The first heat wave of the season came on with a vengeance Tuesday with temperatures predicted as high as 121 degrees in Riverside County and 110 in the inland valleys of Los Angeles County.
The National Weather Service has issued severe weather alerts for Riverside and San Bernardino Counties, saying the heat will result in "very oppressive conditions."
Local news from across Southern California
Forecasters say the hottest part of the heat wave will hit Tuesday and Wednesday. The elevated temperatures and dry conditions have also increased the likelihood of wildfire in the region.
Temperatures are expected to cool off slightly by the weekend, but it will still be hot.
The weather is sending many who don't have to work to beaches. Some summer camps are taking children to cooler, indoor places like skating rinks and movie theaters; others are urging parents to send sunscreen and extra water.
Los Angeles County public health officials opened 55 cooling centers in libraries, senior centers and other gathering places, and urged people who don't have air conditioning to take advantage of them.
The heat was a subject of considerable talk on social media Wednesday, where news stories on the high temperatures were quickly shared and forwarded.
The California Highway Patrol used Twitter to remind people not to endanger children by leaving them alone in hot cars.
"You wouldn't place your baby on the stovetop," the CHP Southern Division tweeted, "so why leave your baby in an oven?"
Others seized the opportunity to link the heatwave to global warming and even fracking, a controversial practice used to extract natural gas from the earth.
One Twitter user, however, urged restraint (even as he forwarded a story on the weather).
"Heat wave?" scoffed Roberto Macias on the social media service. "It's summer at 100 degrees! Not a damn heat wave."