With temperatures expected to be well above 100 degrees again Sunday, California officials were appealing to residents to turn down their air conditioners and hold off on using major appliances until after dark.

The blistering heat wave blanketing California continued to place tremendous strain on the power grid, as some 2,600 homes and businesses in Los Angeles remained without power Saturday after overloaded circuits knocked out power to thousands last week.

Around the state, dozens of cooling centers have been opened in parks, libraries, senior centers and county fairgrounds.

The heat wave wasn't the only extreme weather causing havoc in the state. The misery was being compounded by humidity as moisture moves in from the south, causing concerns about sudden thunderstorms. Flash-flood warnings have been issued for many valley, mountain and desert areas. A funnel cloud touched down in the Antelope Valley desert, but no one was hurt.

The statewide heat wave was expected to bring triple-digit temperatures through the Labor Day weekend.

Highs were expected to reach 113 in Woodland Hills in the San Fernando Valley and well over 100 in many other valley and desert areas of Southern California.

At the same time, unstable weather caused by monsoonal moisture from the south prompted concerns of sudden thunderstorms in valleys, mountains and deserts. A flash-flood watch was in effect through Saturday for those areas in San Bernardino, Riverside and San Diego counties.

In San Bernardino National Forest, a lightening strike from a summer storm was suspected of sparking a wildfire Saturday. Officials said the 150-acre fire, dubbed the "Butler Fire," was burning on steep slopes near Butler Peak at the east end of Big Bear Lake, a reservoir 40 miles northeast of San Bernardino.

The California Independent System Operator, which oversees the state's power grid, said no major shortages were expected through Monday. Still, it urged customers to continue conserving electricity by setting air conditioning thermostats higher and waiting to use major appliances until after dark.

In Los Angeles, which has its own power system, blackouts were reported Friday night as demand overloaded some circuits. About 2,600 customers were still without electricity Saturday afternoon, most of them in the Eagle Rock area near downtown.

There was "tremendous strain" on electrical transmission equipment because nights remain hot and people were running air conditioners around the clock, said Joe Ramallo, a spokesman for the Department of Water and Power.

"I compare it to running a car at 100 mph for 24 hours," he said.

The DWP said its power load peaked at 6,107 megawatts at midafternoon Friday, second only to its all-time record peak of 6,165 megawatts set on July 24, 2006.

Around the state, dozens of cooling centers were open in parks, libraries, senior centers and county fairgrounds. The shelters were sparsely attended on Saturday and hospitals also reported few patients with heat-related problems, said Carol Singleton, a spokeswoman for the state Office of Emergency Services.

However, she urged people to check on vulnerable neighbors, such as senior citizens, to make sure they were getting enough water.

"That's really key, to look out for each other during this heat wave," she said.

In Central California, crews battling a two-month-old wildfire in Los Padres National Forest worried about hot and dry weather that could undo some of their work.

Highs were expected to reach 102 to 107 and coupled with low humidity, "you've got the mixture for extreme fire danger," fire spokesman Larry Comerford said. "We're on alert."

The fire was 97 percent contained and was expected to be fully surrounded on Sunday after scorching 240,207 acres or wilderness, or about 375 square miles.

Several other small fires also were burning in the forest. They were caused by lightning strikes that accompanied dry thunderstorms which hit the area last week. The largest two, at 200 and 30 acres, respectively, were burning in remote areas and no homes were threatened, Comerford said.