​​A four-day, once-in-a-generation weather event is pounding the middle of the U.S. with destructive tornadoes and life-threatening flooding.

Friday marks day three of the devastating storm, with severe flooding in Kentucky and Tennessee and possible intense tornadoes for parts of Texas, Mississippi, Tennessee, Kentucky, Missouri and far southern Illinois. Here's what you need to know:

At least eight people have died across four states.

In Franklin County, Kentucky, a 9-year-old boy died after he got caught in floodwaters on Friday while walking to a school bus stop, officials said.

Authorities in Missouri said a local fire chief, 68-year-old Garry Moore while helping a stranded driver on Wednesday. Moore was the chief of the Whitewater Fire Protection District.

Another death was confirmed in Hendricks County, Indiana, just outside of Indianapolis. A 27-year-old man was driving on Wednesday when he hit downed power lines in the road, and then he got out of his car "and came into contact with the live power lines," the Hendricks County Sheriff’s Office said.

Another five weather-related fatalities were confirmed in Tennessee, according to state officials.

Gov. Bill Lee announced the fifth death in the state during a news conference Thursday evening, where he spoke of the "immense devastation" wrought by a powerful tornado that tore through the small city of Selmer, in the southwestern part of the state, between Memphis and Nashville.

Lee had declared a state of emergency in Tennessee, as did Kentucky Gov. Andy Beshear.

"We are facing one of the most serious weather events we've had forecast," Beshear warned on social media. "Please stay alert, take all precautions, and be prepared."

Since Wednesday, over a half foot of rain has inundated portions of Tennessee, Arkansas and Kentucky -- and the threat isn't over.

Flash flooding will be major concern Friday night and Saturday.

Louisville, Kentucky, Mayor Craig Greenberg warned that this could be "one of the top 10 floods in recorded history in Louisville."

A massive flood watch is now in effect from Texarkana, Texas, to Little Rock, Arkansas, to Memphis, Tennessee, to Louisville to Indianapolis to Columbus, Ohio, to Charleston, West Virginia.

A particularly dangerous situation, or PDS, flood watch has been issued from Texarkana to Little Rock to Memphis to Paducah, Kentucky.

By the time the storm ends, rain totals could be over 15 inches in some areas. Some cities may see record-high four-day rain totals.

Rivers, creeks and other waterways could also advance into major flood stage from Arkansas to Kentucky.

The flooding could pose problems for "days, if not weeks, to come," Kentucky's governor warned Friday.

On Friday, a tornado watch is in effect for portions of Texas, Louisiana, Mississippi, Arkansas, Kentucky, Missouri, Tennessee and Illinois.

Since the outbreak began Wednesday, there have been more than four dozen reported tornadoes from Arkansas to Ohio. This includes an EF-3 tornado in Selmer, Tennessee, with winds of 160 mph, and an EF-3 tornado in Lake City, Arkansas, with winds of 150 mph.

Matt Ziegler documented the moment the tornado hit Lake City.

"I've always heard that they sound like a train on a track, but to be honest with you, it was eerily quiet," he told ABC News. "If you weren't looking, you wouldn't know that there was a major tornado just a field over from us."

On Saturday, the severe threat is labeled "enhanced," a level 3 out of 5 risk, with the potential for strong tornadoes from Louisiana to Tennessee.

The system will finally move east Sunday afternoon, bringing rain to the Southeast on Sunday, Monday and Tuesday.

ABC News' Elyse Smith and Victoria Arancio contributed to this report.

At least 8 killed as devastating storm pounds central US with more flooding, tornado threats originally appeared on abcnews.go.com