Storms have continued to drench the US Midwest, which is already enduring record floods that are reported to have killed at least 25 people.
Meanwhile, a heatwave in south-eastern states is also setting records and has been blamed for a similar death toll.
States from Iowa to Texas have all been deluged, some suffering the aftermath of Tropical Storm Erin, followed by other heavy storm systems.
More heavy rain is forecast in Illinois, prompting flood warnings.
The National Weather Service has warned that residents can expect another 2in (5cm) of rain on Friday afternoon and evening.
"It could turn out to be a pretty bad flooding situation for the area," forecaster Nathan Marsili told the Associated Press.
Earlier in the day, another band of thunderstorms dumped more rain on Missouri, Iowa and Wisconsin.
About 500 flights via Chicago's O'Hare airport were cancelled on Thursday.
Forty people were injured in the same city when the roof of an industrial site collapsed during the violent thunderstorm.
Flood watches or warnings are in place for a number of states from Iowa to Texas.
While some areas have seen the worst of the weather, with a high pressure system expected to dry things out over the weekend, others may still have more rain to come.
"This is unprecedented," a spokesman for the National Weather Service told the AFP news agency.
Diving teams were deployed to search for a boy lost in Oklahoma
The victims included a teenage cross-country runner swept away while trying to cross a flooded watercourse in Oklahoma, and an elderly man in Ohio killed by fumes when flood waters knocked over a can of petrol and started a fire.
In Wisconsin three people, including a toddler, were electrocuted at a bus stop when a live power cable was struck by lightning and fell into a puddle. One of them was killed trying to assist the other two, while a bus driver was also badly hurt as he tried to help.
Hundreds of homes across the region were badly damaged by flooding, while mudslides in some places destroyed houses and hampered rescue efforts.
In Ohio, the rain eased on Friday but temperatures climbed to about 95F (above 30C), making the clean-up operation even tougher.
Texas is suffering its wettest year in more than a century, and 40 people have died there in flash flooding incidents in 2007 - equalling the record set in 1989.
"We've had persistent, ongoing, relentless precipitation pretty much all year," Victor Murphy, a meteorologist with the National Weather Service, told AFP.
Meanwhile, the south-eastern states of Tennessee, Georgia and Alabama continue to suffer from a heatwave and drought.
In Athens, Georgia, the temperature has exceeded 100F (38C) in 13 days this month - compared to the August average of one day.
"These are 100-year-plus records that are being shattered," Mr Murphy said.