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SAN DIEGO (CNS) – Scorching heat will sear the San Diego County deserts Monday, with temperatures in the rest of the county expected to rise quickly throughout the week, according to the National Weather Service.

A high pressure system will strengthen and expand over the Southwestern United States this week, ushering in the sweltering conditions, forecasters said.

An excessive heat warning will be in effect from 10 a.m. Monday through 9 p.m. Saturday in the deserts.

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

The heat in the deserts could reach 122 on Tuesday and Thursday, then 123 on Friday, forecasters said.

Temperatures in the western valleys could reach triple digits on Tuesday, Wednesday, Friday and Saturday, while high temperatures in the mountains are expected to reach 108 on Tuesday and 106 on Wednesday and Thursday, forecasters said.

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.

To help the public beat the heat, the county is offering a cooling center at the Borrego Springs Branch Library, 2580 Country Club Road, from 10 a.m. to 4 p.m. each weekday.

More “Cool Zone” locations will be announced soon, county officials said.

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.

Highs temperatures on Monday are expected to be in the low-70s to low-80s in coastal areas, the mid-80s to low-90s in the western valleys, the mid- to high-90s near the foothills, the mid-90s to low-100s in the mountains and the low- to mid-110s in the deserts.

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