Severe storms threaten the East for Fourth of July holiday travel

Millions of Americans are at risk of experiencing severe weather Sunday as thunderstorms and extreme heat alerts are in effect for multiple states.

Americans in Arkansas, Illinois, Indiana, Kentucky, Tennessee, Alabama, Georgia, Ohio, Mississippi, Missouri and one county in North Carolina are under a severe thunderstorm watch through the evening, according to meteorologists.

Another severe thunderstorm watch is also in effect along the Texas/Oklahoma panhandle, and it includes parts of northeast New Mexico, according to weather experts.

The main threat for millions of people is damaging winds, but some tornadoes could develop, meteorologists said.

The overall risk zone for severe thunderstorms stretches from Arkansas to New Jersey, as several clusters of storms are expected to flare up as the afternoon progresses into the evening.

Just north of the severe weather threat, there is an ongoing flash flood warning for the Chicago area.

A low-pressure system is sitting directly over Chicago, dropping ample amounts of rainfall, according to experts.

Chicago has seen more rain Sunday than throughout the months of May and June combined, meteorologists said. The Midway Airport picked up nearly four inches of rain on Sunday.

The extreme heat also stretches from coast to coast, with temperatures soaring above the 100-degree mark in California, and heat indexes (the feel-like temperatures) up to 110 degrees in the southeastern U.S.

About 68 million Americans from Oregon to Arizona and from Texas to North Carolina are under either excessive heat warnings or heat advisories.

The southwest won’t see a cooldown anytime soon, meteorologists said. The 7-day forecast in Phoenix, Arizona, is calling for temperatures above 110 degrees every day next week, with temperatures on Monday expected to reach 115 degrees.

In the southeast, temperatures will return to more typical summertime levels as we head through the week, according to meteorologists.

In parts of the South on Saturday, the heat index ranged from 105 to 115 degrees in at least eight southern states from Texas to Georgia. The entire state of Mississippi was under an excessive heat warning.

Air quality has improved significantly across the northeast since Saturday, in large part to the storm system bringing scattered rain and thunderstorms to the eastern U.S.

There were 19 states under air quality alerts on Friday. As of Sunday, air quality alerts have been canceled for every state but northern Michigan.

ABC News' Nadine El-Bawab and Kenton Gewecke contributed to this report.