LOS ANGELES (AP) — After weeks of mild summertime temperatures, Southern Californians are sweltering in a heat wave expected to last for days and potentially test an electrical grid lacking one major power plant.

Temperatures topped 100 degrees Tuesday in many valley and inland areas but moderated closer to the coast, where beach cities had readings in the high 70s or low 80s.

The 108 degrees in Woodland Hills on Monday equaled the record high set on Aug 6, 1997, and the 107 degrees in Ontario tied a record set for the date in 1983.

The heat settled in with the San Onofre nuclear power plant, which can produce enough electricity to power 1.4 million homes, still out of action. The twin-reactor plant on the coast between San Diego and Los Angeles has been offline since January, when unusual wear was discovered in its steam generators' tubing, which carries radioactive water.

Despite the outage, there appeared to be ample electrical reserves, said Stephanie McCorkle of the California Independent System Operator, which manages much of the state's power grid.

"The good news is we have all power plants except San Onofre in good working order," she said.

Nonetheless, utilities urged conservation.

Southern California Edison, which serves an area with nearly 14 million people, said in a statement that it anticipated being able to meet forecast electricity demand but that conservation could provide additional reserves in case of an emergency such as mechanical problems affecting a power plant or wildfires shutting down transmission lines.

SoCal Edison and the Los Angeles Department of Water and Power both noted that conservation also helps prevent strain on the power system on hot summer days.

Both utilities urged residents to set air-conditioning thermostats no lower than 78 degrees and limit use of appliances, among other recommendations.

The National Weather Service said the heat wave — not uncommon for this time of year — was being produced by a big high-pressure ridge centered over Utah and covering most of the western half of the United States.

Midweek arrival of monsoonal moisture could make conditions even more oppressive as humidity rises, forecasters said.

Excessive heat watches and warnings were to go into effect in various areas on Wednesday.

Forecasters also said conditions could produce flash flooding in mountain and desert areas.