Texas, Oklahoma, Louisiana face possible tornadoes, large hail

'Pray and Hold On': Family Recounts Riding Out La. Tornado
'Pray and Hold On': Family Recounts Riding Out La. Tornado

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.

SEE ALSO: JetSetJustine's trip to Dubai will give you a serious case of wanderlust

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.

See more of recent storms:

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.

More from NBCNews.com:

The Latest from our Partners