Flash flood warnings were issued in parts of the Southland Tuesday afternoon, including for the Apple Fire burn scar area, as a thunderstorm moved through the region during an intense heat wave.

The first thunderstorm formed near Big Bear Lake around noon, according to the National Weather Service. 

“Activity will continue to pick up over the next few hours in the mountains, and should gradually drift toward the Inland Empire later today,” the weather service tweeted. 

The system moved into other parts of San Bernardino County, including Joshua Tree, where lightning sparked “numerous” vegetation fires, according to the county’s Fire Department. The largest blaze grew to about 150 acres before forward progress was halted.

In Los Angeles County, showers and isolated thunderstorms were initially detected in parts of the Antelope Valley shortly after 1 p.m. About an hour later, a thunderstorm began developing in the area, with activity reported “between Big Pines, Lake Los Angeles, Acton, and Palmdale,” according to NWS.

Around 3:45 p.m., additional storms formed east of the 5 Freeway near where the Lake Fire was burning in the Angeles National Forest, as well as in parts of northwest Ventura County.

A “strong” thunderstorm near Mount Pinos and south of Pine Mountain Club prompted NWS to issue a significant weather advisory late in the afternoon.

Forecasters warned the storms could bring dangerous lightning, gusty winds, brief intervals of heavy rain and even hail to the area.

Flash flood warnings have gone into effect in areas throughout Southern California, including for Oak Glen and Cherry Valley, potentially putting recent burn scarred areas from the recent Apple Fire at risk through late into the afternoon.

“Excessive rainfall over the burn scar will result in potentially deadly and destructive debris flows. The debris flow can consist of rock, mud, vegetation and other loose materials,” the weather service said.

The Apple Fire, which broke out on July 31, has burned about 52 square miles and is almost fully contained.

Hemet, San Jacinto, Valley Vista, Running Springs, Arrowbear Lake And Green Valley were among the areas under flash flood warnings during the afternoon.

Anyone who encounters flooding should immediately move to higher ground and avoid driving through floodwaters, forecasters advised.

Meanwhile, an excessive heat warning was in place throughout the region amid searing, triple-digit temperatures.

By 12:45 p.m., the heat index had already reached 119 degrees in Palm Springs, 111 degrees in Riverside and 109 degrees in Santa Ana, according to the weather service.

In San Diego County, the heat index values along the Interstate-15 corridor eclipsed 115 degrees by early afternoon, thanks to the humidity.

“These are remarkable numbers for inland San Diego County – feeling just as hot as the Coachella Valley,” NWS tweeted.