A trough from the Gulf of Alaska brought Valley rain, mountain snow, and thunderstorms to northern California. The system hit in two waves, the initial frontal passage followed the trough axis moving over northern California.
Every event with a place to put it. Tap one for its entry below.
Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
The episode as NWS STO wrote it up.
A trough from the Gulf of Alaska brought Valley rain, mountain snow, and thunderstorms to northern California. The system hit in two waves, the initial frontal passage followed the trough axis moving over northern California. Clearing behind the cold front allowed for a favorable thunderstorm environment to develop, which would eventually lead to two confirmed tornadoes for interior northern California. Snow levels with this system started around 5,000 and 6,500 feet and dropped down to 4,000 and 4,500 feet.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS Sacramento, CA wrote its last area forecast discussion before the first event at Jan 4, 2:54 AM PST, 1 hour 6 minutes earlier.
Area Forecast Discussion National Weather Service Sacramento CA 254 AM PST Mon Jan 4 2021 .SYNOPSIS... Strong Pacific Storm will bring periods of heavy rain, possible thunderstorms, heavy mountain snow and blustery winds to Norcal today. Tuesday and most of Wednesday will be dry, but additional rain and snow is possible again later in the week. && .DISCUSSION... Moderate TPW moisture plume (AR) will be moving inland with a favorable SW trajectory and orographic effects for heavy precip mainly N of the I-80 corridor, and especially for the Feather/Yuba River basins. Upper trof nearing 130W at 09z will be nearing the N Coast by 18z and moving inland thru 00z. Surface cold front and strong LLJ will precede the trof mainly moving across our CWA from 18z-00z with the tail end of the front hanging back over the Sierra into the evening. The HREF Probability of >40 dBZ REF shows a 100% bulls-eye over/around the North Complex burn scar from 10am-2pm, then a 70%+ from 2pm-4pm. The Probability of 1 hr rainfall >0.50" is also >70% from 11am-1pm, and given these probabilities and forecast QPF of 1.50" to 3.50" over the burn scar today, and the convective potential for the Valley and Sierra Foothills given negative LI's and several hundred J/kg of CAPE, there is the potential for lines of thunderstorms and flooding/flash flooding over that burn scar. Model QPF has generally increased slightly from yesterday with amounts mostly ranging from up to an inch in the Northern Sacramento Valley to 2 to 5 tenths of an inch southward into the San Joaquin Valley and 1 to 2.5 inches in the foothills and mountains with 3+ inches in the aforementioned Feather/Yuba River basins. Snow totals up to a foot to a foot and a half with localized amounts up to 2 feet are forecast over the higher elevations/higher peaks. Light snow will be possible down to 5000 feet this evening, especially in convective showers. The WAD (wind advisory) for mainly the Valley and coastal mtns looks on track given the nearly 50Kts NAM 925 winds ahead/along the front from 15z-21z. the strongest winds are expected to the in the Nrn half of the Sac Vly. The WSW for the mountains also looks on track with periods of heavy snow with 1-2 inch hourly rates possible given the instability forecast over the Sierra during the afternoon, mainly from 21z-after 00z (1pm-5pm or so). Ridging will follow this trof as it moves across the Nrn Plains/Nrn Rockys on Tue, followed by a another trof mainly moving into the Pac NW, but also brushing Norcal, late Wed and Wed night. This is a weaker TPW plume (AR) with much lighter QPF expected in Norcal, e.g. a couple of inches of snow for the Sierra, compared with a foot to a foot and a half with today's system. Another system is forecast to move into our area late Thu/thu nite. JHM .EXTENDED DISCUSSION (Friday THROUGH Monday)... Pacific frontal system moves through interior NorCal Friday bringing widespread rain and mountain snow. Snow levels look to range from 3500 to 5000 feet and thus could impact mountain travel. Drier weather expected over the weekend under upper ridging. Potential for some night and morning valley fog development under moist stable conditions. Models suggest another system moving through on Monday with some uncertainty in timing and strength. && .AVIATION... Wdsprd MVFR with areas of IFR and lcl LIFR tda in intr NorCal as Pac fntl sys movs thru. Sn lvls 040-060. Isold tstm poss btwn 20z-02z in Sac Vly and ern fthls. Areas Sly sfc wnd 25g35kts in Cntrl Vly btwn 16z-23z tda. SWly sfc wnd gsts up to 50 kts poss ovr hyr mtn trrn tda. Conds impvg tngt, but areas MVFR/IFR/LIFR poss in Vly BR/FG Tue mrng. && .STO WATCHES/WARNINGS/ADVISORIES... Flash Flood Watch from 10 AM PST this morning through this evening for Northeast Foothills/Sacramento Valley-Western Plumas County/Lassen Park. Winter Storm Warning until 4 AM PST Tuesday for West Slope Northern Sierra Nevada-Western Plumas County/Lassen Park. Wind Advisory from 6 AM this morning to 8 PM PST this evening for Central Sacramento Valley-Mountains Southwestern Shasta County to Western Colusa County-Northeast Foothills/Sacramento Valley-Northern Sacramento Valley-Southern Sacramento Valley- Western Plumas County/Lassen Park. &&
The Beale AFB radar (KBBX), 79 km away, was scanning. The volume nearest the first event began at 21:36 UTC.
2 tornado, 1 winter storm. Local standard time, as filed.
Local ski resorts across zone 69 reported between 4 and 12 inches of new snow. Squaw Valley reported 12 inches, Sugar Bowl 10 inches, Boreal 8 inches, Sierra-at-Tahoe 6 inches, Tahoe Donner 5 inches, and 4 inches of snow was reported at Kirkwood.
A possible tornado touchdown was reported by a member of the public in western Corning, with multiple trees knocked down, a barn and other buildings damaged, and a power pole knocked down. Calfire dispatched to the area for power line damage also observed downed trees around the intersection of Apple Rd and Simpson Rd. Tornado track, width, and timing estimated by radar and storm reports.
A tornado was sited on the ground out in an open field near Vina by Calfire. No damage has been reported. Multiple photos and videos confirm a tornado in clear contact with the ground. Tornado track, width, and duration estimated by radar and reports.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in California in 2021Every state in 2021This day in weather historyThe tornado archive for California, 2021Warnings in California, January 2021
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 940320, 940315, 940318.