Forecast high temperatures on Friday from the National Weather Service.

This article was last updated Thursday afternoon. For the latest information on the heat wave, published Friday, see: 'Potentially deadly’ heat wave grips two-thirds of U.S., with dozens of records likely to fall

A potentially deadly combination of heat, humidity and stagnant, polluted air has engulfed more than half of the Lower 48 states, with air temperatures climbing toward 100 from the Plains to the Midwest, Mississippi River Valley and eastward to the heavily populated Boston-to-Washington corridor.

“A widespread and dangerous heat wave is building in the central and eastern U.S.,” the National Weather Service said Thursday.

Heat advisories and warnings affect 154 million Americans. In many major population centers, the heat index — how hot it feels factoring in the humidity — is forecast to peak around 110 degrees between Friday and Sunday. The actual air temperature is expected to reach at least 95 for more than half the population of the Lower 48 over the next several days.

The trigger for this heat wave is a sprawling, strong high-pressure area, also known as a “heat dome,” building across the United States. Another high-pressure area in the Western Atlantic, known as the “Bermuda High,” is also a key player, because the circulation around these weather features is pumping hot and humid air from south to north.

Hurricane Barry’s remnants have added to the misery by bringing a surge of sultry, swamp-like tropical moisture that has blanketed the heat wave zone. As a measure of that moisture, many locations along the East Coast reported dew points of 80 degrees Wednesday, about as high as such temperatures go in nontropical locations.

Such extremely humid conditions are expected to continue through Sunday, particularly along the Eastern Seaboard.

The most memorable aspect of this heat wave will be the lack of relief at night, especially in urban areas where the heat island prevents temperatures from falling quickly overnight.

According to the National Weather Service, overnight low temperatures will be in the mid- to upper 70s to 80 degrees, and “dozens of high minimum temperature records are forecast to be set, with a few record high maximum temperatures possible as well.”

The Weather Service projects 123 warm low temperature records to be tied or broken.


(National Weather Service)

For example, the overnight low temperature at New York’s Central Park observing station is not forecast to drop below 80 degrees Saturday and Sunday, and could tie or break the daily records of 82 degrees on both nights. Washington is forecast to have three consecutive nights with low temperatures in the low 80s, from Saturday morning through Monday morning.

Such high overnight lows will exacerbate the public health threat from this event. Heat is typically the No. 1 weather killer in the United States each year, and heat-related illnesses spike when overnight lows stay warm, depriving the human body of a break from heat stress.

The groups most vulnerable to heat-related illnesses include the elderly, chronically ill, children and outdoor workers.


Forecast heat index values at 4 p.m. Eastern on Friday. (WeatherBell.com)

City forecasts

Here are forecasts for several major cities:

  • St. Louis: Hottest day? Friday. Forecast high? 98. Heat index? 109.
  • Chicago: Hottest day? Friday. Forecast high? 96. Peak heat index? 102.
  • Cincinnati: Hottest day? Friday. Forecast high? 96. Peak heat index? 107.
  • Detroit: Hottest day? Saturday. Forecast high? 97. Peak heat index? 111.
  • Washington: Hottest day? Saturday. Forecast high? 100. Peak heat index? 109.
  • Philadelphia: Hottest day? Saturday. Forecast high: 99. Peak heat index? 110.
  • New York: Hottest day? Saturday. Forecast high: 98. Peak heat index? 112.
  • Boston? Hottest day? Saturday. Forecast high: 93. Peak heat index? 102.

The climate context

As the climate warms because of human activities, numerous studies show that heat waves such as this one are becoming more common and intense, as well as longer-lasting.

An expansive study in the journal Proceedings of the National Academy of Sciences in 2017 found a climate change fingerprint in heat waves worldwide. Specifically, it showed that climate change has heightened the chances for record heat across more than 80 percent of the surface area of the globe that has sufficient weather data available. (This research excluded parts of the developing world, where weather monitoring networks are more sparse.)

In addition, a sweeping climate assessment published by the Trump administration last year found extreme heat events are on the increase in the United States and have been since the 1960s. Interestingly, summer nights have warmed nearly twice as fast as summer days in the United States, according to data from the National Oceanic and Atmospheric Administration. This makes heat waves a more formidable threat to public health.

In Washington, for example, lows of 80 degrees or higher have occurred 32 times since 2010, which is higher than the number of instances from 1872 through 2009, according to the Capital Weather Gang’s Ian Livingston.