LOS ANGELES (AP) — Southern California will see dangerously high temperatures this week and a chance of thunderstorms and flooding in the mountains as a building high-pressure system delivers a late-summer scorching, forecasters said Tuesday.

Excessive heat warnings were issued into Thursday night in many areas, with highs expected to reach 102 to 106 degrees in the valleys and lower mountain elevations.

“We do measure temps in the shade, so if you’re standing out in the sunshine it could be 10 to 15 degrees warmer,” said Stuart Seto of the National Weather Service in Oxnard.

It was 95 in downtown Los Angeles by early Tuesday afternoon and 101 in Riverside.

Nighttime won’t cool down much — maybe into the 70s — because high pressure will keep cooler air from moving ashore in the late afternoon and evening, Seto said.

Wednesday and Thursday will likely be the hottest days before cooling sets in during the weekend, forecasters said.

Coupled with the heat was a chance of isolated thunderstorms in the mountains and deserts due to remnant moisture from Hurricane Linda in the Pacific off Mexico. Flash flood watches and warnings were issued through Thursday for the mountains and deserts.

Beaches could see highs of around 90 but swimmers should be careful because there could be a risk of large waves and rip currents, according to the weather service.

Los Angeles County’s health officer declared a heat alert and warned people to be careful of working or playing too hard in the heat.

“When temperatures are high, even a few hours of exertion may cause severe dehydration, heat cramps, heat exhaustion and heat stroke. Others who are frail or have chronic health conditions may develop serious health problems leading to death if they are exposed to high temperatures over several days,” Dr. Jeffrey Gunzenhauser, the county’s interim health officer, said in a statement.

Children, the elderly and pets should never be left alone in homes without air conditioning or in cars with the windows closed, he said.

The county and others designated libraries and other air-conditioned buildings as cooling centers where people could wait out the heat.

“If you have an elderly or infirm neighbor without air conditioning, make sure that they get to a cooling center or other air conditioned space between the hours of 10:00 a.m. and 8:00 p.m.,” Gunzenhauser said.