A day after four San Diego County cities tied or set new records for low temperatures, a heat wave is set to wash over the region beginning Friday, with the weather warming significantly throughout the weekend and into next week.

On Thursday, the temperature in El Cajon reached just 75 degrees at its peak, three degrees cooler than the previous record low for the date set in 2012, according to the National Weather Service. In Alpine, the mercury peaked at 76 on Thursday, also setting a new record low temperature for Aug. 24. The previous record low in the city on the date was 79 in 1990.

Vista and Ramona tied their previous record lows for the date. Vista reached just 74 degrees Thursday, matching the record from 2012. Ramona only got to 79, matching the record low from 1989.

Those cool temperatures were thanks in large part to a low-pressure system and deep marine layer that saw much of the county covered in clouds throughout the day. But a high-pressure system is moving over the western United States today, and with it will come extreme heat that could see temperatures in some areas jump more than 20 degrees in less than 24 hours.

In preparation for the hot weather, the NWS issued an excessive heat watch for San Diego County that will go into effect Sunday morning in most areas and Monday morning along the beaches. That heat watch will last through Tuesday evening throughout most of the county.

The hot weather is expected to last until at least Wednesday but could "carry on the entire week," the NWS said.

"It will be important to prepare for the heat and have a plan ready in case your (air conditioning) goes out or you don't have any (air conditioning)," the weather service said. "And don't physically exert yourself outdoors during the days."

The marine layer along the coast will be shallower today than it was Wednesday and Thursday, but was still expected to keep the weather relatively cool and pleasant at the beaches.

High temperatures today will be 71 to 76 degrees along the coast of San Diego but could rise as high as 88 on Monday and 90 on Tuesday, meteorologists predicted. The same warming trend will be seen throughout the county, with temperatures starting to rise today before soaring into triple digits in most areas on Sunday, Monday and Tuesday.

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