National Weather Service forecast for the maximum heat index on Saturday. (weatherbell.com)

This story was last updated Wednesday afternoon. For our latest story on the heat wave, published Thursday, see: ‘Dangerous heat wave is building’ as temperatures spike in the central and eastern U.S.

A heat wave featuring a life-threatening combination of heat and oppressive humidity has begun to spread across the United States, with excessive heat warnings and heat advisories in effect for at least 22 states and the District of Columbia. According to the National Weather Service, 51 percent of the Lower 48 states are likely to see air temperatures reach or exceed 95 degrees during the next seven days, with 85 percent experiencing temperatures above 90 degrees during the same period.

Washington could see its first high temperature at or above 100 degrees since 2016. In Chicago, the air temperature is also forecast to approach the century mark.

The heat index, which is a measure of how hot it feels to the human body when air temperatures are combined with the amount of moisture in the air, are forecast to climb into rare territory in many cities, from Chicago to Kansas City and eastward all the way north into southern New England.

The moisture from former Hurricane Barry, drawn out of the Gulf of Mexico through the South, is intensifying the humidity levels over a sprawling area.

According to the Weather Service forecast office in Chicago, “The heat is forecast to be oppressive and dangerous everywhere, with possibly some of the hottest conditions since 2012."

The heat index in Washington, for example, is forecast to peak around 110 degrees on Saturday, with similarly sweat-inducing readings predicted for the so-called “Acela Corridor,” up through Philadelphia and New York. Chicago could see a heat index of near 113 degrees on Saturday.

Because this is typically the hottest time of the year, it’s difficult to break daily and all-time high temperature records, so a wave of new temperature benchmarks are not expected. However, this event will be remembered for its wide geographic scope, stretching from the Plains to the East Coast, as well as the high overnight low temperatures that stand a better chance of breaking records.

In New York, the temperature may not fall below 80 degrees at night between Friday and Monday.

Overnight low temperatures are important for determining the health impacts of a heat wave, since the human body needs a respite from hot conditions to stave off heat-related illnesses. The most vulnerable populations to heat waves include the elderly, those with chronic health conditions, athletes who practice outdoors, and children.

Extreme heat is the deadliest form of extreme weather in the United States, beating hurricanes and floods in a typical year.

Climate change is already making heat waves more frequent and severe

While hot weather in mid-July is to be expected, heat waves such as this one are already becoming more common, longer-lasting and intense across much of the world because of human-caused global warming.

For example, a study in the journal Proceedings of the National Academy of Sciences last year found a fingerprint of climate change in excessive heat events worldwide. Specifically, it found that climate change has heightened the chances for record heat across more than 80 percent of the surface area of the globe where there is robust temperature data.


Trend in daily minimum temperatures during the summer months of June through August, 1895-2019. (NCEI)

A major climate assessment published by the Trump administration in 2018 found extreme heat events are on the increase in the United States and have been since the 1960s. Such events, the report found, are already harming people, livestock and infrastructure. Data shows heat waves are affecting 50 major cities in the United States more frequently now than several decades ago, increasing from twice per year during the 1960s to nearly six heat waves per year in the 2010s. (However, the Dust Bowl-era of the 1930s still tops the list of the most extreme heat events in the United States, the report found.)

In keeping with a trend toward more humid heat waves, summer nights have warmed faster than daytime summer temperatures in the United States, according to NOAA data. In Washington, D.C., for example, lows of 80 degrees or higher have occurred 32 times since 2010, whereas there were only 31 instances from 1872 to 2009, according to the Capital Weather Gang’s Ian Livingston.