Dangerous heat wave to grip large swaths of U.S. this week

A significant portion of the United States is bracing for a dangerous and prolonged heat wave this week, with forecasters warning of extreme temperatures and high humidity.

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A long and dangerous heat wave will blast a large swath of the United States this week, the National Weather Service said, with temperatures rising ahead of the July Fourth holiday and feeling even hotter because of the high humidity that's arriving with it.

The National Weather Service has issued warnings for the lower Great Lakes, mid-Atlantic, and the Mississippi and Ohio River valleys. Many major cities, including New York, Philadelphia, Washington, Baltimore, Chicago, St. Louis, and Detroit, are expected to see their highest temperatures of the year so far.

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Several days of high temperatures will settle in across the lower Great Lakes, the mid-Atlantic and the Mississippi and Ohio River valleys, the weather service said.

The heat is forecast to last into next weekend across the Great Plains, Southeast, and mid-Atlantic. High temperatures will reach well into the 90s and low 100s Fahrenheit, with heat indices factoring in humidity potentially soaring to 100-110 F, and as high as 115 F in some areas. Nighttime lows in the 70s are unlikely to offer much relief.

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A number of big cities could see their highest temperatures of the year so far as they host World Cup matches.

This widespread heat wave is the first of the summer, impacting more than half the country. While the central and eastern U.S. grapple with extreme heat and dangerous fire weather conditions in the Four Corners region, the West is experiencing unusually cool temperatures, with some areas in the Northwest and northern Rockies seeing readings 20-35 degrees below average.

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High heat will last into next weekend across the Great Plains, southeast and mid-Atlantic, the weather service said.