Maps show where millions of people in the US will be affected by the late summer heat wave

Meteorologists say a heat wave will hit the Midwest this week with record-breaking temperatures before spreading to the East Coast. At the same time, a series of severe thunderstorms are forecast for the next few days, following a similar path from the Great Lakes region toward the Mid-Atlantic and southern New England.

High, in some cases extremely high, temperatures prevailed across much of the Midwest on Tuesday, including major metropolitan areas such as Chicago and Milwaukee, the National Weather Service said in an alert, noting that parts of the Mid-Atlantic region, including Philadelphia, were also affected. Millions of people were under various types of heat warnings due to the weather.

The humidity combined with the high temperatures resulted in maximum heat indices between 40 and 46 degrees in the regions, according to the weather service, which warned on Tuesday of high and in some places extreme heat risks in the Midwest and the lower Great Lakes regions. The heat index indicates the actual perceived temperature in combination with the relative humidity.

Meteorologists said the spread of the heat wave eastward and eventually southward through Thursday will bring “a brief period of potentially record-high temperatures” extending from the Midwestern states into the Ohio Valley, Tennessee Valley, parts of the Appalachians and the Mid-Atlantic region.

“Extremely hot temperatures are expected today across much of the Midwest, including the Chicago and Milwaukee metropolitan areas,” the weather service said in a social media post Tuesday. “If you're working outside in these areas today, make sure you take plenty of breaks in the shade and drink plenty of water – even if you don't feel thirsty.”

An accompanying map illustrated the spread of the heat wave across a large group of Midwestern states, with Michigan, Ohio, Indiana, Illinois, Iowa, Wisconsin and Missouri feeling the brunt on Tuesday. Huge swathes of land in the region were at risk of heat-related impacts on human health, infrastructure and some sensitive industries.

The dangers in some parts of the area were rated “moderate” by the National Weather Service, which is heat that “affects most heat-sensitive people, especially those without effective cooling and/or adequate hydration” and extended well into the Mid-Atlantic and Northeast at the time of the last weather update. However, the situation posed a general threat to people living almost everywhere in states such as Illinois, Ohio and Michigan, where forecasters rated the dangers in most places as “major” or “extreme.”

The first designation warns of heat levels that would affect “everyone without effective cooling and/or adequate hydration” and would also likely affect “some health systems, heat-sensitive industries and infrastructure.” The latter describes a “level of rare and/or prolonged extreme heat with little or no overnight relief” that could affect everyone in a similar way under certain circumstances.

David Parkinson, senior weather producer at CBS News, reported early Tuesday that heat in the Chicago area would likely feel like 42 degrees, while temperatures in Louisville would reach triple digits, albeit with slightly lower humidity. Nashville will see the worst of the heat wave on Wednesday, with temperatures expected to top 38 degrees, while Washington, D.C., could reach a record 37 degrees — possibly the 47th day above 32 degrees this year. That would be well above the capital's average in 2023, but below the all-time record of 67 days with temperatures above 32 degrees, Parkinson said.

On Wednesday, severe storms will move through New York, Philadelphia, and DC, bringing with them significantly cooler temperatures. It's important to note that none of the major cities in the Northeast will experience a heat wave, as there won't be three consecutive days of 90 degrees. That goes for DC, too. By the end of the week, only the Tennessee River Valley will hold the record heat, while northern areas will return to and nearly maintain normal temperatures.

Heat warnings and advisories were in effect Tuesday for 61 million people in the Mississippi Valley, Midwest and southern New England, the weather service said. Meteorologists added that those warnings and advisories are expected to spread to parts of the Mid-Atlantic by Wednesday as the heat wave moves in that direction.

A heat warning was in effect for southeast Michigan until Tuesday night. Meteorologists expected actual temperatures to exceed 90 degrees and the heat index to rise to 100 in some areas.

According to the Chicago Weather Service, a heat warning was also in effect for much of northwest Indiana on Tuesday, and an extreme heat warning was issued for most of northern Illinois.

“Today is expected to be the hottest day of the heat wave, with heat indices reaching 40 to 43 degrees,” Chicago meteorologists said in a social media post, noting that some of these areas could be hit by thunderstorms starting Tuesday afternoon into Tuesday night.

“An air quality warning is in effect for northeastern Illinois and northwestern Indiana until midnight today,” the post continued. “Air quality is expected to be hazardous to the health of sensitive groups. Limit prolonged outdoor activities, especially for those with chronic respiratory conditions.”