Millions bake under intense U.S. heat

• NEW: One-third of the U.S. population is under some kind of heat warning Friday
• Temperature readings may exceed 100 degrees from St. Louis to Washington
• The scorching conditions may stretch well into next week in some places
• Forecasters and officials urge people to be smart, and to check on those in need

(CNN) -- About one in three Americans sweltered under heat warnings, watches or advisories Friday as extremely high temperatures spread across 27 states, dealing out misery and threatening lives.

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Heat warnings, watches or advisories were posted across 730,000 square miles of the United States, an area roughly the size of Mexico, CNN meteorologist Brandon Miller said. About 100 million people were affected, he said.

High temperature readings were expected to be around or above 100 degrees in locales from Topeka, Kansas, to the District of Columbia on Friday, and were expected to stay around that level in many places through next week if not longer.

In some places, the heat index could reach 115 degrees, Miller said.

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"Heat-related illness such as heat exhaustion or heat stroke are a real threat; dehydration can occur quickly," noted an excessive heat warning from the National Weather Service's bureau in Indianapolis, where highs were expected to hit 101 on Friday. "The extended period of heat also will cause drought conditions to worsen and raise the concern for wildfire development."

The thermostat in St. Louis was forecast to hit 108 degrees and highs are expected to remain above 100 through next Thursday, at least, according to the weather service.

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That's prompted the city health department to remind residents to limit outdoor activities, to leave no children or pets in vehicles, and to watch out for signs of heat exhaustion.

In Memphis, Tennessee, where highs were likely to hit 104 degrees on Friday with a heat index closer to 108, firefighters were going door to door checking on residents to make sure they're bearing the weather well. Churches and faith-based institutions also were being urged to reach out to people and to ask people to check on their neighbors and relatives.

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"Please, if you know of someone who doesn't have air conditioning or who might be struggling with the heat, just stop by and see how they are doing," Mayor A.C. Wharton Jr. urged residents in a news release.

A "code red smog alert" was declared Friday for metro Atlanta, the first of its kind since 2010, according to the Clean Air Campaign, a partnership between employers and the state transportation department.

Other metro areas on the East Coast were also feeling the heat. Philadelphia and Washington both were expected to have highs near 100 on Friday, while New Yorkers were likely to see temperatures soar into the high 90s.

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