When it comes to Southern California’s heat wave, the worst is yet to come

Firefighters labored in scorching heat Sunday against five separate brush fires across Southern California as forecasters warned of more triple-digit temperatures in inland communities.

The punishing heat wave broke records on Sunday in Lancaster, Palmdale and Sandberg, which saw highs of 108, 108 and 99 degrees, respectively. With a dry, high-pressure system parked over the Southwest, temperatures are expected to keep climbing through midweek, according to the National Weather Service.

In Riverside County, firefighters tackled a 10-acre brush fire near Beaumont, a 40-acre fire near Moreno Valley and a 20-acre blaze just north of Lake Elsinore. In San Bernardino County, crews reported 50% containment on the Zermatt fire near Wrightwood, which consumed 11 acres.

North of Santa Clarita, Los Angeles city and county fire departments spent a second day working to control an 800-acre fire near Castaic Lake. By Sunday night, crews reported that the blaze had destroyed two outbuildings and was 20% contained. Fire officials had initially said the Castaic fire reached 1,000 acres.

The weekend’s fires are “a precursor of what we expect through the summer,” said Nathan Judy, spokesman for the U.S. Forest Service, which also had crews fighting the Castaic and San Bernardino County fires.

Authorities said crews were helped by higher humidity and relatively docile winds.

Hotter temperatures are forecast to last through Wednesday night, with moderate cooling through the weekend, said Bonnie Bartling, weather specialist at the National Weather Service. Over the next few days, some desert and inland areas will experience “pretty intense” temperatures that could trigger heat-related illness, she said.

“Obviously, you need to stay hydrated with water — and I emphasize, water,” she said. “Do not leave anybody, pets or anybody, in the car for even a short period of time.”

A brush fire near Castaic Lake burned 1,000 acres over the weekend and was only 10% contained as of Sunday afternoon.

(Francine Orr / Los Angeles Times)

The National Weather Service posted a heat advisory through Wednesday evening for the San Fernando and San Gabriel valleys, as well as the Santa Monica Mountains. The agency issued an excessive heat warning through Wednesday for the Antelope Valley.

Highs in the Antelope Valley are expected to reach 108 to 112 degrees, while highs of 100 to 106 were forecast for coastal valleys of Los Angeles and Ventura counties. San Bernardino could see a high of 107 by midweek, while temperatures could top out at 122 degrees in the Coachella Valley, according to the National Weather Service.

Adding to the discomfort, the South Coast Air Quality Management District is predicting unhealthful air on Monday across the Inland Empire and in the San Fernando and San Gabriel valleys.

“Heat does drive the formation of ozone, our primary summer pollutant,” said Sam Atwood, a spokesman for the air district. “And the fact that [the heat wave] is longer means there are going to be many days in a row with potentially unhealthy, or very unhealthy, air quality.”

Public health experts are advising Southern Californians to drink plenty of liquids, avoid the outdoors if possible during the peak heat of the day and check on elderly relatives or neighbors.

Seniors whose homes don’t have air conditioning have been advised to go to shopping malls or a local “cooling center” — typically libraries, senior centers or parks and recreation facilities — during the midday heat.

Despite the rising temperatures, health officials in Los Angeles County have opted so far not to extend the hours at any of their publicly run cooling centers. That’s due in part to the fact that temperatures have dropped significantly during the evenings.

Dr. Jeffrey Gunzenhauser, interim health officer for the county’s Department of Public Health, said his agency plans to keep a close eye on weather conditions in the Santa Clarita and Antelope valleys — to see if cooling stations should have later operations.

“If we do see that the humidity goes up, or high temperatures are persisting into the early evening, we may extend the hours,” he said.

david.zahniser@latimes.com

Twitter: @DavidZahniser

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UPDATES:

8:35 p.m.: This article was updated with information on a fifth brush fire and with current figures on the growth of a fire in Castaic.

This article was originally published at 6:15 p.m.