Thursday marks the first day of summer in Southern California and it won’t take long for a heat wave to follow.

“We’re talking about a real big warmup for the upcoming weekend,” KTLA Meteorologist Henry DiCarlo said during the final hours of spring.

As the season changes, so will the mild weather the Southland has been enjoying.

Residents will begin to notice warmer weather on Friday but the big jump in temperatures arrives over the weekend and will continue into next week, according to the National Weather Service.

A heat advisory has been issued between 10 a.m. Saturday and 8 p.m. Sunday for parts of Los Angeles County, the Inland Empire and Orange County.

Los Angeles County areas include the San Fernando Valley, San Gabriel Valley and Santa Clarita Valley where temperatures could reach over 100 degrees.

Temperatures in some Inland Empire locations could reach 104 degrees on Saturday and 101 degrees on Sunday.

Triple-digit conditions are expected to continue in some desert locations through at least Wednesday of next week.

“Hot temperatures may cause heat illnesses,” the Weather Service warned. “Drink plenty of fluids, stay in an air-conditioned room, stay out of the sun, and check up on relatives and neighbors.”

The heat wave is not the only thing residents should be aware of.

“Now we’re going to increase the chance of thunderstorms [Sunday] over our local mountains and desert areas. Some of those could creep into some of the valleys, like the Inland Empire,” Henry said.