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SAN DIEGO (CNS) – A heat wave will bring blazing temperatures to the San Diego County mountains and deserts this weekend, according to the National Weather Service.

A strong high-pressure system over the Great Basin will continue to expand westward, ushering in the sweltering conditions through Monday in most of the county, forecasters said.

The NWS issued an excessive heat warning that will be in effect from 9 a.m. Saturday to 8 p.m. Monday in the deserts. A heat advisory is also in effect until 8 p.m. Monday in the mountains.

Highs in the deserts could reach 120 on Saturday and 118 on Sunday, while the mercury in the mountains is expected to reach 102 on Saturday and 101 on Sunday, according to the NWS.

Meanwhile, high temperatures on Friday are expected to be in the low- 70s to low-80s in coastal areas, the low- 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 low- to mid-110s 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.

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.

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