About 47 million people across large swaths of the Midwest and parts of the Ohio Valley were under extreme heat warnings on Tuesday as a dangerous, slow-moving dome of heat pushed eastward, threatening records and straining communities at the start of the July 4 holiday week.
The National Weather Service issued extreme heat warnings for several major cities, including Chicago, Detroit, New York and Boston, with temperatures forecast above 90 degrees and high humidity driving heat index values — what the air actually feels like — past 100. Forecasters expected the heat index to reach about 105 degrees in Minneapolis and near 105 in Chicago by Tuesday, with Indianapolis and Green Bay, Wisconsin, also feeling like the triple digits.
The warnings mark an intensification of a heat wave that has already gripped much of the country. Meteorologists said the dome would expand its reach through the week, with one forecast projecting that well over 200 million Americans could face dangerous heat by the Fourth of July as the system spread across the Midwest, South and East.
Heat of this kind is especially hazardous because of how long it lasts and how warm the nights stay. When overnight low temperatures remain elevated, the human body cannot cool down and recover, raising the risk of heat exhaustion and potentially fatal heat stroke. The danger is greatest for older adults, young children, outdoor workers, and people without reliable air conditioning.
Cities responded by opening cooling centers and urging residents to limit time outdoors, stay hydrated and check on vulnerable neighbors. Some outdoor events and activities were canceled or curtailed across the Midwest as officials warned that prolonged exposure during the hottest hours could quickly become dangerous.
The timing compounds the threat. The stretch around Independence Day is one of the busiest of the year for travel, outdoor gatherings and fireworks displays, putting more people outside during peak heat and increasing demand on power grids already pressed by widespread air-conditioning use. Utilities urged conservation to reduce the risk of strain during the hottest afternoons.
The episode is the latest in a summer of repeated heat waves, part of a broader pattern in which extreme heat events have grown more frequent and intense. As the dome shifts toward the East Coast, forecasters warned that several cities could approach or break daily temperature records before the pattern begins to ease, and that the dangerous conditions would persist into the holiday weekend across much of the eastern half of the country.