Temperatures will decline slightly Wednesday but remain in the 90s and 100s in several communities.

An excessive heat warning, which has been extended since it was first issued last week, will be in force until 9 p.m. Thursday in the San Fernando, Santa Clarita, San Gabriel and Antelope valleys, as well as the San Gabriel and Santa Monica mountains. The National Weather Service said conditions in those areas would be “dangerously hot.” In inland Orange County and the Santa Ana Mountains, the warning was in effect until 10 p.m. Thursday.

A less serious heat advisory was in effect until 9 p.m. Thursday in the Los Angeles coastal zone — beach cities, metropolitan Los Angeles, downtown Los Angeles and the Hollywood Hills.

The worst day of the lingering heat wave — the result of high pressure over Nevada — was Tuesday, when highs reached 113 in Northridge, 112 in Van Nuys, 110 in Chatsworth, 109 at Acton and Pasadena, 108 in Saugus, 107 in Lancaster and 105 in Palmdale.

Daily records were set in Woodland Hills, where the 112-degree high broke the previous record of 109 for Aug. 18 set in 1992; at Hollywood Burbank Airport, where the 109-degree high broke the previous record of 100 set in 1986; at Long Beach Airport, where the 100-degree high was one degree higher than the previous record set in 1986; and UCLA, where the 97-degree broke the previous record of 90 set in 1986.

In Orange County, records were set for an August 18 in Anaheim at 105 degrees, breaking the old record of 101 in 1992, and Santa Ana at 106 degrees, breaking the old record of 95 in 2010, according to the National Weather Service.

Forecasters warned that excessive heat and a heightened risk of wildfires would last at least through Thursday.

“Extreme heat will significantly increase the potential for heat related illnesses, particularly for those working or participating in outdoor activities,” according to the weather service. “Take extra precautions if you work or spend time outside. When possible reschedule strenuous activities to early morning or evening.”

“Very hot conditions will continue through the week with very warm nights, especially for valley and interior areas,” according to the weather service. “Temperatures will cool only slightly later in the week and will remain above normal through early next week.

“There is a slight chance of thunderstorms through Wednesday mostly over the mountains and the Antelope Valley.”

The National Weather Service forecast partly cloudy skies in L.A. County Wednesday and highs of 86 in Avalon; 88 at LAX; 92 in Long Beach and on Mount Wilson; 94 in Downtown L.A.; 95 in San Gabriel; 99 in Pasadena; 100 in Burbank; 105 in Saugus; 106 in Woodland Hills; and 110 in Palmdale and Lancaster. Highs in the 90s and 100s are in the forecast at least through Tuesday.

Sunny skies were forecast in Orange County, along with highs of 79 in San Clemente; 81 in Newport Beach and Laguna Beach; 89 on Santiago Peak; 92 on Ortega Highway at 2,600 feet93 at Fremont Canyon; 94 in Irvine; 95 in Yorba Linda and Fullerton; 96 in Mission Viejo and Anaheim; and 97 at Trabuco Canyon. Temperatures will climb down over the coming days but largely remain in the 90s away from the coast.

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