High temperatures continue to cook parts of Southern California, with many areas reaching well over triple digit temperatures.
High pressure and moisture will raise desert temperatures and humidity to "hazardous'' levels, the watch notification said.
The National Weather Service said the heat wave, which began Tuesday, would persist through early next week as a result of dominant high pressure over the Southwest combined with weak onshore flow.
A dozen cool centers in the Coachella Valley will be open while the heat advisory is in effect. Centers were also open in the Santa Clarita area including the Valencia Library and Santa Clarita Valley Senior Center.
In the Antelope Valley, the Lancaster Soccer Center and Desert Sands Park were open. Cinemark 12 was offering $1 movies for seniors.
"The moisture from southeast Mexico combined with very high temperatures is going to make it feel pretty uncomfortable,'' meteorologist James Thomas said. "The rising heat plus humidity, that's when you get your problems.''
Those prone to heat exhaustion and heat stroke should be extra careful, Thomas said.
The NWS urged people planning outdoor activities to wear lightweight clothing and drink plenty of water, and it warned to never leave children, the elderly or pets in enclosed automobiles, even for a short period of time.
Thomas said the area will "cool off'' slightly over the weekend, with temperatures reaching 113 Saturday and 115 Sunday. Humidity should decrease after Friday, he said.
There is a chance of thunderstorms in the mountains west and northeast of Palm Springs Friday and Saturday afternoon, he said.