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Widespread, dangerous heat wave to expand across much of the U.S.

Visualization of heat and humidity Friday. (

This article was published Tuesday. For our latest article on this heat wave, posted Wednesday, see: Widespread, oppressive and dangerous heat to roast much of the U.S. through the weekend

A stifling heat wave has begun to take shape across large portions of the United States, with millions likely to see temperatures creep toward the century mark, along with even higher heat indexes by this weekend.

The heat wave is already generating excessive heat watches in the central United States, and by Wednesday the national weather map is likely to feature a blanket of heat advisories from the National Weather Service. The combination of sultry dew points and scorching air temperatures approaching will help make this event a dangerous one from a public health perspective.

Temperatures are expected to climb over the weekend in parts of the Midwest and cities along the East Coast. Here’s what you should know. (Video: The Washington Post)

Cities including Chicago, St. Louis, Washington, Philadelphia, New York, Nashville and Kansas City, Mo., are likely to see at least three days with temperatures between 95 degrees and 100 degrees, along with dew points — a measure of the amount of moisture in the air — above 70 degrees.

This is likely to create ideal conditions for setting hot overnight low-temperature records, with the possibility that some records for such temperatures might be broken.

For example, the GFS model, which is generally bullish on hot weather, is projecting a low temperature in Washington of 86 degrees Sunday morning. The all-time warmest low temperature for the District is 84 degrees.

Oppressive heat and humidity, the most extreme this year, may hit Washington late this week

Faster warming of overnight lows compared with the warming rate of daytime highs are consistent with model projections and observations of how global warming is reshaping our weather. In cities, “urban heat island” effects also help keep overnight temperatures higher than surrounding areas.

When overnight lows fail to drop below a particular threshold, it’s harder for the human body to cool down and rest, elevating the health threat.

The sauna-like conditions forecast are the result of a sprawling and intense heat dome or zone of high pressure that will swell over the eastern two-thirds of the nation beginning Wednesday. Temperatures will soar as many as 10 to 20 degrees above normal. That may not sound like a lot, but considering it is the hottest time of the year, the resulting heat will challenge records in some areas.

Regional breakdown

Excessive-heat watches are already in effect for all of Missouri as well as portions of Kansas, Illinois and Iowa, as heat indexes are forecast to range from 100 degrees to 110 degrees from Wednesday through Saturday.

“The prolonged duration of the heat and humidity will potentially become dangerous to those most vulnerable,” according to the National Weather Service. Heat kills more people per year in the United States than any other weather hazard, and typically the elderly, young children and those without air conditioning are most susceptible to heat-related illnesses.

Chicago is predicted to see its hottest temperatures Friday, whereas the hottest days for areas further east may come Saturday and Sunday.

In Philadelphia, the Weather Service says it has “high confidence” in heat index values over 105 degrees Friday through Sunday and has issued an excessive-heat watch due to the “rare prolonged” stretch of heat. “The combination of hot temperatures and high humidity will combine to create a DANGEROUS SITUATION in which heat illnesses are possible,” it cautioned.

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? 108.
  • Cincinnati: Hottest day? Friday. Forecast high? 95. Peak heat index? 109.
  • Detroit: Hottest day? Saturday. Forecast high? 95. Peak heat index? 108.
  • Washington: Hottest day? Saturday. Forecast high? 98. Peak heat index? 112.
  • Philadelphia: Hottest day? Saturday. Forecast high: 98. Peak heat index? 114.
  • New York: Hottest day? Saturday. Forecast high: 97. Peak heat index? 110.
  • Boston? Hottest day? Saturday. Forecast high: 94. Peak heat index? 105.

According to meteorologist Ryan Maue, about 86 percent of the population of the Lower 48 states, or 290 million, may see temperatures at or above 90 degrees during the next seven days, he told The Post.

The climate context

While a heat wave in mid-July is not unusual across the United States, one of the most robust findings of climate change research is that the increase in average global surface temperatures is significantly raising the odds of extreme heat events, causing them to be more severe and longer-lasting. The most recent example of this was the heat wave that gripped Western Europe in late June, breaking numerous national all-time high temperature records.

A peer-reviewed study published Tuesday in Environmental Research Communications found that the frequency of what is now an extraordinarily rare heat event in the United States is projected to dramatically increase even if greenhouse gas emissions are kept below a “business as usual” scenario.

The study, funded in part by the Union of Concerned Scientists, a science research and advocacy group, found that the annual number of days with heat indexes exceeding 100 degrees and 105 degrees are projected to double and triple, respectively, by mid-century, compared with a 1971-2000 average, depending on the amount of greenhouse gas emissions.

In other words, if you don’t like this upcoming heat wave, you probably won’t like what’s coming later.