With triple-digit temperatures forecast in both L.A. and Orange counties Tuesday, a heat advisory indicating temperatures may be hot enough to make some people ill will be in force in much of the Southland from Tuesday morning until Thursday evening.

The heat advisory will be in effect from 10 Tuesday morning until 8 p.m. Thursday in the Santa Monica Mountains and the San Fernando, Santa Clarita and San Gabriel valleys, beach cities, metropolitan Los Angeles and the Hollywood Hills, and inland Orange County, with the National Weather Service forecasting highs of between 100 and 106 in those areas, with the hottest temperatures occurring on Wednesday.

“Hot temperatures may cause heat illnesses to occur,” warned the NWS. “Drink plenty of fluids, stay in an air-conditioned room, stay out of the sun, and check up on relatives and neighbors. Young children and pets should never be left unattended in vehicles under any circumstances.”

No red flag warnings were in effect Tuesday — a red flag warning in the San Gabriel Mountains and the Santa Clarita Valley expired around 5 p.m. Monday — but the weather service warned of near-critical fire conditions through Friday amid Santa Ana winds gusting at 35-40 miles per hour and single-digit humidity levels.

In Los Angeles County, a number of cooling centers will be open from noon to 6 p.m.:

— Platt Branch Library, 23600 Victory Blvd., Woodland Hills, 818-340-9386

— Mid-Valley Regional Library, 16244 Nordhoff St., North Hills, 818-895-3650

— Sun Valley Branch Library, 7935 Vineland Ave., Sun Valley, 818-764-1338

— Lake View Terrace Branch Library, 12002 Osborne St., Lake View Terrace, 818-890-7404

— Eagle Rock Branch Library, 5027 Casper Ave., Los Angeles, 323-258-8078

— Vernon Branch Library, 4504 S. Central Ave., Los Angeles, 323-234-9106

Social distancing requirements due to the coronavirus pandemic have resulted in reduced capacity at the sites.

The NWS forecast partly cloudy skies near the coast, sunny skies over interior locations and highs of 84 in Avalon; 87 at LAX; 88 on Mount Wilson; 95 in Long Beach and Downtown L.A.; 96 in Palmdale; 97 in Lancaster; 99 in San Gabriel; 100 in Burbank and Pasadena; and 104 in Woodland Hills. A cooling trend will begin Friday.

Sunny skies are forecast everywhere in Orange County, along with highs of 80 in Newport Beach and San Clemente; 81 in Laguna Beach; 83 on Santiago Peak; 93 on Ortega Highway at 2,600 feet; 96 at Fremont Canyon; 98 in Mission Viejo, Irvine and at Trabuco Canyon; 99 in Yorba Linda; and 102 in Fullerton and Anaheim. As in L.A. County, a cooling trend will start Friday.

>> Want to read more stories like this? Get our Free Daily Newsletters Here!