The Washington PostDemocracy Dies in Darkness

Exceptional heat is baking the Central U.S. but is soon to relent

About 100 million people remain under heat alerts. Cities under excessive heat warnings include St. Louis, Indianapolis, Louisville, Huntsville, Houston, Dallas and Little Rock.

Heat indexes are near 120 degrees in spots again Friday. (Pivotal Weather)
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As has been the case for much of the summer, a relentless heat dome, or sphere of excessive heat and dangerous humidity, is baking a large swath of the Lower 48. Temperatures between 105 and 110 degrees will be common across the central and southern states again Friday, with heat index values pushing past 120 or 125 degrees in spots.

About 100 million people in the central U.S. remain under heat alerts. Cities under excessive heat warning include St. Louis, Indianapolis, Louisville, Huntsville, Houston, Dallas and Little Rock.

Additional excessive heat watches have returned to the Southwest U.S. as temperatures are set to rise there this weekend.

“[E]xtreme heat and humidity will significantly increase the potential for heat related illnesses,” warned the Weather Service.

The heat comes after Chicago saw its hottest day in a decade on Thursday, while Houston tied its all-time high.

With 100 or more daily records forecast to be broken Friday and Saturday, the intense heat blasting the country will begin to relent by Sunday in many spots, and even more into next week. Right now, no record highs are forecast by the middle of next week.

Where heat is focused now

Friday and Saturday are the beginning of the end of the worst with the sprawling expanse of heat sitting over the Central U.S. Across the Upper Midwest, a cold front has already brought an end to record-breaking high temperatures. Another front on the way will end up pushing the heat even farther south in the days ahead.

St. Louis, where the average late August high is 87 degrees, is expected to hit 102 degrees Friday. That would beat the Aug. 25 record of 101 degrees. Moderation will arrive there by the weekend, with a forecast high of 89 degrees Saturday and 83 degrees Sunday.

In Oklahoma City, where readings around 91 degrees are more typical this time of year, both Friday and Saturday should reach lower 100s before a return to 80s next week.

Houston and Dallas are expecting at least two more days of record heat in the 100s. Parts of Louisiana can expect intolerable heat to persist into early next week.

Through Saturday, a belt from the eastern half of Texas into the South and along the Gulf Coast will see the potential for record highs before the heat contracts and shifts more toward the Southwest for at least a few days.

The heat alone is meteorologically impressive, but even more remarkable is the fact that it’s occurring in late August. July tends to the hottest month in most of the country. By now, there is about 60 to 90 minutes less daylight compared to the summer solstice in June, with lower sun angles and lesser peak heating.

That means the parent heat dome has to be sufficiently intense to compensate for the reduction in solar heating. Indeed, weather balloons and models indicate that it has been of record strength aloft.

Heat records keep on falling

The Midwest and Great Lakes have been sizzling with the hottest weather in years over recent days, including Chicago, which hit a daily record 100 degrees Thursday, reaching the century mark for the first time since 2012.

A number of other impressive heat marks have been reached, including a number of all-time highs the last few days:

  • Alexandria, La., hit 110 degrees Thursday. This ties the all-time record set Aug. 19 there. So far, all but two days in August have featured record highs in the city.
  • Houston logged a high of 109 degrees Thursday, tying the city’s hottest day ever observed since records began in 1889.
  • Beaumont-Port Arthur, Tex., reached 108 degrees Thursday, tying the all-time record there.
  • New Orleans made it to 100 degrees Thursday, after tying an all-time high of 102 degrees Wednesday. The city has now seen 14 days at or above 100 degrees in 2023, compared to the old annual record of five, and is on pace to shatter the previous record for warmest summer by 1.7 degrees.

Many other places posted calendar day record highs Thursday, including Tyler, Tex., which hit 108 degrees. Dallas-Ft. Worth and Waco made it to 107 degrees, and Jackson, Miss., reached 106 degrees. Record warm nighttime lows were also widespread, including 82 degrees in St. Louis and Tulsa, as well as 87 degrees in Dallas, which is one degree below the all-time record warm low.

In August, almost 1,900 calendar day record highs have been set. The leaders for most record highs so far this month are: Del Rio and New Orleans with 17 each, Austin with 16, Baton Rouge with 13 and Brownsville, San Antonio and Abilene with 12 each.

Days and days of obscene heat indexes

During this week’s heat wave, the Corn Belt has seen some of the most extreme conditions, where heat index values — a combination of heat and humidity — have pushed past 120 degrees daily.

The exceptionally high heat indexes are due, in part, to excessive moisture emanating from corn, which “sweats,” or evaportranspirates, as much as 3,000 to 4,000 gallons of water per acre per day. Boundaries such as approaching cold fronts can also pool moisture over a region.

The Windy City’s dew point — a measure of how much moisture is in the air — hit a downright steamy 79 degrees Thursday, a value characteristic of the Amazon Rainforest. The combination of blistering heat and oppressive humidity led to a heat index of 120 degrees, a record that surpassed values seen during the historic 1995 heat wave.

Buttressed by dew points near and above 80 degrees, dozens of other locations reached or surpassed heat indexes of 120 degrees during this stretch across the central U.S. Lawrence, Kan., hit 133 degrees Sunday and 134 degrees Monday. Algona, Iowa, got to 131 degrees Tuesday. Newton, Iowa, made it to 131 degrees Wednesday. Macomb, Ill., reached 127 degrees Thursday.

What’s ahead?

The stubborn heat dome is finally shrinking, and its core will ultimately slip back toward the Southwest through the weekend. Much of the upcoming week may see near- and even below-average temperatures across the Midwest and East Coast.

Moving deeper into the first week of September, current signs point to an expansion of the high pressure across the Southwest U.S., perhaps again focusing across much of the southern tier of the country.

By that point, most spots east of the Rockies have usually seen their last 100-degree day. That might be some solace in that the hot season is waning. But in a year like this one, there’s no guarantee that triple-digit heat might not bleed into the fall.