Afternoon temperatures soared above 100 degrees in parts of Arizona, California, Colorado, New Mexico and Texas. The national high was 115 degrees in Death Valley, Calif.
Highs reached the 90s from the Gulf of Mexico states northward to Illinois, Indiana and Nebraska.
Accuweather.com said the heat would persist into Wednesday, when summer officially begins at 7:09 p.m. EDT, and move to the east. Baltimore, Boston, New York City, Philadelphia, Pittsburgh and Syracuse, N.Y., all are expected to reach the 90s.
Looking for someplace to cool down? Try Stanley, Idaho. The national low was 23 degrees there Tuesday, the National Weather Service said.
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