LOS ANGELES — Californians struggled to keep cool Thursday as temperatures again soared into triple digits and power grid operators girded for possible record electrical demand amid a heat wave that was nearing its peak in the southern half of the state but just firing up in the north.

The temperature in downtown Los Angeles shot past 90 degrees early in the day and a spectator collapsed just as Mayor Eric Garcetti was about to start a late-morning ceremony to mark the reopening of the city’s historic Angels Flight funicular railroad.

Garcetti came to the aid of the man, who appeared to recover but was taken away by paramedics.

Monsoonal moisture flowing into Southern California added another dimension to the torrid conditions, unleashing small but intense thunderstorms and a barrage of lightning bolts in scattered spots. One downpour helped douse one of several small brush fires that erupted in the region, some of which were sparked by the lightning.

Colton, a city of 53,000 about 60 miles east of Los Angeles, widely lost power when lightning struck its main electrical substation and full recovery was expected to take hours. In Los Angeles, the Department of Water and Power said 14,000 customers had outages overnight.

Downtown LA topped out at 102 but other parts of the metropolitan area fried at temperatures as high as 111 while inland cities to the east and the deserts were hotter.

Beaches offered limited relief as the weeklong heat finally extended to the shoreline, sending temperatures up into the 80s.

The National Weather Service said the high pressure system spawning the heat would continue dominating the West into next week. The only question was where it would be dangerously hot and where it would just be hot.

Friday was expected to be the hottest day of the heat wave in Southern California, but just the start of broiling weekend in Northern California.

The operator of the state’s power grid predicted an all-time record electrical demand of 50,950 megawatts on Friday and issued a statewide Flex Alert — a call for voluntary electricity conservation — from 1 p.m. to 10 p.m. Friday.

The California Independent System Operator Corp. said that “all available resources are on line and ready to respond.”

The current all-time record demand was 50,270 megawatts, set on July 24, 2006.

Forecasts show the ridge of high pressure expanding over Northern California, bringing record or near-record temperatures, the San Francisco National Weather Service office said.

Temperatures exceeding 110 will be possible in interior valley areas east of San Francisco Bay on Friday and Saturday afternoon. Among high numbers shown by various models, Livermore could range from 111 to 116 degrees.

“Considering that 115 is the all-time record high at that spot (dating back to 1950), being that close and potentially breaking the record is notable. Even spots closer to San Francisco Bay will see hot conditions,” the weather service wrote.

In preparation for the coming heat, several dozen emergency Red Cross workers who had planned to fly to Texas to help with flood relief were told to remain in the Bay Area, said Cynthia Shaw, spokeswoman for the Northern California chapter of the American Red Cross. If the heat wave triggers wildfires, she said, they will be needed to staff shelters here.

“We have to be mindful that it’s going to be a very, very hot weekend and we want to make sure we have resources here in our neck of the woods,” she said Thursday.

Coordinators were lining up possible shelter sites and putting volunteers on standby across the region.

Seventeen volunteers left for Houston Thursday morning. Those who stayed behind may be deployed to the Gulf Coast after the California heat wave abates.

Forecasters were also keeping an eye on Tropical Storm Lidia, which was south of Baja California on Friday. It is expected to move northward and dissipate, but some of its moisture could continue on into Southern California.

Associated Press writers John Rogers and Christopher Weber contributed to this report.

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