NBC's Lester Holt reports on the destruction, while TODAY's Al Roker looks at the next severe weather system.
As towns cleaned up and survivors recalled near-death tornado experiences, forecasters on Thursday warned of a new round of severe weather Thursday night and Friday that could produce even more twisters.
"We've got a really bad system starting to develop, just as bad if not worse for tomorrow," NBC weather anchor Al Roker reported on the TODAY show, citing "a strong risk of storms from Huntsville, Alabama, to Indianapolis and on into central Ohio."
Parts of Illinois and Mississippi are also at risk, he noted, and any twisters could be several miles long due to the system's strength.
Thirteen people were killed Wednesday in Illinois, Missouri, Tennessee and Kansas by a system that spawned more than a dozen twisters across the Midwest. Hardest hit was Harrisburg, Ill., where six people died, some 300 homes were destroyed or damaged, and residents had stories of survival and tragedy.
Son finds bodies of parents
Jeff Rann had ample warning early Wednesday. A frantic call to his wife from his mother-in-law alerted them to reports that a tornado was barreling down, and Rann heard the deafening wail of storm sirens.
Rann's home was untouched, but just two blocks away, his parents were not as fortunate.
Rann raced through the darkness to his parents' duplex, but saw instantly there was nothing left, natural gas whistling eerily as it spewed from the property's severed meter. In the mud of a debris-strewn field, Rann found the body of his dad, 65-year-old Randy Rann, and his mother, 62-year-old Donna Rann.
"She just said, 'It hurts. It hurts,'" Rann said of his mother, who had been looking forward to early retirement next month but who died a short time later at a hospital.
The National Weather Service preliminarily listed the tornado as an EF4, the second-highest rating given to twisters based on damage. Scientists said the tornado was 200 yards wide with winds up to 180 mph.
Across the road from the Ranns, a co-worker of Donna's, Amanda Patrick, was roused by the sirens about five minutes before all hell broke loose. She called Donna Rann — her co-worker at the U.S. Forest Service — to alert them but got no answer, then thrust herself into a bathtub as the twister she described as sounding "like a bulldozer and Hoover vacuum at the same time" ripped through.
How to help tornado victims
PHOTOBLOG: Destruction across Midwest
"Not trying to be holy, I got on my knees and said, 'God, watch over me,'" she said.
The winds shifted the tub as the walls buckled above her. In a gray T-shirt and pink-striped pajama pants, she crawled shoeless out into the rain and muck.
She called out for the Ranns but heard nothing back.
Hours later, tears streamed down Patrick's face as she grieved for the late couple.
"A couple weeks ago, there was a bad storm and I looked out the window to check on them," she said, sobbing. "Donna texted me and said, 'I saw you in the window.' She was checking on me. That's the way we were, always just looking out for each other."
This time, she said, "they didn't have a chance."
Hospital patients moved in time
At Harrisburg Medical Center, staffers were alerted to the tornado's approach by the sheriff's department some 20 minutes before the severe weather finally threw its punch, the center's CEO Vince Ashley said.
"We get these calls periodically, and often it's a false alarm," Ashley said. "But we get them often enough that everyone knows what to do."
Nurses hustled the patients into the hallways and away from their room's windows, closing the doors behind them, and were fighting to close the last of the heavy, steel fire doors at the end of the hallway when the tornado came, Ashley said. Seconds later, he said, windows started shattering, walls shook and ceiling tiles rattled.
The fierce winds blew some walls off some rooms, leaving disheveled beds and misplaced furniture but miraculously no injuries. Hours later, Ashley said some of the destroyed portions of the hospital will have to be razed and rebuilt.
Adding to the danger, it hit as many slept — a timing research meteorologist Harold Brooks called unusual but "not completely uncommon."
Brooks, with the NOAA National Severe Storms Laboratory in Norman, Okla., said perhaps 10 percent of tornadoes happen between midnight and 6 a.m., a time when the danger level rises because the storms are harder to spot and it's harder to get the word out.
"If you're asleep, you're less likely going to hear anything, any warning message on the danger," Brooks said.
Deaths, damage elsewhere
In southern Missouri, one person was killed in a Buffalo trailer park while two more fatalities were reported in the Cassville and Puxico areas.
A tornado hopscotched throughthe main thoroughfare of Missouri country music mecca Branson, damaging some of the city's famous theaters just days before the start of the town's crucial tourist season.
It went "bouncing from business to business to business -- tens if not hundreds of millions in property damage," Missouri Gov. Jay Nixon told NBC.
Three people were reported killed in eastern Tennessee — two in Cumberland County and another in DeKalb County.
And in Kansas, much of tiny Harveyville was in shambles from what state officials said was an EF2 tornado packing wind speeds of 120 to 130 mph.
A man whose Harveyville home collapsed on him was taken off life support Wednesday evening.
The Associated Press contributed to this report.
More content from msnbc.com and NBC News