Those areas will see a little relief early week with most temperatures returning to near average while the heat and humidity expand elsewhere.
A shifting upper air pattern will plunge the East Coast into dangerous heat and humidity later in the week. Even nighttime temperatures will be warm and provide little relief.
Sunday and Monday
More than 46 million Americans from California to Florida are under heat alerts in effect until at least late Monday. This number is an improvement from Thursday's peak of more than 110 million under heat alerts.
The FOX Forecast Center said more than 37 million people across the U.S. saw high temperatures reach the triple digits Sunday, with more than 8 million of them seeing temperatures of 110 degrees or higher.
Excessive Heat Warnings and heat advisories are currently in effect across the West and southern tier of the U.S. In some portions of South and Southwest Florida, heat alerts have been issued for 20 straight days.
Portions of the Mid-South, Southeast and Gulf Coast felt a little reprieve on Sunday but excessive heat and humidity will rebuild across the areas later this week.
"This takes us into the end of the month, and then we have August ahead of us," Herrera said.
Heat expands to the east this week
The heat will begin to build in the middle of the country as an area of high pressure expands and pushes off to the east this week. As it does, temperatures across the Plains and the Midwest start to rise.
"Here’s what’s concerning. This area of high pressure, this big bubble, starts to blow up and cover a huge chunk of the country," Herrera said.
Temperatures in the Plains will approach or exceed 100 degrees in some major cities starting on Monday.
"Across the Pacific Northwest and across to New England, those are the two spots in the week ahead that will be close to average," Herrera continued. "But this will take us to the end of the month. So, we’ve got a lot more heat to contend with.
People should take the heat seriously and avoid extended time outdoors. Temperatures and heat indices will reach levels that would pose a health risk and be potentially deadly to anyone without effective cooling and adequate hydration.