National Weather Service heat map shows forecast Tuesday high temperatures.

Scorching heat will spread throughout San Diego County Tuesday as a potentially record-breaking heat wave builds this week, according to the National Weather Service.

A high pressure system strengthening and expanding over the Southwestern United States is ushering in the sweltering conditions for all areas except along the coast.

“Strong high pressure over the southwestern states will continue a long-duration heat wave for inland areas this week, peaking today through Friday. Inland days will be very hot with nights very warm,” forecasters said.

An excessive heat warning will be in effect through 9 p.m. Saturday in the deserts.

The warning will also include the western valleys and the mountains starting at 10 a.m. Tuesday and lasting through 9 p.m. Saturday.

Highs temperatures Tuesday are forecast to be 76 to 81 along the coast, 85 to 90 inland, 89 to 94 in the western valleys, 99 to 104 near the foothills, 97 to 105 in the mountains and 116 to 121 in the deserts.

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.

Starting Tuesday, the county will open eight new “Cool Zone” locations to help the public beat the heat. The zones will be 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.