Sunrise from Mount Woodson near Poway on Monday morning. Image from UCSD HPWREN camera

Sweltering heat will linger in the San Diego County mountains and deserts Monday accompanied by a statewide Flex Alert, but relief will begin arriving Tuesday.

The San Diego office of the National Weather Service said temperatures will remain hot everywhere except coastal areas, but the strong high-pressure system centered near Las Vegas will finally weaken Tuesday.

An excessive heat warning will be in effect until 8 p.m. Monday in the deserts. A less severe heat advisory is also in effect until 8 p.m. Monday in the mountains.

High temperatures on Monday are expected to be in the low 70s to low 80s in coastal areas, the high 70s to mid 80s in the western valleys, the low to mid 90s near the foothills, the low-90s to low 100s in the mountains and the high 100s to mid 110s in the deserts.

The California Independent System Operator, which manages the state’s power grid, has extended the Flex Alert — a call for voluntary conservation in hopes of reducing strain on the system and preventing outages — for a fourth day, from 4 p.m. to 9 p.m. Monday.

“Demand is expected to increase on Monday, July 12. The ISO has called for power plants to delay any planned maintenance and to be available. Californians are asked to remain vigilant in case we need conservation help,” the ISO tweeted early Sunday afternoon.

The weather service said the extreme heat will significantly increase the potential for heat-related illnesses, particularly for those working or participating in outdoor activities.

The county has opened nine “Cool Zone” locations to help the public beat the heat. They are located in Alpine, Borrego Springs, Fallbrook, Lakeside, Potrero, Ramona, Santa Ysabel, Spring Valley and Valley Center.

City News Service contributed to this article.

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Chris Jennewein

Chris Jennewein is Editor & Publisher of Times of San Diego.