It was a record-setting weekend, where dozens of daily record highs were set across portions of the lower-48 stretching from the Northern Plains to the Northeast.
When Sioux Falls, South Dakota, soared above 100 degrees on Saturday, it was their hottest day in nine years.
Meanwhile, Milwaukee set record highs three days in a row.
This record-smashing heat continues Monday, where more than a dozen record highs could be set across the Northern Plains and New England.
About 19 million people woke up under Heat Advisories Monday covering parts of the Upper Midwest, Northeast and New England. The Heat Advisory included Minneapolis, Philadelphia and Boston.
Temperatures were forecast to soar more than 20 degrees above average leading to highs in the upper 80s and low 90s. It was also forecast to be muggy with high humidity in some spots, making temperatures feel more like the mid-to-upper 90s and close to 100 degrees, especially in the big cities.
Philadelphia could experience its first official heat wave of the year, with 90 to near-90 degree temperatures forecast Monday, Tuesday and Wednesday. The Northeast is the only region with an official definition for a heat wave, which is three consecutive days of 90 degree temperatures.
This scorching heat lasts through Wednesday, before cooler temperatures arrive Thursday and Friday.
The high heat and humidity will also fuel summertime thunderstorms throughout much of the week.
About 11 million people are under flood alerts across portions of Texas, Oklahoma, Arkansas and Louisiana and include the large metro areas of Dallas and Little Rock.
In these areas, afternoon thunderstorms capable of torrential downpours and high rainfall rates could produce isolated flash flooding, especially in urban areas.