Maps show where late-summer heat wave will affect millions in U.S.

A heat wave was expected to bring record-high temperatures to the midwestern United States this week before spreading to the East Coast, forecasters said. At the same time, a bout of severe thunderstorms was forecast to follow a similar path from the Great Lakes region toward the Mid-Atlantic and southern New England over the next couple of days.

High, and in some instances excessively high, temperatures settled in Tuesday over much of the Midwest, including major metropolitan areas like Chicago and Milwaukee, the National Weather Service said in an advisory that noted portions of the Mid-Atlantic region, including Philadelphia, were also affected. Millions were under various types of heat alerts in response to the weather.

Humidity combined with soaring temperatures to create maximum heat indexes between 105 and 115 degrees across the regions, according to the weather service, which warned of major, and, in a number of places, extreme, heat risks Tuesday across the Midwest and into the lower Great Lakes. The heat index is what the actual temperature feels like when it's combined with relative humidity.