WASHINGTON — A dangerous and potentially historic heat wave has blanketed large swaths of the eastern United States ahead of the nation's 250th Independence Day celebrations, placing more than 160 million Americans under heat alerts and forcing cities to shorten parades, cancel train routes and activate emergency response plans as temperatures soar to levels not seen in more than a decade.

The National Weather Service described the event as a "potentially historic heatwave across the eastern third of the country" in its forecast discussions over the weekend, warning that Thursday and Friday carried "the possibility of all-time record highs" in multiple cities. Real-feel temperatures have reached as high as 100 to 115 degrees Fahrenheit across parts of the Midwest, Northeast and South, driven by a sprawling heat dome that meteorologists say has trapped extremely hot air over the region and prevented the overnight cooling that would typically offer some relief.

National Weather Service forecaster Bryan Putnam framed the danger in terms that went beyond the raw temperature readings, emphasizing how humidity compounds the threat even after the sun sets.

"Your temperatures might stay in the 80s and the 90s in the heat in the evening, as well as the fact is with the humidity, that's going to keep those heat indices high as well," Putnam told NPR.

The timing could not have been more disruptive. This year's Fourth of July marks the 250th........

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