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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.

Michael Walter, a spokesman for Houston Emergency Management, confirmed three deaths Tuesday afternoon, raising the death toll to 11. At least 12 people were still missing, and countless more were evacuated amid the deluge that has inundated Texas and Oklahoma with record-breaking floods since Saturday.

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.

Flash #flood emergency thru 6a for part of Houston metro. ~11" rain SW Harris Co. #Houstonflood pic.twitter.com/URo7mISi5O -TWC Breaking (@TWCBreaking) May 26, 2015

530 AM: Here's a look at where some of the heaviest rain fell overnight for Southeast TX: #txwx #houwx #bcswx pic.twitter.com/r6TMJuaDRu — NWSHouston (@NWSHouston) May 26, 2015

Wow: When was the last time a flash flood emergency was issued for Houston? @NWSHouston: First time. Never been done before. #houstonflood — Justin Abraham (@jjabraham) May 26, 2015

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.

Crazy flooding on I-45 N at North Main in #Houston. Cars stranded, flooded on interstate. #houstonflood pic.twitter.com/7iJ08nKR9w — Justin Abraham (@jjabraham) May 26, 2015

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This is not just friendly advice. Stay off the roads and avoid high water. This is not your typical rain storm. #HoustonFlood #houwx — TxDOT- HOU District (@TxDOTHoustonPIO) May 26, 2015

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.

"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.

LIVE NOW: Michael Walter of #Houston EMA says #lightning has sparked building fires in city; floods slowing response. pic.twitter.com/5uZiG4T8Ju — The Weather Channel (@weatherchannel) May 26, 2015

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.

#BREAKING: Harris County TX emergency mgmt says hundreds of homes are now flooded in western part of county. #flood pic.twitter.com/FTX9CL5Apx — The Weather Channel (@weatherchannel) May 26, 2015

11:30p CDT: White Oak Bayou has climbed 25 feet in 2 hours and will #flood I-10 just N of downtown #Houston shortly. pic.twitter.com/hEg0WcFygp — The Weather Channel (@weatherchannel) May 26, 2015

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.

Today's early tornado probabilities include parts of the Midwest pic.twitter.com/HIWc2q2jql — Jim Cantore (@JimCantore) May 26, 2015

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.