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SAN DIEGO (CNS) – Sweltering heat will linger in the San Diego County mountains and deserts Monday — accompanied by a statewide Flex Alert, urging residents to conserve power in hopes of preventing outages.

Relief will begin arriving Tuesday, according to the National Weather Service.

A strong high-pressure system centered near Las Vegas will weaken through Tuesday, but Monday temperatures will remain hot everywhere except coastal areas, forecasters said.

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.

Meanwhile, the California Independent System Operator, which manages the state’s power grid, has declared a Flex Alert — a call for voluntary conservation in hopes of reducing strain on the system and preventing outages — 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.

Residents are being asked to keep thermostats at 78 degrees or higher, to turn off unnecessary lights and avoid using major appliances in the time when the Flex Alert is in effect. In previous Flex Alerts called in June and during last summer’s heat wave, the agency said consumers “collectively made a significant reduction in their energy use.”

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.

People should be prepared to drink plenty of fluids, stay in an air- conditioned room, stay out of the sun and check up on relatives and neighbors.

While young children and pets should never be left unattended in vehicles under any circumstances, the weather service said that’s especially true during warm or hot weather — when car interiors can reach lethal temperatures in a matter of minutes.

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.

A full list of the locations can be found here.

Highs in the mountains are expected to drop to the mid-80s to low-90s by Thursday, while the mercury in the deserts will gradually drop to about 111 by Thursday, forecasters said.

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