Wednesday's highs in Northern California are forecast to be in the upper 50s at the coast and lower to mid-60s inland, under mostly sunny skies. Increasing high clouds in the afternoon and evening will signify the changes in the atmosphere ahead of the approaching storm.
A prolonged storm will begin Thursday and is anticipated to continue through Sunday. In the Sierra Nevada, blizzard warnings are in effect for potentially historic snowfall, with up to 12 feet possible. While the mountains will take the brunt of the impacts, the Bay Area can expect the same weather system to bring periods of heavy rain, gusty winds and accumulating snow above 2,500 feet.
Snowfall forecasts in the Sierra Nevada have remained consistent over the past few days, with the probability of higher-end totals increasing as the storm approaches. Up to 12 feet of snow is possible along the western Sierra crest, including Donner Pass, with up to 4 feet in the Lake Tahoe Basin.
Locations along Interstate 80 in Placer County , including the UC Berkeley Central Sierra Snow Lab, could set single-day snowfall records, topping the previous high of 49 inches, set at Squaw Valley (now Palisades Tahoe) in 1958. A blizzard warning is in effect from Thursday morning through Sunday morning.
A powerful jet stream will take aim at California, tracking straight from Alaska. The jet stream's ribbon of fast-moving air in the atmosphere will cause cold westerly winds to slam into the Sierra Nevada, dumping massive amounts of snow in the mountains.
Light snow is expected to begin Thursday morning along Interstate 80 and Highway 50 and become much heavier by the afternoon. Snow will continue to increase Thursday night. Extreme snowfall rates of 2 to 5 inches per hour and gusts of 45 to 60 mph Friday through Saturday morning will create whiteout conditions. Travel will be nearly impossible, and highway closures are likely. Ridge tops could see gusts over 100 mph, and ski resorts will probably close due to wind and avalanche danger.
While snow is expected to gradually taper off Sunday morning, latest forecast guidance indicates heavy snow may continue 12 to 24 hours more than previously indicated. If these trends continue, moderate to heavy snow may not stop until Monday morning, and the storm may approach historical territory. Stay tuned for updates to the forecast.
Bay Area
This storm is an outlier this winter, with the most significant impacts expected in the Sierra Nevada rather than at the coast. While widespread flooding and wind damage is not anticipated in the Bay Area, periods of impactful weather are still possible. Additionally, this sort of weather pattern can result in unusual precipitation types in the Bay Area such as hail and snow.
Light rain showers are possible Thursday morning in the Bay Area, but an approaching cold front will cause southerly winds to pick up in the afternoon. A period of heavy rain will likely accompany a cold front in the evening. Rainfall rates of a quarter- to half-inch per hour are possible as this front moves west to east. Gusts of 30 to 40 mph are also expected.
In the wake of Thursday's cold front, widespread rain will transition to hit-or-miss thunderstorms Friday through Sunday. Hazards from thunderstorms include quick-hitting downpours, erratic wind gusts, a mix of rain and snow above 2,000 feet, small hail, possible lightning and a slight chance of a coastal waterspout. Another period of widespread 30 to 40 mph gusts is expected Friday evening.
By Saturday, the freezing level is expected to fall to 3,000 feet in the Bay Area. Heavy showers will drag the snow line down even farther, and could cause minor snow accumulations down to 2,000 feet. Several inches of snow are expected above 3,000 feet on Mount Saint Helena, Mount Diablo, Mount Hamilton and the Santa Cruz Mountains.
Rainfall totals are expected to range from 1 to 3 inches from this storm. River flooding is not expected, but brief periods of heavy rainfall could cause minor urban flooding.
Wednesday breakdown
San Francisco: Aside from the lack of fog, Wednesday will feel like a May day in San Francisco, with a pleasant early afternoon before westerly gusts pick up into the evening. Highs will climb to the upper 50s in the Richmond and Sunset districts and lower 60s in downtown, SoMa and the Mission District under mostly sunny skies. Clouds will increase overnight with lows in the upper 40s.
North Bay: Wednesday's weather is expected to be nearly identical to Tuesday, with a chilly morning warming for a seasonal afternoon. A frost advisory is in effect until 9 a.m. in Napa County, eastern Marin County and interior Sonoma County. Afternoon highs throughout the North Bay will reach the lower 60s under sunny skies. Increasing clouds in the evening will act as a blanket overnight, preventing lows from dropping below the lower 40s.
East Bay: Another chilly morning and pleasant afternoon is in store for the East Bay. BART riders in Antioch and Dublin should wear a warm coat on the platform, as temperatures will be in the upper 30s to lower 40s in the morning. Highs are expected to climb to the lower 60s in Oakland and Hayward and mid-60s in Fremont, Walnut Creek and the Tri-Valley. Lows will be warmer than the past couple nights, in the mid-40s.
Pacific Coast and Peninsula: Wednesday is expected to be the last day of pleasant weather until at least next week in San Mateo County. Sunny skies are expected, with highs in the upper 50s in Daly City, Pacifica and Half Moon Bay, and lower 60s in San Bruno, Burlingame and Redwood City . Gusts of 15 to 25 mph are expected in the afternoon, especially in South San Francisco, like on an early summer day. Lows will be in the upper 40s with increasing clouds.
South Bay and Santa Cruz: A gorgeous late February day is expected in the Santa Clara Valley, with highs in the mid- to upper 60s in San Jose , Milpitas, Palo Alto and Cupertino. Mid-60s are also forecast in Boulder Creek and Scotts Valley, and lower 60s in Santa Cruz. Sunshine will prevail for most of the day, but clouds will increase overnight with lows in the mid- to upper 40s.
Reach Anthony Edwards : anthony.edwards@sfchronicle.com
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