Residents of low-lying towns in the central U.S. stacked sandbags, grabbed belongings and evacuated their homes on Wednesday after 30 cm of rain pushed rivers and creeks out of their banks.
At least 13 deaths had been linked to the weather, and three people are reported missing.
Record or near-record flood crests were forecast for several towns in Missouri. Flooding was reported in large areas of Arkansas and parts of southern Illinois, southern Indiana and southwestern Ohio, and schools were closed in parts of western Kentucky because of flooded roads.
"We've got water rising everywhere," said Jeff Korb, a municipal official in Vanderbugh County, Ind.
The U.S. National Weather Service posted flood and flash flood warnings from Texas to Pennsylvania.
After two days, rain had finally stopped falling Wednesday afternoon in much of Missouri and Arkansas, as the weather system crawled toward the Northeast, drenching the Ohio Valley and spreading snow over parts of northern New England. A parallel band of locally heavy rain stretched from Alabama and Georgia to the mid-Atlantic states.
Atlanta police closed some downtown streets in case the stormy weather knocked down more broken and debris from buildings damaged by Friday's tornado.
In Ohio and other areas, the rain fell on ground already saturated from heavy snowfall less than two weeks ago.
'He was going down the creek screaming'
Five deaths were linked to the flooding in Missouri, five people were killed in a highway wreck in heavy rain in Kentucky and a 65-year-old Ohio woman appeared to have drowned while checking on a sump pump in her home. In southern Illinois, two bodies were found hours after floodwaters swept a pickup truck off a rural road.
Searches were under way in Texas for a teenager washed down a drainage pipe, and two people were missing in Arkansas after their vehicles were swept away by rushing water.
Searchers in Missouri found the body of Mark Speir on Wednesday about three kilometres downstream from where he was seen being swept into a creek the previous evening.
"He was going down the creek screaming and hollering," Lawrence County emergency management chief Mike Rowe said.
An estimated 300 houses and businesses were flooded in the Missouri town of Piedmont, where dozens of people were rescued by boat.
In Valley Park, outside St. Louis, resident Chandra Webster and her children ran bags of toys and clothes to their car, and stacked belongings on the second floor of their house. The nearby swelling Meramec River was threatening the entire community.
"It's a lot of work, but it's worth it to save your stuff," Webster, 34, said.
Valley Park alderman Steve Drake helped fill sandbags with other volunteers.
"We've got everybody working together," Drake said. "It's going to be interesting."
Meanwhile, widespread flooding in Arkansas had washed out some highways and led to evacuations in some areas, while residents in southern Illinois also had to evacuate. In the Illinois city of Marion, firefighters in some cases used their own fishing boats to rescue 13 stranded residents.
Key roads were closed in the Cincinnati area, where water more than a metre deep was reported in businesses in the nearby suburb of Sharonville, police said.