Thunderstorms, fire risks, and snowfall in the Sierra
Fall officially begins Friday night, and Northern California is sure to feel the change of seasons this week. A mix of weather conditions are in store from the Bay Area to the Sierra Nevada.
The Bay Area will see partly cloudy skies with seasonable temperatures through Wednesday, but changes are coming in the second half of the week. By Thursday, northerly winds are expected to clear the skies and increase fire danger in the Sacramento Valley. However, these winds will bring cold air into the mountains, likely cold enough for the first snowflakes of the season to fall in the high Sierra.
Low annoying cut
A low pressure system continues to swirl off the California coast, deepening the marine layer and sparking thunderstorms in the mountains.
Baron/LynxA pesky low pressure system that brought widespread morning clouds to the Bay Area over the weekend continues to impact local weather.
The lowest cutoff levels are areas of low atmospheric pressure that have been displaced by the jet stream’s channeling winds, meaning they often remain in place for several days. The continuing decline has affected the weather in the Gulf region since Saturday and will continue until Wednesday. Locally, morning clouds are expected to continue, while thunderstorms associated with a low depression over the Sierra will continue on Monday and Tuesday.
High temperatures will range from the 60s on the coast to the upper 70s and 80s inland through Wednesday, with overnight lows in the 50s.
Fire and Ice Thursday
Thursday will see a change in the weather pattern, as a stronger low pressure system moves south from the Pacific Northwest into the Great Basin. While temperatures in the Bay Area won’t change significantly, the system will bring dry winds from the north, possibly creating clouds and fog on Thursday and Friday.
Humidity levels on Thursday will be in the teens in the Sacramento Valley, increasing the risk of fire weather.
Baron/LynxFine fuels such as grasses and small brush may be at risk for the fire Thursday above 2,000 feet near Mount St. Helena. However, the primary fire threat should remain in the Sacramento Valley, where winds exceeding 30 mph and humidity levels in the teens could trigger a red flag warning.
While the Valley and Gulf deal with scorching weather conditions, the Sierra Nevada region will feel the change of seasons on the last full day of summer on Thursday. A low-pressure system will circulate directly over Reno, bringing cool air into the Sierra.
A US weather model expects snow to fall in the Sierra highlands on Thursday evening.
Baron/LynxHigh temperatures Thursday in Lake Tahoe will be more than 15 degrees below normal, in the mid 50s with a low to the 30s. Elevations above 8,000 feet in the Tahoe National Forest could see the first snowflakes of the season, and possibly dust accumulation.
Monday collapse
San Francisco: Mostly cloudy skies will gradually clear up to a partly sunny afternoon. Highs will range from the upper 60s in the Embarcadero, Marina District, Richmond and Sunset areas to the lower 70s in the Mission District and Noe Valley.
In the afternoon and evening, wind gusts of up to 20 mph are possible in the Presidio, Twin Peaks, and along Ocean Avenue. Overnight lows will be in the upper 50s with overcast skies.
East Bay: Morning clouds are expected to extend all the way to Mount Diablo but dissipate quickly in Concord, Walnut Creek, San Ramon, Dublin and Livermore. Highs will be in the upper 70s to near 80. Winds are expected to reach 20 mph near Delta in the evening.
Clouds will burn off more slowly along the Gulf Shore, and skies will become mostly sunny in the afternoon. Highs will be a few degrees below normal, in the lower 70s in Oakland, Alameda, San Leandro and Hayward. The 60s are expected to be higher in Richmond and Berkeley. Mid-70s will be more prevalent near Fremont.
Overnight lows will drop into the upper 50s in Alameda and Contra Costa counties, with overcast skies returning to Gulf Beach, and some clouds filtering in east of the Caldecott Tunnel.
North Bay: Clouds are expected to spread back into the Napa, Sonoma and Petaluma Valleys Monday morning before calming down throughout the day. High temperatures will be fairly uniform in Marin, Sonoma and Napa counties, in the low to mid 70s. Napa, Fairfield and Vacaville will range from the upper 70s to low 80s.
Evening wind speeds of up to 25 mph are possible in Benicia, Vallejo, Suisun City and other locations near the Delta. Overnight lows will drop to the mid 50s as clouds return to much of the North Bay.
Pacific Coast and Peninsula: Skies are expected to be mostly cloudy along the peninsula Monday morning, with less cloud coverage south of San Mateo. Regardless of location, skies along Highway 101 should be clear on a generally sunny afternoon. Highs in Daly City, South San Francisco, San Mateo and Redwood City will be in the upper 60s to mid 70s.
Patches of midday sun are possible in the afternoon along the Pacific Coast. High temperatures will be in the mid 60s in Pacifica and Half Moon Bay. Overnight lows in San Mateo County will drop into the upper 50s as overcast skies return.
South Bay and Santa Cruz: The sunniest part of the Bay Area will likely be the Santa Clara Valley on Monday morning as clouds struggle to move east of the Santa Cruz Mountains. High temperatures in Sunnyvale, Cupertino, Santa Clara and San Jose will range from the mid to upper 70s. Overnight lows will drop into the upper 50s.
Residents of Santa Cruz, Filton and Scott Valley will likely wake up to morning clouds. By midday, the clouds will disperse for a sunny afternoon, with high temperatures in the upper 60s at the beach and low to 70s in the mountains. Overnight lows will be in the mid 50s.
Contact Anthony Edwards: anthony.edwards@sfchronicle.com; Twitter: @edwardsanthonyb
