Thursday into early Sunday is currently projected at 5 to 9+ feet for elevations above 5,000 feet.
- Significant snow accumulations are expected above 3,000 feet
- Cold air will drop snow levels down to 2,000 by the weekend, with light accumulations possible at those levels.
- Wind gusts to 50 to 60 mph are expected over the mountains. Snow rates of 2-3+” per hour are possible. These will combine to bring whiteout conditions at times.
- Mountain travel will be nearly impossible, especially Friday.
- Strong winds are in the forecast Thursday through Friday in the Valley, gusting from 35 to 55 mph from Marysville northward.
Latest radar and cameras indicate the scattered mountain snow showers have dwindled across the Sierra. As we move into Tuesday, dry northerly flow behind the front will bring some breezy north winds across the Sacramento Valley this morning through the afternoon.
Winds have begun to pick up tonight, with some gusts of 15-25 mph across portions of the northern Sacramento valley. Strongest winds are forecast to be north of Sacramento along the I-5 corridor, where gusts to around 30 mph will be possible later this morning-afternoon. This morning’s low temperatures will drop to near freezing across the northern Sacramento Valley, which will allow for locally patchy frost to develop. Afternoon high temperatures will climb into the upper 50s and low 60s for the Valley.
On Wednesday, an anomalously cold system approaches the area from the PacNW, beginning to bring some precipitation chances mainly for the Southern Cascades, northern Coastal Range, and Shasta County by Wednesday night.
As the trough continues to quickly dig into the area Thursday, winds and precipitation will continue increase and spread southward. By Thursday morning, a strong upper- level jet will create significant orographic lift/upslope enhancement across the Sierra leading to impressive snow rates of 1-3 inches per hour and extreme accumulations in the mountains are likely.
This same jet will create southerly to southwesterly winds to increase across the region, with strongest winds expected across the northwestern Sacramento Valley, Southern Cascades, and the Sierra bringing gusts to 35-55+ mph across portions of the Valley with gusts over 60 mph possible in the foothills and mountains.
The strong foothill/mountain winds will accompany the heavy snowfall, creating white- out/blizzard conditions. Precipitation will continue through Friday, as another re-enforcing low digs into the region, bringing even colder air, and continued orographic enhancement from a strong upper-level jet.
This will lead to even more impressive snowfall rates of 2-3+ per hour across the Sierra. High snow rates and strong winds of 55-60+ mph will make mountain travel impossible.
Snow levels will initially be around 5000-6000 feet Thursday, decreasing to 3000-5000 feet through Friday, then decreasing even further with the frontal passage Friday night- Saturday morning to 1500-2500 feet.
Given how anomalously cold and dynamic this system is, confidence in the snow level forecast is still somewhat low at this point, and snow levels may locally drop lower with the heavy snow rates Thursday-Friday.
The time is NOW to prepare for this *significantly* impactful winter storm.
Extended Discussion (Saturday through Tuesday)
By Saturday morning, ensemble guidance is in agreement that the upper level closed low will begin moving inland, centered primarily over the Pacific Northwest. This will transition the flow pattern aloft over interior NorCal from west-southwesterly to north- northwesterly throughout the day, which will lead to a slightly less favorable pattern for orographic enhancement that will accompany comparatively less moisture influx into the region on Saturday.
Major impacts from heavy mountain snow and accumulating impactful foothills snow are still expected through Saturday, and will likely be compounded by the period of Extreme impacts anticipated Friday into early Saturday.
As the trough moves further inland, precipitation impacts will become more confined to the foothills and mountain locations Saturday into early Sunday.
That being said, there will be some conditional potential (10-20% chance) for isolated thunderstorm activity in the Valley, which could result in brief periods of higher impact weather throughout Saturday.
Otherwise, extremely heavy snow (snow rates of 3+” at times) is expected to linger into early Saturday morning prior to this, before transitioning toward heavy to moderate snow (snow rates closer to 1 to 2″ per hour) later in the day Saturday into Sunday.
Snow levels on Saturday morning will also have the most variability as a push of colder air filters into the region. At this time, snow levels down to 1000 feet will be possible across the northern Sacramento Valley, with snow levels down to 1500-2000 feet along the Sierra and a slight chance (20-40%) of even lower snow levels.
Snow levels then drop again by Sunday morning as the final push of cold air arrives. Northern Sacramento Valley snow levels look to drop to sub-1000 feet values, with snow levels along the Sierra nearing 1000 feet by Sunday morning.
While much lighter snowfall is expected by Sunday morning, some low elevation accumulating snow will be possible. After the early Saturday morning heavy precipitation event (2 to 4″ of liquid equivalent) along the Sierra and northern Sacramento Valley within this storm, an additional 1 to 2″ of liquid equivalent precipitation is expected throughout the day Saturday into Sunday. This equates to a 60 to 80% chance of exceeding an additional 12″ of snowfall above 5000 feet along the Sierra from midday Saturday through midday Sunday.
Elsewhere, liquid equivalent precipitation totals look to generally range from an additional 0.25 to 1″, with higher totals in any areas where thunderstorms develop. So, overall individual impacts comparatively across the bulk of the weekend will be lower (Major vs. Extreme), but again, these impacts will be likely be exacerbated by the Extreme impacts just prior to this period and the lowering snow levels introducing snowfall potential into the lower elevations.
Breezy to gusty south-southwesterly winds will also continue Saturday into Sunday for much of the region, but will remain much lower than winds expected on Thursday and Friday.
Otherwise, the persistent pushes of cold air masses into interior NorCal will result in low temperatures dropping to near freezing throughout the Valley (coldest across the northern Sacramento Valley) on Sunday and Monday mornings. This will introduce the potential for Valley frost formation each of these mornings as well.
Mountain snow showers will likely linger through Sunday, before tapering off Sunday evening.
There is currently high ensemble uncertainty in the upper level pattern across the early next week time frame, but an overall trend toward drier and quieter weather is expected.