Flash-flood watches and warnings were issued across seven states early Tuesday as an unprecedented downpour of torrential rain triggered "extremely dangerous and potentially life-threatening" conditions in Houston.
More than 30 million Americans were told to brace for dangerous thunderstorms — including flooding, hail and possible tornadoes — as meteorologists warned the weather that has centered on Texas and Oklahoma since Saturday could expand to other areas.
In Houston, more than 80,000 people were without power and the flood waters closed roads including Interstate 10 and Interstate 45. Houston was among 24 counties where Texas Gov. Greg Abbott declared a state of disaster on Monday.
Houston Intercontinental Airport smashed its all-time record for most rainfall in one day on Monday — its 4.34 inches almost doubling the previous milestone set in 1946.
"The rain just kept coming, and coming, and coming," said Ashley Aivles, a 25-year-old call center worker who struggled to make it back to her home in a Houston suburb early Tuesday.
"I lived here during Hurricane Ike [in 2008] and this was a close second. We've had really bad rain in the past but this was something else," said Aivles, who said many of her co-workers were still trapped in the office.
Around 200 basketball fans remained trapped inside the city's Toyota Center at 4 a.m. local time (5 a.m. ET), having watched the Houston Rockets' playoff win over the Golden State Warriors.
Asif Noorani, a 22-year-old student, took his chances and left the stadium after the game despite a warning on the Jumbotron to wait until the storm passed.
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"We didn't realize it would be that bad," he said. "We were just about able to make it home."
Lightning triggered blazes including two-alarm fire in the Memorial area of the city, officials said.
All Houston METRO rail and bus services were canceled until the flood waters receded and conditions were deemed safe by the city's Office of Emergency Management, the transportation service announced at 4:20 a.m. local time (5:20 a.m. ET).
The Houston Independent School District also announced that classes would be delayed by two hours.
Target employee Joshua Cooper said he and a dozen workers were stuck inside a store after become surrounded by the waters.
"We are basically an island right now," the 24-year-old told NBC News. "There's no way for us to get out. The waters are so high that we have a bayou that's basically a lake and buses are getting stuck on the roads."
Cooper added: "I've lived in Houston all my life and I've not seen flooding this bad in a very long time."
An unknown number of people were also stranded in Houston's The Galleria mall, after parts of the building and the surrounding streets were drenched by the deluge.
The Weather Channel warned of "extremely dangerous and potentially life-threatening flood situation" in Houston.
At least 12 people were still missing, eight people have been killed, and countless more evacuated amid the deluge that has inundated Texas and Oklahoma with record-breaking floods since Saturday.
Around 32 million Americans in the Plains and South were at risk from dangerous thunderstorms Tuesday, meteorologists said.
Flood watches and warnings were in effect at 6:30 a.m. ET in parts of Texas, Arkansas, Oklahoma, Kansas, Missouri, Louisiana, and Mississippi.
There were also 19 reported tornadoes in Texas, Arkansas, Oklahoma and Mississippi on Monday — and another twister over the Mexico border that killed 13 people.
And there appeared to be no let up in sight, with Weather Channel meteorologist Kevin Roth predicting the storm would likely refocus on central Texas, as well as branching out to the Ohio Valley and further north later on Tuesday.
"There will be another round of thunderstorms developing in central Texas in the mid-afternoon today, which could then clip Houston and Dallas by the evening," he said.
However, whereas a storm lingered over Houston on Monday night, Roth said Tuesday's round could pass over the city quicker and therefore not dump as much rain on the city.
Severe thunderstorms were also possible throughout the day in the Ohio Valley and as far north as Chicago, Milwaukee and Green Bay, Roth said.
"This is likely to mean wind, hail, and isolated tornadoes cannot be ruled out," he said.
At least eight people were confirmed dead over the holiday weekend's storms and flooding, including a 14-year-old boy in Texas who was found inside a storm drain and believed to have drowned and a homecoming queen who was driving home from her prom.
Four were confirmed dead in Oklahoma, including a Claremore firefighter who died during a water rescue, and a 33-year-old woman who died in a storm-related traffic accident in Tulsa.
The 12 missing in the small town of Wimberley, Texas, between Austin and San Antonio, included members of two families who were vacationing together. The house they were staying in was swept away by flash floods on Sunday, relatives told NBC News.