LOS ANGELES (AP) — Southern California roasted in triple-digit temperatures Monday and forecasters predicted potentially record-setting heat for at least another day — along with extreme fire danger for baking hillsides.

The National Weather Service issued heat warnings or advisories through Tuesday for valleys, mountains and inland areas from Ventura County to the Mexican border. Increasing humidity beginning later Monday would do nothing to cool things down and, in fact, will make heat illnesses more likely, forecasters said.

“The body’s not able to cool itself” because more moisture in the air makes it harder for perspiration to evaporate, said Kathy Hoxsie, meteorologist with the National Weather Service in Oxnard.

The heat began building over the weekend. By noon Monday, temperatures hit 106 in Mission Hills in the San Fernando Valley area of Los Angeles; 101 in San Fernando; 102 in Riverside and 107 in Palm Springs. It was 93 in downtown Los Angeles and 97 in San Diego.

The heat wave, caused by a ridge of high pressure over the area, kept temperatures 5 to 15 degrees above normal for the season, Hoxsie said.

Los Angeles County designated dozens of libraries, senior centers and recreational buildings as cooling centers where people could go to beat the heat.

Officials urged people to drink lots of water, take it easy and stay out of the heat if possible.

“When temperatures are high, even a few hours of exertion may cause severe dehydration, heat cramps, heat exhaustion, and heat stroke,” said a statement from the Los Angeles County Department of Public Health.

Children, the elderly and pets shouldn’t be left unattended in homes without air conditioning or cars, even if the window is left open, agency said.

The San Diego Unified School District said all of its schools that don’t have 100 percent air conditioning in classrooms would be on a minimum-day schedule on Monday.

The temperature will begin to fall Wednesday but at the same time, moisture from Hurricane Odile in Mexico’s Baja California would increase Southern California humidity.

“Most people won’t notice a big difference on Wednesday,” Hoxsie said of the heat. “When it’s more humid, it feels just as miserable.”

Thursday will see even more humidity — perhaps as much as 40 percent in some areas — and with it, a chance of thunderstorms in the mountains.