By Steve Gorman

LOS ANGELES (Reuters) - Wildfire warnings were posted across parts of three Western U.S. states on Sunday as a heat wave baked the region in record, triple-digit temperatures, stoking flames in California from the coastal foothills outside Santa Barbara to desert brush near the Mexican border.

Excessive heat advisories and "red flag warnings" for extreme fire conditions were in effect across southern portions of California, Nevada and Arizona, the National Weather Service reported on the eve of the first official day of summer.

In the Los Angeles suburb of Burbank, the mercury topped out at 109 degrees Fahrenheit (42.8 Celsius), shattering the prior record high for the date of 104 degrees set in 1973. In Phoenix, Arizona, the temperature climbed to 118 degrees, 3 degrees above the previous high mark for the date reached in 1968.

With rising demand for air conditioning expected to test the region's generating capacity, the California Independent System Operator, which runs the state's power grid, urged consumers to conserve daytime electricity on Monday.

Forecasters said record-breaking heat would persist through Tuesday, especially in the Desert Southwest, where temperatures could reach as high as 120 degrees.

"These extreme temperatures can be life-threatening," the Weather Service said on its website.

Fire officials said the heat was a major factor in worsening a wind-driven blaze roaring through dry brush and chaparral about 50 miles east of San Diego, north of the Mexico border, forcing evacuations of dozens of homes in the desert community of Potrero.

The blaze, which erupted Sunday morning, had blackened about 1,500 acres and was still burning unchecked over steep terrain and drought-parched vegetation by evening, San Diego County Fire Captain Kendal Brotisser said.

About 200 miles to the north, excessive heat also continued to plague crews battling the so-called Sherpa Fire, burning for a fifth day in the canyons and foothills near Santa Barbara.

That blaze, which has charred nearly 7,900 acres and forced hundreds of people from their homes, was 51 percent contained as firefighters took advantage of abating "sundowner" winds that had initially propelled the flames.

A much smaller brush fire flared briefly beneath a freeway interchange near downtown Los Angeles, destroying three storage sheds, damaging two homes and snarling traffic in the vicinity as firefighters rushed to douse the blaze.

Meanwhile, in New Mexico, local authorities declared a state of emergency due to a five-day-old timber fire that has consumed some 17,615 acres (7,129 hectares) and destroyed about two dozen homes southeast of Albuquerque.

(Additional reporting by Ian Simpson in Washington; Editing by Andrew Bolton and Himani Sarkar)