January 21, 2021 – Dry weather today, then periods of rain and snow Friday and into next week, bringing potential winter weather driving hazards.
Air mass gradually recovering some moisture after the strong Nly wind event earlier in the week. Under clear skies (or cirrus clouds) and light winds, dewpoints are recovering and RHs up some 30-50 percent. With temps dropping into the 30s in the Valley, can’t rule out some light frost in open areas this morning. Otherwise, an increase in mostly high cloud cover is forecast today ahead of Friday’s wx system, with the closed low center currently near 48N/133W (W of VCRISL).
The low center is forecast to drop SEwd into Norcal during the day on Fri, preceded by its associated cold front. So today will be the last day of widespread max temps in the 60s in the Valley for several days as max temps return to near normal on Fri with highs in the 50s in the Valley, mostly 30s and 40s in the foothills and mtns. Min temps will cool into the 30s in the Valley, and teens and 20s in the mtns for the rest of the week.
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 a couple hundred J/Kg for Fri afternoon with the GFS consistently highlighting instability over portions of the Srn Sac Vly/Nrn SJV since Mon/Tue, so can’t rule out an isolated thunderstorm or two, or some graupel showers in the Mother Lode. This is also hinted in the NAM 3km REF prog.
Heaviest precip amounts are forecast over the orographically favored Sierra with liquid amounts up to around a third of an inch or so, and 2-4 inches of snow, and max amounts up to 5-7 inches in Srn zone 69. A dusting of snow is possible down to the upper foothills, around 3000 feet in heavier showers. The Valley should see a few hundredths N to a tenth or two tenths of an inch over the east side of the Valley.
Saturday should see some clearing and modestly warming temperatures as the trof departs. A few light snow showers could linger over the Sierra Crest south of I-80 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.
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.
Extended discussion (Monday through Thursday)
Long wave trough sets up along the West Coast allowing short waves to drop into it resulting in unsettled weather through the extended forecast period.
Below normal temperatures expected with snow levels in the foothills and possibly upper elevations of the Northern Sacramento Valley Monday and Tuesday. Overall QPF looks light during this period.
A much wetter Pacific storm is forecast midweek with snow levels rising into the upper foothills late Wednesday into Thursday.