Texas, Oklahoma, Louisiana face possible tornadoes, large hail

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More than 15 million Americans were facing the threat of a severe thunderstorm capable of producing tornadoes, large hail and high winds which took aim at several states Tuesday.

The storm originated in Mexico and began crossing the border in Big Bend, Texas, early Tuesday. The storm started increasing in strength and size as it moved stateside.

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By 3:30 a.m. ET, a severe thunderstorm watch was in effect for a 350-mile-long chunk of Texas -- threatening "very large hail," wind gusts of 70 mph, and "a tornado or two," the National Weather Service said.

As the storm was set to increase, most of Texas -- including Austin, Dallas, and Houston -- as well as most of Oklahoma and small parts of Kansas, Missouri, Arkansas and Louisiana, were expected to be under threat of severe thunderstorms through Tuesday, according to The Weather Channel.

The NWS said that people "in these areas should be on the lookout for threatening weather conditions and listen for later statements and possible warnings. Severe thunderstorms can and occasionally do produce tornadoes," it added.

This threat of severe thunderstorms was forecast to have shifted eastward Wednesday, straddling the Texas-Louisiana border and covering Shreveport, Lake Charles and Baton Rouge.

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Texas, Oklahoma, Louisiana face possible tornadoes, large hail
WAVERLY, VA - FEBRUARY 25 The remains of a home lie in the path of destruction from a tornado that barreled through the small community of Waverly,the day before on February 25, 2016 in Waverly, Virginia. Virginia's Governor McAuliffe declared a state of emergency after a storm system spawned tornados that killed four people on February 24, and the Virginia National Guard expects to have more than 50 soldiers on duty to help emergency response. (Photo by Jay Paul/Getty Images)
WAVERLY, VA - FEBRUARY 25: Paul Logan, 21, talks about the tornado that barreled through the small community of Waverly, VA the day before on February 25, 2016 in Waverly, Virginia. Logan said it sounded like a bomb exploded in the front of his home during the storm. Virginia's Governor McAuliffe declared a state of emergency after a storm system spawned tornados that killed four people on February 24, and the Virginia National Guard expects to have more than 50 soldiers on duty to help emergency response. (Photo by Jay Paul/Getty Images)
WAVERLY, VA - FEBRUARY 25 The remains of a home lie in the path of destruction from a tornado that barreled through the small community of Waverly,the day before on February 25, 2016 in Waverly, Virginia. Virginia's Governor McAuliffe declared a state of emergency after a storm system spawned tornados that killed four people on February 24, and the Virginia National Guard expects to have more than 50 soldiers on duty to help emergency response. (Photo by Jay Paul/Getty Images)
WAVERLY, VA - FEBRUARY 25: A tree lays on a house a day after a tornado barreled through the small community of Waverly, on February 25, 2016 in Waverly, Virginia. Virginia's Governor McAuliffe declared a state of emergency after a storm system spawned tornados that killed four people on February 24, and the Virginia National Guard expects to have more than 50 soldiers on duty to help emergency response. (Photo by Jay Paul/Getty Images)
WAVERLY, VA - FEBRUARY 25 Sussex, County deputy, Larry Anthony, right, and Investigator, Eddie Oburn look over the remains of a home where two men and a child were killed when a tornado barreled through the small community of Waverlym the day before on February 25, 2016 in Waverly, Virginia. Virginia's Governor McAuliffe declared a state of emergency after a storm system spawned tornados that killed four people on February 24, and the Virginia National Guard expects to have more than 50 soldiers on duty to help emergency response. (Photo by Jay Paul/Getty Images)
WAVERLY, VA - FEBRUARY 25: Sussex, County deputy, Larry Anthony, (C) and Investigator, Eddie Oburn, (L), look over the remains of a home where two men and a child were killed when a tornado barreled through the small community of Waverly on February 25, 2016 in Waverly, Virginia. Virginia's Governor McAuliffe declared a state of emergency after a storm system spawned tornados that killed four people on February 24, and the Virginia National Guard expects to have more than 50 soldiers on duty to help emergency response. (Photo by Jay Paul/Getty Images)
WAVERLY, VA - FEBRUARY 25: The remains of the house that held 3 victims, two men and a child, when a tornado tore through the structure are seen February 25, 2016 in Waverly, Virginia. Virginia's Governor McAuliffe declared a state of emergency after a storm system spawned tornados that killed four people on February 24, and the Virginia National Guard expects to have more than 50 soldiers on duty to help emergency response. (Photo by Jay Paul/Getty Images)
WAVERLY, VIRGINIA Ã FEBRUARY 25, 2016: Sussex, County deputy, Larry Anthony, left, and Investigator, Eddie Oburn look over the remains of a home where two men and a child were killed when a tornado barreled through the small community of Waverly the day before February 25, 2016 in Waverly, Virginia. Virginia's Governor McAuliffe declared a state of emergency after a storm system spawned tornados that killed four people on February 24, and the Virginia National Guard expects to have more than 50 soldiers on duty to help emergency response. (Photo by Jay Paul/Getty Images)
WAVERLY, VA - FEBRUARY 25: The remains of the house that held 3 victims, two men and a child, when a tornado tore through the structure are seen February 25, 2016 in Waverly, Virginia. Virginia's Governor McAuliffe declared a state of emergency after a storm system spawned tornados that killed four people on February 24, and the Virginia National Guard expects to have more than 50 soldiers on duty to help emergency response. (Photo by Jay Paul/Getty Images)
WAVERLY, VIRGINIA Ã FEBRUARY 25, 2016: Sussex, County Investigator, Eddie Oburn walks past a truck that was crushed by a tree during a storm the day before February 25, 2016 in Waverly, Virginia. Virginia's Governor McAuliffe declared a state of emergency after a storm system spawned tornados that killed four people on February 24, and the Virginia National Guard expects to have more than 50 soldiers on duty to help emergency response. (Photo by Jay Paul/Getty Images)
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According to NBC News meteorologist Bill Karins, the storm is rare because is being fueled by the jet stream dipping unusually low. "A strong jet stream rarely dips all the way into Mexico in March -- or anytime -- but that is exactly what is happening today," he said.

This pattern meant the storm would not be swept up in the typical west-east "flow" of weather that moves across the U.S. "With nothing to push the storm along, it will linger ... for three days producing periods of strong to severe thunderstorms," Karins added.

Meanwhile, showers and thunderstorms were expected to dump between 3 and 12 inches of rain through Friday across a huge area from the Gulf of Mexico to Illinois, and central Texas to Mississippi.

The heaviest rain was forecast in Arkansas, Louisiana and Texas, and a flash flood watch was in effect early Tuesday across those states plus Oklahoma.

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