California is facing a prolonged late-summer heat wave this week, with widespread triple-digit temperatures starting in the south and spreading northward, the National Weather Service said Monday.

Excessive heat watches will go into effect Wednesday morning and remain in effect through Sunday evening in a large swath of Southern California, including much of the normally temperate coastline, forecasters said.

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Temperatures were predicted to top 100 degrees in many valley and mountain locations.

"This heat may be record breaking and will likely produce a very high heat illness risk," the Los Angeles-area weather office wrote.

The torrid conditions will be caused by high pressure that was already pushing into the state and making it difficult for onshore flow of marine air.

"These trends are forecast to continue and will likely set up (a) prolonged and likely dangerous heat event," the office said.

The heat wave will expand into Northern California later in the week.

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Temperatures in the Sacramento Valley could range as high as 112 on Sunday and Monday.

"Be cautious about planning outdoor activities for the holiday weekend!" the San Francisco Bay Area weather office wrote.