Cities that will be hottest as nearly 100 million people are under alerts

One of the most significant heat waves of the summer continues to build across the eastern half of the country on Wednesday.

Major to extreme heat risk — the highest threat categories — target areas along the Mississippi River from the Upper Midwest to the Gulf Coast and are set to shift eastward in the days ahead. Some of the worst mix of heat and humidity will occur Wednesday as the heat wave’s footprint grows and intensifies.

Heat alerts are in place for just shy of 100 million people in two dozen states. Temperatures in the Midwest region will soar into the 90s, and it will feel 10 degrees hotter than that.

The highest risk alerts — extreme heat warnings — encompass much of the mid- and lower Mississippi Valley region, including St. Louis, Memphis, Little Rock and New Orleans. There, roughly 12 million people will face heat index values of 110 degrees to perhaps as high as 120 degrees during a brutal afternoon. While unusually high temperatures will cover more areas through the rest of the week, this region is likely to see the most persistent and longest-lasting effects from the current event.

“There is high confidence that heat will reach levels that would affect anyone without sufficient cooling and/or adequate hydration,” the National Weather Service warned. It added, “Long duration heat waves that last multiple days in a row, with little to no overnight relief and high humidity levels, often pose a greater danger.”

Additional alerts are popping up in the Northeast, including in and around Boston. Temperatures there and into the mid-Atlantic are expected to rise well into the 90s to potentially around 100 degrees late in the workweek.

Below are some of the cities most at risk for dangerous conditions Wednesday and into Thursday.

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Baton Rouge

An extreme heat warning is in place for Baton Rouge through Wednesday. High temperatures near 100 degrees will have a heat index approaching 110. It should be about five degrees less intense on Thursday.

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New Orleans

Sultry conditions grip the Big Easy amid an extreme heat warning Wednesday for high temperatures around 95 degrees and very high humidity that will cause feels-like values around 110. A few storms could break the heat late day, and it should be a touch less intense Thursday.

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Gulfport, Mississippi

There’s no relief even right on the coast. Extreme heat warnings are in place, with high temperatures in the mid-90s and a toasty Gulf bubbling moisture into the air. Heat indexes will be around 110 degrees in the afternoon. More clouds and rain by Thursday should help moderate temperatures.

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Huntsville, Alabama

The city should hit the mid-90s through at least Thursday, with heat indexes forecast to be between 105 and 110 degrees on Wednesday. There’s only a slight chance of a storm and little change in the forecast beyond Thursday.

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Memphis

Also under extreme heat warning Wednesday, Memphis is forecast to see heat indexes around 110 degrees in the afternoon. Actual temperatures will end up right around 100, with similar ones expected Thursday and Friday.

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Little Rock

An extreme heat warning is in place for the capital of Arkansas, where highs are forecast to rise above 100 degrees on Wednesday and Thursday. Heat indexes are likely to reach 110 to 115 across the region.

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St. Louis

Extreme heat warnings are in place through Thursday. Wednesday’s forecast is for a high of 96 degrees with peak heat index of 116. Temperatures will probably rise near 100 again on Thursday.

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Chicago

Heat advisories are in place for the Windy City and surrounding region through Thursday. Highs in the low and mid-90s Wednesday will rise several degrees further on Thursday, with heat index values within a few degrees of 105 expected. Overnight lows will only fall to near record values in the upper 70s.

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Des Moines

Magnified by corn sweat, unbearable humidity makes highs in the low 90s feel more like 105 degrees. Late-day storms are likely, and 80s should replace 90s on Thursday.

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Minneapolis

A high right around 90 degrees Wednesday will make it feel more like 100 degrees in the afternoon. The air mass will moderate somewhat into Thursday with a cool front passing the region and delivering rainfall late Wednesday into early Thursday.