Some areas recorded 10 to 15 inches (25.4 to 38.1 cm) of rain. Photograph: (AFP)
Around 1,000 people have been evacuated as parts of Louisiana brace for more rain over the next couple of days
Torrential rains have caused record flooding in parts of the southern United States, officials said on Saturday, with US media reporting at least three deaths.
Flooding in southeast Louisiana and southern Mississippi shut down roads, cutting off at least one town as an area of low pressure slowly moved west along the Gulf Coast.
Louisiana Governor John Bel Edwards declared a state of emergency, calling the flooding "unprecedented."
"We have record levels of flooding along rivers and creeks," he told reporters during a news conference, urging residents who have been advised to evacuate to leave their homes.
Emergency services were transporting residents by high-water vehicles, boats and aircraft, he said. More than 1,000 residents had been evacuated in Louisiana, the authorities said.
The floods killed at least three people on Friday, media reported. Among them, a man in the Louisiana town of Zachary, near the capital Baton Rouge, drowned trying to escape flood waters, local television station WAFB reported.
"We were walking out and he slipped and fell," his roommate Vernon Drummond told the station. "He went under the water. We tried to save him, but we couldn't."
The area recorded 10 to 15 inches (25.4 to 38.1 cm) of rain, David Roth, a meteorologist with the National Weather Service, told AFP. Another 10 inches were expected in parts of Louisiana over the next two days.
"Even for them it's very unusual," Roth said.
One of the worst storms 'ever'
Layton Ricks, president of Livingston parish in the Baton Rouge area, told reporters that "we're experiencing one of the worst storm events we've ever had, with flash flooding."
Roads that had never flooded were under water, he said, adding that the backlog of people waiting to be rescued.
The National Weather Service reported Saturday afternoon that the storming had "resulted in catastrophic flash flooding across Louisiana," and was continuing to produce very heavy rainfall.
The threat of heavy rain would expand westward with "at least a slight risk of flash flooding tonight over a large area from the southern plains to the mid-Mississippi/Ohio valleys and even the Northeast," the National Weather Service said.
Showers and thunderstorms would continue into Sunday and Monday, it predicted.
Entergy Louisiana said more than 11,700 customers were affected by power outages as of Saturday evening.
On the East Coast, meanwhile, millions of American residents were sweating through a heat wave amid extreme weather warnings in New York, Philadelphia, and Washington.
The combination of heat and humidity would make it feel as hot as 110 degrees Fahrenheit (43 degrees Celsius) in those cities.
Temperatures would be 5 to 15 degrees above average on Sunday across much of the Mid-Atlantic and Northeast, the National Weather Service said.
Authorities warned of heat-related health problems, especially for the elderly and those with chronic health problems, and for people who work outdoors.
Americans were advised to stay inside and use air conditioning where possible, check on vulnerable friends and neighbors, drink plenty of fluids, and not leave children or pets unattended in vehicles.
Overall, the heat wave stretched from southwest Ohio to western Virginia and Washington, and north through Philadelphia, New York and Boston, the National Weather Service's Roth said.