A heat wave will kick in for San Diego mountains and deserts beginning Saturday. Photo credit: @NWSSanDiego, via Twitter

The California Independent System Operator, which manages the state’s power grid, has declared a statewide Flex Alert, in effect from 4 to 9 p.m. Friday, in advance of a weekend heat wave.

The alerts call for voluntary conservation in hopes of reducing strain on the system and preventing outages.

Cal-ISO officials urged residents to set their thermostats to 78 degrees or higher during the alert, avoid using major appliances and turn off all unnecessary lights. Residents should also consider pre-cooling their homes before the alert takes effect and use window coverings to keep rooms cool.

Forecasters expect temperatures to warm throughout Southern California, particularly inland and in the mountains and deserts.

The National Weather Service issued an excessive heat warning beginning 9 a.m. Saturday for San Diego County deserts and the Coachella Valley.

Dangerously hot conditions with high temperatures of 116 to 121 are expected. Low temperatures during the overnight hours will only drop to the upper 80s to lower 90s.

In mountain communities, including Julian and Pine Valley, forecasters expect highs to range from 92 to 102 degrees each day.

The NWS warning is to remain in effect through 8 p.m. Monday.

A continuing onshore flow will keep temperatures cooler along the coast.

City News Service contributed to this report.

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