Valley rain and mountain snow – potential high-impact storm next week

January 22, 2021 – Periods of rain and snow Friday and into next week, bringing potential winter weather driving hazards. A much wetter storm is possible by the middle of next week.

Discussion

Upper low just off the Nrn CA coast will be dropping SSEwd over the region today with scattered-numerous showers. This low pressure system is not associated with an AR so it does not have an abundance of moisture associated with it, but it will be associated with a cold pocket and steep mid level lapse rates. A few of the SREF CAPE plumes for SAC show nearly a couple hundred J/Kg for this afternoon with the GFS consistently highlighting instability over portions of the Srn Sac Vly/Nrn SJV for the last several days. Although forecast soundings are not impressive, can’t rule out an isolated T-storm or two, or some graupel showers in the Mother Lode as short wave energy moves inland during the day. This is also been hinted by the higher resolution CAMs the last couple of days. Cumuliform cloudiness associated with the cold pocket could result in some breaks in the clouds later this morning and afternoon behind the front which is moving through this morning.

Heaviest precip amounts are forecast over the orographically favored Sierra with liquid amounts up to around a third of an inch to a half inch or so, and 2-4 inches of snow, and max amounts up to 5-7 inches in Srn zone 69. Heavier convective snow showers may occur down to 3000 to 3500 ft, otherwise our snow level is forecast to be around 4 kft. The convective nature of the snow showers is likely to cause some travel problems over the Sierra and will issue a WSW. Given the lack of moisture, will trend the QPF to the SREF mean/average for the Valley, but a little above the average in the orographically favored areas.

Today’s high temps will be at least 5-15 degrees cooler than yesterday, and we have seen the last of widespread 60s in the Valley for quite a while. Saturday should see some clearing and modestly warming temperatures as the trof departs, otherwise colder wx is ahead. A few light snow showers could linger over the Sierra Crest south of I-80 on Sat as secondary energy drops Swd along the W coast reforming the closed low circulation to our south. This could cause backwash clouds/showers to linger into Sat. Otherwise, clearing behind the initial system could allow some patchy frost in the Valley Sat morning.

Another trof and cold front are forecast to bring a reinforcing shot of cooler and showery weather to Norcal Sun/Sun nite, and lingering over the Sierra into Mon. This is a modestly wetter system, but not associated with an AR. However, an AR storm looks more promising around the middle of next week according to the GEFS mean, less so according to the ECMWF ensemble mean.

Extended discussion (Tuesday through Friday)

Decreasing shower threat on Tuesday as short wave upper ridging moves through. Next Pacific frontal system follows with more widespread precipitation spreading across the CWA Wednesday afternoon into night.

Abundant moisture associated with this system resulting in significant QPF. Several feet of snow is possible in the higher elevations through midweek. Threat of showers continues Thursday into Friday with upper trough.