Temperatures soared in Chicago and combined with the humidity to make it feel even hotter there and in other sweltering cities.
More than 100 million people were expected to be affected by the middle of week and authorities warned residents to stay hydrated, remain indoors when possible and be aware of the health risks of high temperatures.
Strong storms brought heavy rain and damaging wind to many of the affected areas on Monday, and more than 500,000 customers remained without power on Tuesday, including more than 350,000 in Ohio.
Excessive heat warnings are in effect for much of Illinois and Indiana along with parts of Minnesota, Iowa, Michigan and Ohio through to Wednesday night, according to the National Weather Service.
Heat index values – which take into account the temperature and relative humidity and indicate how hot it feels outdoors – approached 40C (105F) in some locations, including Chicago, the weather service said.
Much of south-eastern Michigan – from just south of Flint to the state lines with Ohio and Indiana – was put under an excessive heat watch through to Thursday morning as the warm front is forecast to move east.
A heat advisory was also issued, stretching from as far north as Wisconsin down to the Florida Panhandle on the Gulf coast.
In Chicago, where a ferocious storm on Monday night heralded temperatures that were expected to exceed 32C (90F) on Wednesday, the May deaths of three women when temperatures climbed into the 90s served as a fresh reminder of the dangers of such heat – particularly for people who live alone or are dealing with certain health issues.
“The next two days will require that we all look out for one another and provide extra attention and resources for our vulnerable neighbours,” said Alisa Rodriguez, managing deputy commissioner for Chicago’s Department of Family Services and Support.