California's central and southern regions are baking in a late summer season heat wave that's knocked out power in some areas and left others sweltering in temperatures above 100 degrees Fahrenheit this week.

A hot-air mass is settling over California's Central Valley, while moisture surging north will increase humidity, the National Weather Service in Hanford, California, said in a heat advisory posted earlier today. The heat index, a measure of temperature and humidity that represents how hot it feels, will remain above 110 degrees Fahrenheit (43 degrees Celsius) through tomorrow before cooler air moves into the region, the service said.

Triple-digit temperatures in Southern California helped set power-demand records for the year, according to the Los Angeles Department of Water and Power, the nation's largest municipal utility. About 164 customers have lost power today. Yesterday, after 2,000 customers lost power in the Sun Valley area, the utility installed a temporary transformer and restored service.

''Peak demand today is expected to be 6,100 megawatts as we go into the hottest part of the afternoon,'' said department spokeswoman Gale Harris in an interview. The highest demand so far today was 5,786 megawatts at 12:52 p.m. local time. One megawatt can power 800 average U.S. homes.

Yesterday's energy peak was 6,039 megawatts, the utility said. When temperatures soared in 2006, the utility provided a record 6,165 megawatts on July 24.

The utility, which has 1.4 million electricity customers and 680,000 water customers in Los Angeles, said it has crews ready to respond to power failures.

With hot weather expected to last through the weekend, utility officials and Los Angeles Mayor Antonio Villaraigosa asked customers to reduce consumption.