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Labor Day heat wave begins in California with expectations of scorching triple-digit temperatures up to 115ºF

Millions in U.S. facing excessive heat
More than 50 million Americans facing excessive heat 02:01

A heat wave started to set in on the West Coast on Thursday as California anticipates some of its hottest days of the season through Labor Day weekend. Triple-digit temperatures are expected in the "prolonged" heat, according to the National Weather Service, with much of the state under excessive heat warnings and watches. 

One of the heat warnings said temperatures will be 8 to 12 degrees above normal in northwest Arizona, southeast California and southern Nevada, while some of the northernmost areas of California are under excessive heat watches. 

"Minimum temperatures will struggle to fall below 80 degrees for many locations, which could set late August and early September records, including in Las Vegas," the National Weather Service said. "This, coupled with the increased recreation due to the upcoming holiday weekend, make this heat event particularly unique and potentially impactful."

NWS Los Angeles said in its area forecast that the heat could be "record breaking" and "will produce a very high risk of heat illness." The heat wave sets in as about 200 homes in Castaic, northwestern Los Angeles, had to be evacuated on Wednesday because of a wildfire. The Route Fire burned more than 4,600 acres as of Wednesday night. 

San Diego's excessive heat warning went into effect on Tuesday morning and is not expected to end until Monday night, with temperatures forecast to reach 97ºF,  while San Bernardino and Riverside County will see temperatures as high as 110ºF, and Coachella Valley and San Diego County deserts could reach 115ºF. 

San Diego, the Santa Ana mountains and Orange County are also expected to hit triple-digits, and portions of southwest Arizona are expecting afternoon temperatures between 102ºF and 115ºF. 

Several other areas in the Catalina and Santa Barbara islands are under an excessive heat watch, with areas expected to hit "dangerously hot conditions" up to 100ºF. 

More than 50 million people in the U.S. are at risk of extreme heat based on the agency's advisories, warnings and watches, according to data from

The Weather Channel meteorologist Kelly Cass said that the above-average temperatures could set all-time records, even beyond California. 

"For example, with upper 90s expected in places like Great Falls, Montana, for several days in a row; their hottest temp is usually in July with an average of 85 degrees. Usually by September we are down in the mid 70s," she said. 

Residents in affected areas are advised to stay in air conditioning as much as possible and avoid the sun as much as possible. People should also be wary heat exhaustion and heat stroke, which can be fatal if left untreated. The elderly, children and those with chronic illnesses are most at-risk of developing heat-related illnesses, as well as those in urban areas that don't have as much shade and cooling infrastructure. 

And these kinds of weather events, as well as the droughts that have diminished reservoirs in the West, are likely to become more normal as global temperatures continue to rise. 

As global temperatures warm, Climate Central predicts that San Francisco, California, which is set to be scorched by a heat wave this week, will have summer temperatures nearly 6ºF warmer on average by 2100, feeling the equivalent of Southern California's Long Beach by 2060.  Climate Central

"Sadly, as our climate changes, the drought worsens and the fire season gets longer," Cass said. "Without storms to give us the much needed rain, we feel the effects of more heat waves: more intense and longer duration." 

By mid-century, nearly one-third of U.S. adults will live in areas that experience hazardous heat, according to nonprofit group the First Street Foundation. This "heat belt" will go from the Gulf Coast through Chicago, they predict, as cities already grapple with the effects of the heat island effect.

Cities on Fire: The Urban Heat Island Effect 09:13

"With urbanization, there is less vegetation and more concrete and pavement that leads to more absorption of heat," Cass said, pointing to the notable increase in heat of cities, such as Los Angeles and New York, which can be roughly 20ºF warmer than surrounding suburban or rural areas. 

"And this goes for both daytime highs and nighttime lows," Cass said. "This kind of prolonged heat in the city takes its toll on our bodies, even just a few degrees difference. It's important to seek out cooling shelters, or perhaps the mall or library, to take breaks from the heat. And also to check on our neighbors and keep pets inside."

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