Warmth wave may worsen air high quality

A high pressure ridge is triggering the first major heat event of the summer, with residents of inland Bay Area cities facing some heat risk Thursday afternoon. But as hot weather sets in over northern California, pollutants also increase.

The high-pressure system will cause hot air to pour down over Northern California. Residents of inland cities like Sacramento, Santa Rosa and Livermore can expect temperatures to hit the upper 80s and mid 90s on Thursday afternoon.

Air pollution is also likely to be picked up by clockwise air movements on Thursday. Pollutants normally blown away by bay and delta breezes accumulate under the influence of the high pressure system.

The National Weather Service is forecasting a moderate heat risk for the inland valleys on Thursday afternoon due to the sudden rise in temperature in the summer’s first heat event. That means the hot weather poses a threat to residents who are sensitive to heat-related illnesses, such as the elderly, day laborers and the homeless.

The North American Weather Model wind forecast for Thursday evening in the Bay Area. Clean, cool air from the Pacific Ocean should lower pollution levels in cities like San Francisco and Oakland. Calm conditions in the North Bay and East Bay will cause pollution levels to gradually increase through Thursday. By sunset, some pollution relief could reach the inland valleys of the East Bay.

Baron/Lynx

AirNow forecast moderate air quality for most of the Bay Area on Thursday. This means residents can expect airborne haze and health concerns for a small number of sensitive individuals. However, Thursday afternoon’s combination of sea and bay breezes are likely to blow the worst of the haze out of the San Francisco Bay area and into the inland valleys.

Inland residents will be hit hardest by hot weather and poor air quality as breezes will be more confined to the coast in the coming days.

The best choice for cool, clean air Thursday through Saturday is to visit coastal and bay towns on the Way of the Sea Winds. Otherwise, high pressure ridges will continue to cause steady increases in temperature and pollution in cities in the interior of the Bay Area and most of California.

glitch on Thursday

San Francisco: The ocean layer will continue to thin, meaning San Franciscans can expect a quick transition from cloudy skies to sunnier skies by 11 am. Some fog may hang around the Great Highway and Twin Peaks in the afternoon, but a sunny day is forecast for most of the city.

Look for daily highs in the mid-60s on the West Side, the Presidio, Pier 39, and the Embarcadero, while downtown San Franciscans along most of Market Street between Castro and Powell Streets have highs in the upper 60s can expect. However, a light breeze is expected to pick up after 2pm, cooling most of the city to the low 60s by evening.

Some scattered clouds could return towards sunset, but weather models are predicting a mostly clear night with overnight lows in the mid-50s.

Pacific Coast and Peninsula: A thin veil of sea clouds is expected along Highway 1 and parts of the San Bruno Gap.

By the afternoon, the gray skies will be replaced by sunshine and warmer air, allowing maximum temperatures to reach the high 60s in Daly City, Pacifica and Half Moon Bay and lower 70s around San Francisco International Airport. Those south of Millbrae and along the I-280 and 101 corridor can expect mostly sunny skies and light winds throughout the afternoon. Keep an eye out for daily highs in the mid to high 70s in Redwood City, Menlo Park and Atherton — 7 to 10 degrees higher than at the start of the week.

Dry air is forecast to blow gently across the peninsula at night, clearing any fog and sea clouds. Expect clear skies as overnight lows fall to mid-50s on the waterfront and lower 50s on the slopes.

North Bay: Patches of sea fog may be observed along the coast of Sonoma and Marin counties in the morning. Light northeasterly winds are then expected to blow into the inland valleys later in the day, clearing most of the cloud cover and making way for a warm, mostly sunny afternoon.

Residents of the Napa, Santa Rosa, and Petaluma Valleys can expect daily highs in the upper 80s, while temperatures for residents of inland Solano County — Fairfield and Vacaville — will likely hit the mid-90s. The sea layer will persist over parts of Marin County and the Delta region, keeping daytime temperatures closer to the low 80s in Novato and San Rafael and in the low 70s in Tiburon and Vallejo.

Light northeast winds will continue to sweep away sea clouds and make for clear skies tonight as overnight lows drop to the mid 50’s.

east bay: In the morning the sun will slowly break through the layers of fog in the hills of Oakland and Berkeley. In the afternoon, the sky is likely to clear up across the region, with the weather models in Alameda and Contra Costa counties predicting mostly sunny skies and light north-easterly winds.

It might be time to unpack the shorts. Rapid warming is forecast for the afternoon in the San Ramon and Livermore Valleys, with residents of Walnut Creek, Dublin and Pleasanton likely to experience highs in the upper 80’s. Some hotspots around Brentwood and Ulmar could hit the lower 90s just before the delta breeze cools in the evening.

Residents closer to the San Francisco Bay and along corridors I-80 and I-880 — Oakland, Alameda, Richmond, Hayward — can expect a much cooler afternoon thanks to a bay breeze. Watch for highs in the mid-70s and gusts of up to 20mph along the water. Skies remain clear as temperatures plummet to as low as 50C overnight across the region.

South Bay and Santa Cruz: Commuters along the Highway 101 corridor between Gilroy and Morgan Hill could catch fog and haze just before sunrise. Dry northeast winds will quickly dispel most of this, leaving mostly sunny skies and warm air. Look for daytime highs in the low 80s throughout the Santa Clara Valley, with mid-80s possible in downtown San Jose just after 1 p.m.

Expect more sea clouds and relatively cooler air in the Santa Cruz Mountains, with weather models pointing towards daily highs in the upper 70’s. Even cooler air and heavier cloud cover are expected along Highway 1 between Santa Cruz and La Selva, shielding the coast from the region’s warmest air and keeping daytime temperatures closer to the mid-60s.

Some of the cloudiest skies in the region will occur at night just along Monterey Bay, while residents of the Santa Cruz Mountains and Santa Clara Valley can expect clear skies. Look for overnight lows in the upper 50s on the waterfront and mid 50s inland.

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