Another more potent winter storm is expected later today and through the weekend, bringing widespread Valley rain and significant mountain snow. Gusty southerly winds will develop tonight into Saturday. A Winter Storm Warning is in effect from 4 pm today – 4 am Sunday for the Coastal Range and 10 pm today – 4 am Monday for the Sierra, above 3,500 feet elevation. Forecast: 2-5 feet of snow, locally up to 6 feet, above 5000 feet. 6-24 inches of snow above 3500 feet in the Sierra. 1-3 feet of snow, locally up to 5 feet, above 3500 feet in the Coastal Range & Shasta County Mountains. Mountain travel is highly discouraged. Dry conditions return early next week.

Discussion

Latest satellite and radar imagery shows the area of scattered showers has weakened and significantly decreased in coverage, and the Winter Weather Advisory was canceled early. Looking at the handful of automatic snow reporting sites, preliminary observed snowfall through midnight for Thursday has been 2-5″ for the Sierra with 9-14″ in the Shasta County Mountains.

Behind the isolated area of light showers, fairly dense cloud cover prevails, and is expected to remain through much of the day across the region.

The next Winter Storm will begin to impact NorCal (initially Shasta County and the Coastal Range) by this afternoon/early evening, before spreading southeastward across the rest of the region through tonight.

An upper-level low in the Gulf of Alaska will phase with a southern stream shortwave off the west coast by this afternoon, bringing with it a fetch of moisture (weak AR). Anomalous PWAT‘s (2+ sigma/180-200% of Normal), strong sw-ne oriented jet streak, plus added enhancement from orographic lift will lead to very heavy snowfall across the mountains for elevations mainly above 3500-4500 feet.

Latest hires has snowfall rates of 1 to 3 inches per hour for several hours through Saturday and into Sunday, leading to significant snowfall totals of 2 to 5 feet.

Dangerous to near impossible mountain travel is expected, with chain controls, travel delays, and road closures likely.

A strong jet streak (60-70 knot at 700mb) is expected to move across NorCal late tonight through Saturday, producing strong southerly surface winds with gusts of 40 to 50 mph across the Sacramento and Northern San Joaquin Valleys.

Similar, but slightly stronger winds are expected across the higher terrain with gusts to 60 mph possible. Strong winds plus heavy snow on Saturday will create near whiteout conditions, making mountain travel near impossible.

In addition to heavy mountain snow and gusty winds, widespread moderate rainfall will result in rainfall totals in the Valley of 1 to 2 inches, locally up to 2.50 inches possible. Impacts expected from the long-duration Valley rain include, minor roadway ponding and urban flooding of poor drainage areas, as well as longer than normal commutes.

Rainfall rates are not currently expected to be a concern for burn scars. Strong dynamics will be overhead late Saturday afternoon through the evening, which may allow for some embedded lightning strikes with heavier showers.

As the low center moves overhead by Sunday afternoon, slightly better instability will create slightly better chances for seeing thunderstorms, especially south of I-80.

Given the strong south to southeasterly flow ahead of the low, significant bulk shear, and 200-300 0-1km SRH, funnel clouds cannot be ruled out. To put this weather system into perspective, this next system will be much stronger than yesterday’s system, and is expected to be even more impactful than last weekend’s system. Avoid mountain travel if possible.

By Monday, the low center will be centered over northern Nevada, bringing in dry, northerly flow across the region, creating breezy northerly winds for the Valley. Morning low temperatures will return to being near to below freezing Monday morning as well.

Extended Discussion (Tuesday through Friday)

A potentially brief, upper-level ridge then builds off the CA coast keeping quiet weather through at least Tuesday. Cluster Analysis and the latest ensembles are advertising another trough may impact the region sometime mid to late week, but the timing, strength, and impacts with this system are still unclear.