At the beginning of the month, a high-velocity band of high-speed winds known as the jet stream helped carry cold cyclones from the Pacific to northern California, but that is about to change.
The jet stream winds will begin to shift toward Idaho this weekend, pushing the low-pressure region away from California. A high pressure system from the southwest is then expected to take its place, making hot, muggy weather likely in the Bay Area this weekend.
buck the trend
As the jet stream moves away from Northern California this weekend, weather models predict a gradual weakening of the cold wind flow over the Bay Area. Fewer winds brushing off the coast and less fog carry less moisture into the San Francisco Bay.
With this change in pattern, residents of San Francisco, Oakland, and the Peninsula will likely see mostly clear, mild weather early into the weekend. Temperatures on Friday afternoon are expected to reach the high 60s on the coast and the mid-70s on the Gulf. Inland residents such as Walnut Creek and Gilroy can expect daytime highs of just over 80 degrees.
And when the low pressure moves away from the Bay Area, a high pressure system forms.
U.S. weather models predict that the heat dome centered on Arizona and Nevada will once again extend into the Central Valley from Saturday through Sunday. Bay Area residents can expect daytime temperatures to soar by the end of the week, up to 15 degrees Celsius, unless the jet stream winds blow a low pressure system onto the coast.
Most inland residents expect daytime highs in the low 90s by Saturday and in the high 90s by Sunday. For residents of Livermore, Concord, Fairfield and Vacaville, temperatures reaching triple digits are no exception. Most of California’s interior will see another weekend of moderate heat hazards, with highs in the 105-110 degrees range across the Central Valley.
what about the coast?
Weather models are predicting temperatures in the low 70s along the San Francisco Bay this weekend, despite an area of strong anticyclone in Northern California. Natural air conditioning from ocean breezes will still manage to break through the heat, blowing into San Francisco, Oakland, and much of the Delta in the afternoon.
If these ocean winds continue to push against the high-pressure ridge, they may manage to cool the Gulf Coast until early next week when the jet stream turns back toward California.
By Monday, weather models predict the jet stream will bend back toward the Bay Area and Central Valley, pushing heat back toward the Southern California desert from the ridges this weekend.
Baron/LynxEuropean weather models predict that the jet stream will begin to bend toward northern California again by Monday. When that happens, fast winds will pick up right over the Bay Area. And those winds will help push cold, foggy “fog” air back into the region.
weekend breakdown
San Francisco: West San Francisco can get a typical fog and haze on Friday morning, but mostly cloudy skies east of the Sutro Tower. Friday afternoon will be sunny with highs in the low 60s in Twin Peaks and Diamond Heights, and highs in the mid 60s in Pacific Heights, the Embarcadero, the Outer Sunset District and the Richmond District. Downtown and residents of SOMA, Mission and Castro are expected to see highs in the low 70s before winds pick up along the market after 3 p.m. Most residents living downwind of the zone can expect cloudy nights and cool lows in the mid-50s.
The veil of fog will gradually dissipate this weekend, leading to sunny and warm weather. San Francisco residents can expect daytime temperatures to hit the low 70s on Saturday and Sunday, about 3 to 5 degrees above the August average.
Around Chrissie Field, Pier 39 and Ocean Beach, winds typically hit up to 30 mph between 1pm and 6pm, and subside in the evening. Nighttime temperatures are expected to hover in the high 50s this weekend.
East Bay: Heavy fog is forming in the Oakland and Berkeley hills Friday morning. Residents of Alameda, Richmond, Fremont, and Hayward will expect mostly cloudy skies with some afternoon sun. Daytime highs are expected to reach the low 70s along the coast on Friday. Warm, dry weather arrives in the Gulf region this weekend, with sunshine pouring in and highs in the high 70s, rising to 80s on the hillsides by Sunday.
Residents east of the Caldecott Tunnel can expect mostly sunny skies and temperatures of 10 degrees this weekend. Expect highs in the mid to high 80s on Friday in the Walnut Creek, Pleasanton and Dublin Tri-Valley areas, followed by a rise to the low 90s on Saturday and Sunday. Residents of Livermore, Brentwood, and Concord could experience 100-degree heat after 3:00 p.m. until delta winds cool. After sunset, expect clear skies and nighttime lows in the 60s.
North Bay: A veil of fog is likely over the Sonoma and Napa counties vineyards over the next few mornings. Warm air will gradually trickle into the area Friday through Sunday, clearing the way for a foggy, hot afternoon. Note that with mostly clear skies, daytime highs on Friday to high 80’s to high on Saturday and high 80’s to high 90’s on Saturday.
The hottest air in the area settles downwind of the Mayacamas Mountains, where Fairfield and Vacaville residents can expect temperatures in the high 90s to low 100s on Saturday and Sunday. Some of that heat could push temperatures to the mid-90s in Napa and Santa Rosa by the weekend. Cold winds from the delta bring nighttime lows to around 60 degrees.
Gulf Coast residents will escape the heat inland this weekend, but temperatures of 80 to 85 degrees are still expected for Marin County residents in Mill Valley, Novato and San Rafael on Friday and Saturday. A brief spell of 90 degrees Celsius through Sunday could allow shops to open in the sun, but the Gulf breeze won’t keep daytime temperatures from rising any further. Residents of Tiburon and Sausalito expect daytime highs to hit the mid-to-high 70s this weekend. After that, overnight lows are likely to fall to the mid-50s.
Pacific Coast and Peninsula: Residents along Interstate 280 and Highway 101 in Redwood City, Menlo Park, Atherton, Palo Alto, and elsewhere can expect fog Friday morning. The rest of the weekend is expected to be mostly sunny with daytime temperatures rising to between 7 and 10 degrees Celsius. Look for highs in the mid 70s on Friday, high 70s on Saturday and low 80s on Sunday.
Marine fog will recede over San Bruno Gap and the coastline this weekend. Sea breezes reach 30 mph in the afternoon hours, keeping temperatures near 70 degrees for residents of Millbrae, Colma, and South San Francisco on Friday and Saturday. Slightly warmer and drier air arrives at San Bruno He’s Gap by Sunday, with sunny skies and he ushering in a breezy afternoon followed by highs in the mid-70s.
The coldest air in the area sticks to the coast. Daytime temperatures for Pacifica and Daly City residents will remain in the high 60s this weekend due to remnants of fog and haze. By night, minimum temperatures across the peninsula typically drop to the mid-50s.
South Bay and Santa Cruz: Residents along Monterey Bay will experience a weak ebb and flow in the oceanic strata this weekend. Most clouds will clear by mid-morning, with some sun on Friday and Saturday, and mid-day temperatures in the high 60s. Some fog will persist, but most coastal residents of Capitola, Aptos, La Selva and Santa Cruz can expect mostly clear skies, especially through Sunday. Daytime temperatures can briefly hit the low 70s on Sunday afternoon, but then Gulf breezes cool the temperature down again.
Warm, dry weather will ripple through the Santa Cruz Mountains, the Santa Clara Valley, and Highway 101 between Morgan Hill and Gilroy. Friday and Saturday will be mostly sunny with temperatures in the low 80’s and Sunday he’ll be in the high 80’s. Residents of downtown San Jose and Santa Clara can expect highs in the low 90s on Sunday afternoon, while remote mountain residents above 1,000 feet elevation could see highs near the mid-90s. There is In the evening sea air returns, humidity levels rise and temperatures drop to the 60s.
Contact Gerry Díaz: gerry.diaz@sfchronicle.com; Twitter: @geravitywave
