Millions likely to see strong nocturnal tornadoes or thunderstorms this weekend

More than 17 million residents across parts of the Mississippi, Ohio and Tennessee Valleys could see moderate to enhanced risks of severe weather Friday night, according to the National Weather Service Storm Prediction Center. The agency also warned about the danger of "nocturnal" tornadoes — tornadoes that occur overnight — in cities throughout Missouri, Tennessee and Indiana, telling residents to make a plan as soon as possible. 

"Make sure folks know about this threat," the Nashville branch of the National Weather Service tweeted. "It's going to be an *overnight* event and those can be the most deadly."

The Storm Prediction Center issued a level four, or moderate, risk for severe weather on Friday. The Center's hazardous weather outlook threshold ranges from level one, or minimal risk, to level five, which is considered very significant, and a level four suggests a few tornadoes, strong wind and isolated incidences of severe hail. Cities under the issued risk account for more than 2.3 million people and include Memphis, Tennessee; Jonesboro, Arkansas; Indianapolis, Indiana; Nashville, Tennessee and St. Louis, Missouri.

"We believe the storms tonight, some of them, will have the potential to produce fairly long-track tornadoes, meaning several miles in length," Bill Bunting, a Storm Prediction Center meteorologist, told CBS News. "That is capable of doing significant structural damage."

He said the overnight storms should move eastward and be off the coast by Sunday and that the "threat will have shifted" for some areas as early as Saturday morning. 

In a study conducted by the National Institute for Risk and Resilience as well as the Storm Prediction Center, researchers said a tornado that occurs overnight is more than twice as likely to kill someone than one that happens during the day. 

"Nocturnal tornadoes are difficult to forecast, difficult to see (and therefore confirm,) and difficult to respond to because much of the population is asleep when they occur," the researchers said. 

Residents in affected areas for Friday's forecasted storms are urged to have multiple ways to receive warnings that will wake them up, to quickly move to the basement in the event of a tornado, to prepare an emergency supply kit and to know where to go when warnings are issued. 

In:
• Tennessee
• Tornado