January 24, 2021 – Moderate rain and snow expected today into Monday as a cold system moves across Northern California. A long duration, high impact winter storm is expected Tuesday evening into Friday bringing periods of gusty winds and moderate to heavy rain and mountain snow.
Discussion
An active pattern is anticipated for the next week with multiple weather systems impacting interior Northern California.
Mostly clear skies are seen across the CWA early this morning with a few mid level clouds noted over far northern California as the next weather system approaches the area. Slightly below average low temperatures are expected by sunrise across the area with some patchy frost possible in the Valley.
The first weather system of the week arrives today with precipitation beginning across the northern and coastal mountains early this afternoon. Snow levels across much of Shasta County and the northern Sacramento Valley will be around 2.0-2.5 kft with the onset of precipitation, although most precipitation will quickly move east to western Plumas County and the Sierra Nevada tonight. With that said, an inch or two of snow in this area may be seen this afternoon into the early evening.
As for the Sierra/Lassen area, precipitation begins slightly later with the heaviest QPF expected this evening into Monday morning. Snow levels will be around 3.0-3.5 kft at the onset of precipitation and a Winter Weather Advisory remains in effect for this area through the morning as 4 to 8 inches of snow are expected, with locally higher amounts possible at peaks. Snow levels do drop early Monday morning so some foothill locations near the Central Sierra may see some small accumulations down to 2000 feet. Light mountain snow showers may linger through the afternoon.
For the Valley, expect anywhere from 0.10-0.25″ between this afternoon and Monday morning. Behind the system on Monday expect northerly winds to develop generally between 10 and 20 mph. The coldest morning will be on Tuesday with lows in the mid 20s to lower 30s for the Valley to single digits to teens for the mountains.
On Tuesday, a wetter system is expected to approach NorCal bringing significant impacts to the area including gusty winds and periods of heavy rain and mountain snow. This system approaches from the NW as early as Tuesday afternoon with very low snow levels forecast for far northern California (around 0.5-1.5 kft). Depending on the exact timing of precipitation, the far northern Sacramento Valley does have a chance to see a rain/snow mix for a brief period Tuesday afternoon before snow levels slightly rise. Snow levels look to remain lower through Wednesday morning around 1.5-2.0 kft.
Rain and snow become heavy Tuesday night into Wednesday. This, along with increasing southerly winds, will bring extremely hazardous travel conditions to much of the area. Expect ponding of water on roadways and slick conditions in the Valley. For the mountains, expect near white out conditions, travel delays, chain controls, and possible road closures.
Precipitation may become a bit lighter Wednesday afternoon in the Valley, although mountain snow looks to continue with slightly higher snow levels (3.5-4.0 kft). Rain and snow are likely to continue through Friday morning.
See the extended discussion for more details and stay up to date with the forecast as exact snow levels and precipitation amounts may change.
Extended discussion (Thursday through Sunday)
Stormy pattern likely to continue across NorCal late in the week through next weekend. Favorable pattern for epic northern Sierra snow event in place during the middle of the week into Thursday with both GEFS and EC ensemble indicating a trough off the coast with strong southwest flow over the region. Mountains snow amounts will be measured in feet!
A relative break will be possible Friday as the mid-week trough shifts east, but precipitation will likely increase again next weekend as the long-wave trough over the eastern Pacific reloads with more upstream energy dropping down from the Gulf of Alaska.
A prolonged period of severe mountain travel impacts likely due to heavy snow and strong winds, and with snow levels around 3-4k ft, winter weather impacts will extend down to the edge of the upper foothill elevations.