January 4, 2021 – 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.


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.

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.