Temperatures in the valleys were expected to top 100 degrees Thursday as the region briefly emerged from its prolonged "June gloom" into a sticky, sweltering summer.
As the high pressure to our east intensifies it's raising inland temperatures up into the high-90s and possibly even 100-degree range.
It’s also pushing sub-tropical, monsoonal moisture over the county which is adding high humidity to the already high temps, making things even more uncomfortable.
The heat and 50 percent humidity follows weeks of unseasonably cool weather.
"A lot of people a week or two ago were complaining about the colder-than-normal temperatures. Well, they just got their wish," weather service forecaster David Sweet said.
Skies are mostly clear and should remain that way through the weekend with maybe a little more low cloudiness and fog showing up by Sunday morning.
This afternoon should be the peak of the heat wave with some spots maybe 10 degrees or more above average. The expected high in El Cajon is 98 but we could see triple digits in areas like Ramona, Santee or the San Pasqual valley and with the mugginess it will "feel" even hotter.
"The culprit, if you will, is a large upper-level ridge of high pressure that's over the desert southwest, and it's pushed into Southern California," Sweet said. "It's basically a big dome of warm air."
The mountains will be almost as hot. The expected high in Julian is 95 with a slight chance of afternoon thunderstorms. Continuing east it only gets worse. Yesterday Palm Springs hit 115, Borrego Springs 114 and today will be just as warm and maybe a degree or two hotter. Over the weekend temperatures could top out at or near 118.
The coastal zone is offering the only relief from the heat. Beach highs should stay around 75-77 with an 82 expected at Lindbergh Field.
Over the weekend we should begin feeling a little relief west of the mountains: less humidity and high temperatures running 5-7 degrees cooler.
Authorities urged people to avoid strenuous outdoor exercise and to drink plenty of fluids.