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By Alexander Smith, Gabe Gutierrez and Elisha Fieldstadt

Tornadoes and dangerous thunderstorms continued to race across the south-central U.S. after menacing Texas and Oklahoma on Monday with historic flooding that sent rescuers searching for 12 people still missing, including at least three young children.

Flash flood warnings were in effect early Tuesday for parts of eastern Texas and much of Western Arkansas, and tornado watches were issued for parts of Louisiana, Missouri and Arkansas through the morning. Power was out to nearly 70,000 customers in the Houston area.

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.

Across the border in Mexico, a twister killed at least 13 more.

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.

Among them were Laura McComb and her two children. Laura’s husband, Jonathan, was hospitalized with a collapsed lung, broken ribs and a broken sternum, his father, Joe McComb, told NBC News.

Grandfather Holds Out Hopes That Family Swept in Texas Floods Will Be Found

"They think a tree came along and knocked the house off, it was on piers," he said. "At some point the house hit a bridge or something. They were being tossed around the house and then it hit something. They became separated at that point."

He said his son was “still in a state of shock because of everything that just happened."

Up to 72 structures in Wimberley were washed away, and 1,200 structures were damaged, said Kharley Smith, the emergency management coordinator in Hays County. Waters rose so quickly Sunday that roads turned to torrents, and 1,000 people were forced to evacuate.

"We have roads full of slabs now," Smith said.

The mayor of Wimberley said Sunday afternoon that search operations were suspended so crews could be on hand for possible rescue situations.

The search was complicated by another round of severe weather. At least eight tornadoes were reported in Texas on Monday, including one that touched down on the LBJ Ranch in Johnson City, according to the National Weather Service. A tornado was suspected in the death of a man in a mobile home in the small community of Pettibone, according to Milam County Judge Dave Barkemeyer. A tornado also believed behind the death of a woman in Blue, Oklahoma, on Monday afternoon.

In North Texas, forecasters warned of the possibility of the weather phenomenon known as a derecho — wind damage of more than 240 miles, coupled with wind gusts of 75 mph or greater. The threat was greatest for late afternoon.

On Monday, flash-flood warnings stretched from the Texas-Mexico border to western Tennessee and northern Missouri. Much of Oklahoma was under a flood warning.

Forecasters also warned that widespread heavy showers and thunderstorms could occur across most of Oklahoma, central and eastern Texas, Louisiana and Arkansas, possibly including damaging wind gusts, hail and isolated tornadoes.

The flooding over the weekend was described as "catastrophic."

In Hays County, the Blanco River rose to 34 feet in just three hours on Sunday — its height of 40.2 feet breaking a record crest dating to 1929 by nearly six feet. Memorial Day events were canceled in the nearby Hays County city of San Marcos, which said in a statement that the flooding was the "the most severe in recent memory."

Gov. Greg Abbott declared a state of disaster in 24 counties, including Hays and Houston, where heavy winds damaged an apartment building and left two people injured Sunday.

Abbott flew over parts of the Blanco River on Monday, a day after heavy rains pushed the river into surrounding neighborhoods. Abbott said the storms had "relentless tsunami-type power." He urged communities downstream to monitor flood levels and take the threat seriously.

About 200 miles to the west, a twister left 13 people dead and at least 230 injured in Ciudad Acuna, a city in Mexico situated across the border from Del Rio, Texas, according to The Associated Press.

A reported tornado in Amory, Mississippi, downed large trees, but no injuries had been reported, according to the Mississippi Emergency Management Agency.

As the storms moved north and east through Texas, Austin and surrounding Travis County experienced considerable flooding.

The Austin-Travis County EMS said it responded to 21 rescue calls on Sunday, including 17 in a three-hour period.

The storms were the latest in what has been a particularly wet year for the Plains, with several towns and cities already breaking their all-time wettest month records this May.

Records continued to tumble Sunday, with the 3.3 inches that fell in Dallas making it the wettest May 24 in 117 years. Oklahoma City added to what is already its wettest ever month, the 18.69 inches to fall this May far outweighing the 14.92 inches that fell in May 2013, according to The Weather Channel.

Across the nation, 21 river gauges were recording a "major flood" and 47 were showing "moderate flooding," most of which were in Texas and Oklahoma, according to Weather Channel lead meteorologist Kevin Roth.

In Broken Bow, Oklahoma, 13 people were trapped in a rental cabin on a river. The Oklahoma Highway Patrol said they were not in danger, but were stuck on an island when authorities opened the flood gates at a nearby lake and the water level rose.

The weather system also triggered 36 reported tornadoes on Sunday in Kansas, Nebraska, Illinois, Colorado and Iowa, according to the National Weather Service. Roth said that while that twister threat would persist in some areas on Monday it would be significantly lower than over the weekend.

The storms were expected to calm significantly from Tuesday through Friday. As of 10 p.m. EDT Sunday, much of the storms' fury had dissipated with active tornado warnings in just three counties on northwest Louisiana.

Erin Calabrese and Shamar Walters of NBC News contributed to this report.