US heatwave puts nearly 120 million under extreme heat warnings ahead of July 4 holiday

A severe and prolonged heatwave is affecting much of the United States this week, with nearly 120 million people under extreme heat warnings as the country heads into the Fourth of July holiday weekend. The National Weather Service says the dangerous spell is stretching from the Midwest to the East Coast and is expected to bring sweltering daytime temperatures, high humidity and very warm nights. Officials have warned that the combination of heat and duration makes the event particularly dangerous.

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The weather service said temperatures are expected to range between 32C and 40C in many areas, with some locations potentially reaching around 46C. It said the heat is being driven by a heat dome, with the hottest conditions likely to peak in the Midwest and Mississippi Valley from Tuesday to Thursday. The heat is then expected to spread into the Ohio Valley and the East Coast before continuing through the holiday weekend.

The warning comes as Americans prepare for Fourth of July celebrations, a period that typically brings large outdoor gatherings and increased travel. In New York City, the mayor has put in place a heat emergency plan that includes hundreds of cooling centres opening from Wednesday. In Detroit, around a dozen recreation centres with air conditioning have already been opened for residents seeking relief from the heat.

The scale of the warning makes this one of the most significant weather events affecting the US at the start of July. Extreme heat can place pressure on power systems, public health services and local emergency planning, especially when it lasts for several days and does not cool much overnight. The risk is heightened when high humidity makes temperatures feel even more oppressive and limits the body's ability to recover.

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The National Weather Service said the stretch of record-breaking heat will be especially dangerous because of both the daytime highs and the prolonged overnight warmth. That combination can increase the risk of heat-related illness, particularly for older people, outdoor workers and those without access to air conditioning. Local authorities in affected cities are responding by opening cooling centres and urging residents to take precautions.

What remains unclear is how long the hottest conditions will persist in each region and whether additional cities will expand emergency measures as the holiday weekend continues. The weather service's forecast suggests the heat will move eastward after peaking in the central US, but the full impact will depend on local conditions and how quickly temperatures ease. The main focus now is on public safety, cooling access and whether the warning area changes as the system develops.

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