‘Heat dome’ to sizzle San Diego for a few more days before shifting east

San Diego County will continue to bake under a high-pressure “heat dome” that has settled over much of the nation, and it won’t be budging until early next week.

A heat advisory for San Diego County’s valleys — including Escondido, El Cajon, San Marcos, La Mesa, Santee and Poway — has been extended through 8 p.m. Friday, the National Weather Service said. And an excessive heat warning is in effect for Southern California deserts, including Borrego Springs.

Temperatures for the inland valleys are expected to hit the 90s and reach up to 102 degrees, the weather service said. Borrego Springs is forecast to continue hitting temperatures of 115 and 116.

The desert slopes will be at an elevated risk for wildfire due to low humidity and winds that could kick up in the afternoon, according to the weather service.

The hot weather has not led California’s electric grid operator to declare a Flex Alert — a call for utility customers across the state to voluntarily reduce their energy use for specific periods of time.

But the California Independent System Operator sent an alert Wednesday to energy providers, encouraging them to bid into the market because supplies are getting tight.

The California ISO, which manages the electricity system for about 80 percent of the state, said it “is seeing some supply uncertainty this evening, because of heat potentially pushing up demand, and high electricity demand across the western U.S.”

The county coastline will be relatively cooler, with highs in the upper 70s to low 80s over the next few days and low clouds and fog in the mornings that are expected to burn off later in the day, said Samantha Connolly, a weather service forecaster. Some beaches may remain socked in throughout the day, she added.

“We will start a very gradual cooling trend Saturday, but it won’t feel significantly cooler until early next week,” Connolly said. That’s when the heat dome is expected to continue weakening and shift to the east.

The high-pressure system has also delayed the typical mid-July start of monsoon season across the Southwest.

People are encouraged to stay out of the sun, take frequent breaks if working or spending prolonged time outdoors, and check up on relatives and neighbors.

Staff writer Rob Nikolewski contributed to this report.