U.S. heat wave prompts weather service urgent warnings.

Heat wave spanning from Florida to California prompts urgent warnings Extreme heat is the number one weather-related killer in an average year in the U.S. Andrew Freedman Axios

An expanding, intensifying heat wave prompted the National Weather Service to issue heat alerts for more than 112 million people on Thursday morning, with no letup in sight for some areas.

The heat will topple longstanding records, especially in the Southwest, and the duration of this event that's set to continue well into next week increases public health dangers.

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