A heat advisory and excessive heat warning are in effect Friday for San Diego and neighboring counties, according to the National Weather Service.
Strengthening high pressure above the desert will bring hot weather inland Friday and Saturday.
An excessive heat warning is in effect until 9 p.m. Saturday.
A warning like this means heat illnesses are likely if preventative measures are not taken.
At Cowles Mountain, some people were out early trying to get in some exercise before the heat set in. Some of the hikers were up before dawn, tackling the trail in the dark.
"It's a little dangerous," said hiker Lindy Castillo. "You get tired a lot faster. The earlier, the better."
Temperatures in the Inland Empire could reach 105 to 110 degrees with Palm Springs expected to see 119 degrees on Saturday.
Officials issued red flag warnings for fire danger in Santa Barbara County's mountains and south coast due to dry, hot winds starting Thursday evening.
The Cleveland National Forest has announced it will close Three Sisters and Cedar Creek Falls for public and rescuer safety.
To the north in the Sierra Nevada, fire warnings were in effect from Sequoia to Yosemite, while flood warnings due to snowmelt are posted in northern Mono County in California and southern Lyon County in Nevada.
Excessive heat warnings were also in place in Arizona, where temperatures in Phoenix could hit 117 Friday. That's only a few degrees less than the record-breaking heat that baked the state last month.
Las Vegas, where the mercury could hit 113 degrees during the day Friday, won't get much of a release at night, said weather service meteorologist Chelsea Kryston. Overnight temperatures will linger at around 90.
The heat building into the weekend results from high pressure over the desert.
Spiking temperatures will be accompanied by more moisture moving up from Mexico that could add humidity to the heat wave in some areas.
In addition, a heat advisory has been issued for Friday with temperatures reaching triple digits in Ramona, Escondido, El Cajon and Alpine.
Drink plenty of fluids, stay in an air-conditioned room or stay out of the sun and check on relatives and neighbors.
Young children and pets should never be left unattended in vehicles under any circumstances.
Cooling zones have been set up across the county.