California heat wave: These areas will face dramatically high temperatures today

山脊的高压将热量risks for residents across coastal cities likeSan Franciscoand San Diego, but more widespread high temperatures will be concentrated inland by Thursday and into the weekend.

Heat risk outlook

Temperature is one of many elements at play when it comes to how vulnerable communities are to heat.

美国国家气象局预测热风险based on climate and societal factors like age, sensitivity to warm weather, income and access to cooling shelters. In places like the Bay Area this is important since microclimates and socioeconomics vary widely between neighborhoods and cities.

The National Weather Service is calling for a slight heat risk across much of the Bay Area this week, with temperatures likely reaching the 70s and lower 80s along San Francisco Bay. These are places where temperatures rarely go beyond the upper 70s, making residents more susceptible to spikes in warm weather compared to communities in more inland regions like San José and Walnut Creek.

Outside San Francisco, daytime highs are likely to reach the upper 80s to lower 90s by Wednesday afternoon, with more widespread 90s possible on Thursday and Friday. And because this is the first big warm-up of the year, sensitive populations, including outdoor laborers and those who have limited access to shade, will face heightened risk of heat-related illnesses. This moderate heat risk includes places like the Salinas Valley, where daytime temperatures are likely to reach the lower 90s through Saturday.

The moderate heat risk will also extend into parts of the Central Valley by Thursday and into Saturday, where residents in cities like Fresno and Bakersfield can expect daytime temperatures in the 90s.

Still, this ridge of high pressure responsible for the hot weather isn’t set to have major impacts on nighttime temperatures, so residents can get some relief once the sun goes down.

Nighttime temperatures are expected to remain consistent throughout the duration of the heat wave, with most of the coast and inland areas in the Central Valley likely to experience lows in the 40s and 50s. This heat wave is then looking to flow out of California by Saturday night, allowing temperatures to return to normal by the end of the weekend.

Wednesday breakdown

San Francisco:A clash between cool, moist air and warm, dry air is slated for most of the city on Wednesday. Patches of fog are on tap for the hills and west side through 9 a.m. before dissipating in the sunshine. Temperatures should quickly rise over the morning, reaching the upper 60s by noon. Downtown is expected to reach the lower 70s after 2 p.m. — that’s 7 to 10 degrees higher than April’s average daytime temperature of 63 degrees.

San Franciscans looking to enjoy the sunshine and warmth will want to check out Dolores Park and Alamo Square, where upper 60s to lower 70s are likely to persist into midafternoon. A faint sea breeze and some high clouds are likely to filter into Twin Peaks, Diamond Heights and most of the Sunset and Richmond districts, keeping daytime temperatures on the west side closer to the mid-60s.

Some of Wednesday’s warmth will linger into the evening as temperatures fall to the upper 40s, though a few hills could stay closer to the lower 50s overnight.

Pacific Coast and Peninsula:A mix of fog at ground level and wisps of cirrus clouds are looking to spill onto the coast along Highway 1 between Pacifica and Half Moon Bay on Wednesday morning. Warmer, drier air should dissipate most of the clouds, raising temperatures to the upper 60s. Residents in Daly City, Millbrae and around San Francisco International Airport can expect a similar warming trend as daytime highs reach the lower 70s.

An afternoon sea breeze will shave off some of the warmth on the coast and the San Bruno Gap, but that won’t be the case for residents shielded by east-facing hills and mountains. Daytime temperatures along most of the I-280 and 101 corridors south of San Mateo are expected to reach the upper 70s — about 7 to 10 degrees higher than April’s average daytime high of 68 degrees in Redwood City.

Stray clouds are likely to spill onto the Peninsula after sunset as temperatures fall back to the lower 50s overnight.

North Bay:The microclimates of Marin, Sonoma, Napa and Solano counties will be on full display over the course of the day as cool air undercuts some of the hot air mass on tap for Wednesday afternoon. Look for a light breeze near Point Reyes, Bodega Bay and along the bayside towns of Sausalito and Tiburon; the cool air by the water will keep a few clouds around and daytime temperatures closer to the lower 70s. The story will be a bit different closer to Novato, San Rafael and Mount Tam, where the lack of a breeze and sunnier skies will allow daytime temperatures to climb to the upper 70s to lower 80s.

Residents by the delta region, including Vallejo, will also experience daytime highs in the lower 80s as warm, dry air outperforms the cool, misty winds off San Pablo Bay. Afternoon temperatures are looking to rise higher as you head into Fairfield, Vacaville and the Napa, Petaluma and Santa Rosa valleys — mid- to upper 80s are expected after 2 p.m.

Downtown Napa, whose average daytime temperature for the month of April is 72 degrees, is looking to reach the upper 80s.Temperatures are expected to gradually decrease by the evening, but some of the warm air from earlier in the day is likely to linger, keeping nighttime temperatures from falling below 50 degrees across most of the region.

East Bay:Aside from a few high clouds in the afternoon, Wednesday is looking to be mostly sunny and warm across Alameda and Contra Costa counties. A faint breeze off San Francisco Bay will help keep daytime temperatures in Richmond, Alameda and Emeryville in the lower to mid-70s Wednesday afternoon, while warm air off the Oakland and Berkeley hills raise temperatures across the rest of the I-80 and I-880 corridor to the upper 70s. Downtown Oakland should reach the upper 70s by 1 p.m., roughly 10 to 12 degrees warmer than April’s average daily high temperature of 67 degrees.

Residents east of the Caldecott Tunnel are in for even warmer weather, with hot, dry air raising daytime temperatures in Walnut Creek, Dublin and most of the San Ramon and Livermore valleys to the upper 80s. Lower 90s are possible near Livermore and Concord after 2 p.m.

The warm air will continue to flow over the region even after sunset, keeping nighttime lows in the lower 50s by the water and mid-50s farther inland.

South Bay and Santa Cruz:The cool breeze and low clouds off Monterey Bay will struggle to fend off the warmth that’s set to spill into Santa Cruz County on Wednesday. Capitola, Soquel, Aptos and Pasatiempo can expect daytime temperatures in the lower 70s this afternoon as clouds burn off. Much warmer air will set up shop over the Santa Clara Valley, where the air mass is likely to reach temperatures closer to the upper 80s. Downtown San José is set to reach the upper 80s by 1 p.m. That’s almost 20 degrees higher than April’s average daytime temperature of 69 degrees at San José International Airport.

The Santa Clara Valley’s warm, dry air will snake into the Santa Cruz Mountains and the 101 corridor between Morgan Hill and Gilroy, steadily raising daytime temperatures to the lower 80s by the afternoon. Conditions will improve after sunset as temperatures slowly fall back to the mid-50s across the valleys and by the water.

Reach Gerry Díaz: gerry.diaz@sfchronicle.com; Twitter: @geravitywave