Triple-digit heat is coming to parts of California

A hot spell of weather will engulf much of California this weekend into early next week, marking this year’s first true bout of heat in many places. Some cities may hit 100 or even 105 degrees earlier in the year than ever before.

A slow-building high-pressure system over the northeast Pacific will nudge into California on Thursday, but it’ll ramp up considerably Sunday once it settles over the Desert Southwest.

The hot column of air beneath the high-pressure system is expected to be among the most extreme on record in mid-May, “indicating a very rare event,” according to the National Weather Service Los Angeles.

Daily high temperature records could be challenged anywhere from Sacramento to Las Vegas to Oklahoma City between Sunday and Tuesday.

Heat is forecast to be most extreme in the Central Valley, Antelope Valley and the Mojave and Sonoran deserts. Mid-90s to low 100s are forecast in the Sacramento Valley from Sunday through Tuesday, with temperatures as high as 105 in the San Joaquin Valley. A 105-degree reading in Fresno would be nearly a week earlier than the previous record, and more than a month before such temperatures are measured in a typical year.

Temperatures in the 70s to mid-80s in Tahoe will melt away much of what little snowpack remains in the Sierra Nevada, and nighttime temperatures will remain well above freezing.

Unlike the record-shattering March temperatures, this won’t be a coastal heat wave.

San Francisco is unlikely to warm beyond 80 degrees. Santa Barbara, Long Beach and San Diego will be in the 70s to low 80s, with downtown Los Angeles possibly flirting with 90 degrees on Mother’s Day and Monday. Temperature gradients between the coast and valleys tend to be most pronounced from May through August as the marine layer, often filled with clouds, keeps San Francisco and Los Angeles mild.

A 30-minute drive from the beach will yield vastly different weather.

While fog lingers at the coast, Wine Country, Silicon Valley and the interior East Bay are expected to push into the 90s as early as Sunday. By Monday and Tuesday, high temperatures could be as much as 20 degrees above normal in Santa Rosa, Napa, Walnut Creek, Dublin, San Jose and Redwood City.

While these temperatures are frequent in midsummer, the early arrival could pose danger to people, as human bodies aren’t yet acclimated to summerlike heat. There’s a moderate risk of heat-related illnesses in much of the Bay Area on Monday and Tuesday. Overnight temperatures should yield more relief than a July-like heat wave. Lows are forecast to be in the upper 50s to mid-60s in the Central Valley and the 50s in the Bay Area, except in the hills above 1,500 feet, where the mercury will remain in the 60s to low 70s at night.

Heat waves are becoming bigger, more severe and lasting longer as a result of human-caused climate change, according to the Environmental Protection Agency.

Next week’s spell of heat doesn’t appear to have the longevity of the record-shattering March event that lasted for weeks. An active jet stream — with two branches, one directed toward the Pacific Northwest and another over Baja California — should help shove this high-pressure system east. That’ll open the door for milder marine air to flood the Golden State and take an edge off the early-season heat.

San Francisco: It’s May in San Francisco: Morning clouds, afternoon sunshine and a pronounced temperature gradient across the city. Highs will get stuck around 60 degrees at Ocean Beach, with low to mid-60s in the vicinity of Twin Peaks and near the Golden Gate, with mid- to upper 60s in the Mission and downtown. Afternoon gusts of 15 to 25 mph are expected, strongest along Ocean Avenue and near the water. Overnight lows will be in the low to mid-50s with increasing clouds and fog.

North Bay: Low clouds are expected to cover all of Wine Country and extend into Solano County in the morning. These clouds could manifest themselves as fog along the Highway 101 corridor with low visibility. Mostly sunny skies will emerge by late morning, except along the coast. Highs will be in the mid- to upper 70s in San Rafael, Santa Rosa, Napa, Vallejo and Fairfield and around 80 degrees in Vacaville, Calistoga and Healdsburg. Gusts up to 20 mph are expected in the afternoon. Lows will be in the low to mid-50s with increasing clouds.

East Bay: Clouds are expected to push toward Alameda County and spill into parts of Contra Costa County in the morning. Blue sky should emerge between 9 and 11 a.m. Highs will reach the upper 60s in Richmond, Oakland, Berkeley and Hayward, then low 70s in Fremont and Lamorinda and the mid-70s in Concord, Walnut Creek, Antioch and the Tri-Valley. Afternoon gusts up to 25 mph are possible over Altamont Pass, with slightly lower speeds elsewhere. Overnight lows will be in the low to mid-50s with increasing clouds.

Pacific Coast and Peninsula: Marine layer stratus clouds should be fully developed in the morning, with overcast skies extending across San Mateo County. Clouds should retreat to the coast midday, leaving the Highway 101 corridor beneath sunny skies. High temperatures will be in the upper 50s to low 60s in Daly City, Pacifica and Half Moon Bay, the mid-60s in South San Francisco and Daly City and upper 60s to low 70s in San Carlos, San Mateo and Redwood City. Gusts up to 25 mph are likely in the afternoon. Overnight lows will be in the low to mid-50s with increasing clouds.

South Bay and Santa Cruz: A cloudy morning will yield to a sunny afternoon. Highs will be about 5 degrees above seasonal norms, in the low to mid-70s in Mountain View, Sunnyvale, Milpitas and Santa Cruz. Upper 70s are expected in San Jose, Scotts Valley and Boulder Creek. Highs should reach low 80s in Morgan Hill and Gilroy. Gusts up to 15 mph off the bay are anticipated in the afternoon. Overnight lows will be in the mid-50s in Santa Clara Valley and the upper 40s to low 50s in Santa Cruz County.