Extreme heat grips the U.S. ahead of Fourth of July weekend

More than 160 million people were under extreme heat warnings or heat advisories as dangerous, record-breaking heat intensified across most of the central and eastern U.S. Heat indices would likely exceed 100 degrees in many areas as the hot air mass spread eastward and pressed into the Fourth of July weekend, raising the risk of heat-related illness across a broad swath of the country.

The sharpest heat was expected to settle over the East and Mid-Atlantic. Forecasts put parts of the I-95 corridor from southern New England to northern Virginia in triple-digit territory from Thursday through Saturday, with New York City possibly logging its first 100-degree high in almost 14 years if the forecast holds. Some outlooks also showed the Mid-Atlantic heat rivaling or exceeding long streaks of extreme temperatures in Philadelphia.

The same weather pattern was also carrying severe-thunderstorm risk in the central U.S. and Northeast, adding another layer of disruption as holiday travel ramps up. Oppressive humidity, extreme heat and storm chances raise the risk for people spending long periods outside, for commuters on crowded transit corridors and for anyone moving through cities where pavement and building heat can push temperatures higher than the official reading.

NOAA’s HeatRisk product flags impactful heat in the seven-day forecast for decision makers and people who are sensitive to heat. It is supplementary to official heat watches, warnings and advisories, giving local officials and residents another signal to watch as conditions worsen.

Extreme heat can quickly trigger heat-related illness when high temperatures combine with high humidity, and the National Weather Service counts it as the weather phenomenon that has killed more people in the last 10 years than any other.