Central US steams in a 'heat dome.' Mid-Atlantic braces for same.

A 'heat dome' has settled over 17 states in the central US, where it is likely to stay for several days. Temperatures will feel hotter than 110 degrees F. in some places.

Charles Cherney/AP
Chicago Cub sfan Ross Perry of Waukesha, Wis., cools off during 95-degree heat under the mist maker in the right field bleachers before Chicago Cubs played the Florida Marlins in a baseball game on July 17, in Chicago.

Triple-digit temperatures are in store for the central United States this week, as a giant “heat dome” settles over 17 states before shifting to the mid-Atlantic region as early as Wednesday.

The sweltering heat is the result of a high-pressure system that has anchored itself in the middle of the country, where it will remain for several days. Temperatures between 100 to 110 degrees F. are expected each afternoon this week throughout the southern and central Plains states, particularly Illinois, Wisconsin, Minnesota, Iowa, Missouri, Kansas, Nebraska, Oklahoma, as well as South and North Dakota. Heat index values mean it will feel even hotter.

With the jet stream shifted to the north, the heat dome will remain “entrenched,” meaning large swaths of the country will bake for several days in a row, says Jim Keeney, a meteorologist in Kansas City, Mo., with the National Weather Service. Although back-to-back heat waves like the ones last week and this week are “unusual,” temperatures would have to remain above 100 degrees F. through next week to start breaking duration records.

“When something sets up in the center part of the country, it’s going to take a while for it to move out,” Mr. Keeney says.

Seventeen states are under heat advisory. On Sunday, the highest heat index value – which is a reading of how hot it feels – was 120 degrees F. in Mitchell, S.D. The lowest in the affected zone, 105 degrees, was recorded in Hays, Kan.

The searing temperatures have the potential to adversely affect crops. This is especially the case in Texas, where the period between February and June was the driest on record.

The combination of seasonal dryness and extended heat could “kill or at least greatly stress any kind of plant that’s supposed to be growing right now,” says Pat Slattery, a spokesman with the National Weather Service’s central region office. “The top part of that plant that’s not sitting in irrigated water is going to cook,” he says.

Cities throughout the Midwest are expanding the number of emergency cooling centers. In addition to the 120 centers operated by the Illinois Department of Human Services, the agency is also directing people to the many highway rest stops, where restaurants and other amenities are air-conditioned, spread throughout the Chicago area.

The heat is also prompting ozone alerts in many states, signaling problems with smog. The Southeast Michigan Council of Governments, for example, is asking people to pause from daytime actions that may raise those levels, such as filling gasoline tanks and grilling.

As scorching temperatures reach the mid-Atlantic coast, where heat index values are expected to exceed 100 degrees, pressure on America’s power grids is expected to rise, but not to dangerous levels.

PJM Interconnection, which operates the biggest power grid in the mid-Atlantic and the Midwest, from Illinois to New Jersey, forecasts that peak electric usage will reach 145,500 megawatts Tuesday, well below 158,448 megawatts, the grid’s all-time peak set in August 2006.

New York ISO, which operates the power grid in that state, forecasts that peak usage will reach 31,300 megawatts Tuesday, which is also below the record peak of 33,939 megawatts, also set in August 2006.

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