Originally Published: 22 AUG 22 07:50 ET

Updated: 22 AUG 22 12:26 ET

By Rachel Ramirez, Caroll Alvarado and Alisha Ebrahimji, CNN and Brandon Miller, CNN Meteorologist

(CNN) -- More flash flooding threatens the Dallas area after cars and trucks were inundated early Monday by sudden, climate crisis-fueled storms that stunned parts of Texas afflicted this summer by "flash drought."

Flash flood warnings for Dallas and Tarrant counties have been upgraded to reflect a "considerable" threat and extended to 1 p.m. CT (2 p.m. ET), the National Weather Service said. Over 4 million Texans in Dallas, Fort Worth and Arlington are covered by the warnings, which call for imminent and ongoing urgent action to protect lives and property.

Nearly 15 million people from northeastern Texas into northern Louisiana and far southern Arkansas are covered by flood watches from the same system that unleashed heavy rain and flash floods this weekend in parts of the Southwest.

Dallas got an entire summer's worth of rain between Sunday afternoon and Monday morning, with over 7 inches, according to National Weather Service observations at Dallas-Fort Worth International Airport. That rate is only expected once in 50 years, on average.

Dallas also got over 3 inches of rain in just one hour overnight -- also a roughly a 1-in-50-year rainfall rate, according to National Oceanic and Atmospheric Administration estimates.

Fast-rising water trapped vehicles around 3 a.m. CT (4 a.m. ET) on Interstate 30 in Dallas, said Cassondra Anna Mae Stewart, who took video of the dark, watery scene.

"I was able to back up on a ramp to get off the highway," she said. "I took an alternate route home ... although most streets are flooded down there as well."

Around that time, "trained weather spotters reported major flash flooding ongoing across Dallas with numerous roads and cars submerged, including Interstate 30 at Interstate 45 near downtown Dallas," according to a flash flood warning statement issued at 3:21 a.m. CT (4:21 a.m. ET).

Two to 4 inches more rain is possible by midafternoon Monday in places that already got 10 inches between midday Sunday and midday Monday, radar and rain gauge reports indicate. "Additional flash flooding in the immediate Dallas/Fort Worth area may result in life threatening flash flooding," the Weather Prediction Center warned.

Other major cities in Monday's flood watch zone include Austin, Texas, and Shreveport, Louisiana. The region is under a moderate -- Level 3 of 4 -- risk of excessive rainfall. Rainfall rates of 2 to 3 inches per hour have been observed as storms move slowly across the area, creating the potential for up to 3 to 5 inches of rainfall.

A sign of 'climate whiplash'

The Dallas-area deluge comes in concert with a "flash drought" that has developed over this extremely dry year across Texas. Exceptional drought -- the highest designation -- is present over Dallas and Tarrant counties and blankets more than a quarter of the state.

"Over the last half-year, rainfall deficits of 8 inches to locally over a foot have affected areas of central Texas near and south of Dallas/Ft. Worth to the Gulf Coast," the Drought Monitor summary said Thursday.

But those rainfall deficits will be essentially erased after Monday in Dallas, though large deficits will still remain for other areas of Texas.

Human-caused climate change has increased the potential for this sort of weather whiplash, in which dramatic swings in periods of drought and high precipitation can occur more often.

A larger share of precipitation in recent years has come during "intense single-day events," and the likelihood of sudden shifts from severe drought to heavy rain will become more common on a warming planet, scientists say. Indeed, nine of the top 10 years for extreme one-day precipitation events have occurred since 1996.

Rainfall over land has become more frequent and more intense with every degree of warming since the 1980s. It's because warmer air can hold more water, causing storms like Hurricane Harvey, which hit Texas in 2017, to not only produce storm surges and damaging winds but also cause more intense inland deluges.

Monday's rainfall has pushed this month to the third-wettest August on record for the Dallas-Fort Worth area, with more than 7 inches in all so far -- the most rainfall seen in the month since 1915.

Ahead of the Texas flooding, rain persisted Sunday in parts of Arizona and New Mexico following floods in prior days in parts of the Southwest.

In Utah, hikers Friday were "swept off their feet" in Zion National Park by a flash flood. Search and rescue team members were working to find a missing hiker near the Virgin River, the park said Saturday.

In New Mexico, about 160 people had to shelter in place for several hours at Carlsbad Caverns National Park Saturday because of flash flooding, the City of Carlsbad said in a Facebook post.

The park was closed Sunday, the National Park Service said. "Maintenance crews will begin to assess and clean debris from the roadway," the National Park Service added.