The National Weather Service office in San Diego issued separate heat advisories for the mountains and inland valleys through Friday.
The first advisory covers the county mountains, including the cities of Julian and Pine Valley, from Wednesday morning through 9 p.m. Friday.
The second advisory encompasses the inland valleys and cities of Escondido and Poway beginning at 10 a.m. Thursday and continuing to 9 p.m. Friday.
“High pressure will continue above-average high temperatures for inland areas into the weekend,” the weather service said.
Highs on Thursday were forecast to be 75 to 80 at the beaches, 83 to 88 farther inland, 86 to 91 in the western valleys, 94 to 99 near the foothills, 90 to 96 in the mountains and 105 to 110 in the deserts.
Because of excessive heat across the West, the California Independent System Operator issued a Flex Alert for 4 p.m. to 9 p.m. Wednesday and later extended it to Thursday.