Washington, Philadelphia, New York City are in the bullseye with a forecast says up to 6 inches of rain will fall in some areas by Friday.
Maureen Braun considered plowing through a foot of water on a Fairfax, Virginia, street, but changed her mind.
"I'm turning around," she told CNN affiliate WUSA. "I'm not an idiot!"
It was much the same in the Philadelphia area.
Bob Bonenberger found himself cut off at every pass trying to get to his home in Skippack, Pennsylvania.
"It's just terrible," Bonenberger told affiliate KYW. "The flooding is everywhere."
In the New York City area, the heavy rain caused a mudslide that covered three of four Metro-North commuter rail tracks in Yonkers on Wednesday night, delaying train service, affiliate WABC reported.
Robbs Barbota got stuck in high water in northern New Jersey. The experience left him shaken.
"I'm too young to die," he thought as the rains swamped his car. "Please someone help me."
Conditions were precarious outside the nation's capital as floodwaters threatened to overwhelm a dam in Laurel, Maryland.
Authorities ordered evacuations as they opened flood gates on the T. Howard Duckett Dam to relieve pressure. Residents from an apartment complex, a senior center and a hotel were moved to a community center as waters rose.
Florida sets rain record
As much as 18.9 inches of rain fell over 24 hours in Alabama and Florida, according to the National Weather Service.
At the airport in Pensacola, Florida, an estimated 15.55 inches of rain fell on Tuesday -- the largest amount to fall in a single calendar day since officials started tracking rainfall in 1880, the National Weather Service said.
"We've seen flooding before, but never flooding that washes the back of a house away," said CNN iReporter Matt Raybourn of Pensacola. "There are no words for what we are seeing here."
The rushing waters reduced some streets to rubble, gouged huge gashes in others and left stretches of many others submerged, including parts U.S. 98, the main east-west route along the coast. It was closed in several places between Fort Walton Beach and Panama City.
CNN iReporter Randy Hamilton said the scene felt like the aftermath of a hurricane with "abandoned and flooded cars just littering the streets."
"Debris from trees everywhere. Standing water all around, gray skies, and wind gusts that make you fear something will blow down on you," he said.
Along the coast, water pushed ashore by wind-driven waves and unusually high tides lapped at sand dunes as red flags warned swimmers to stay out of the water.
Tides up to 2 feet higher than normal were causing minor coastal flooding and dangerous rip currents, the National Weather Service said.
There was one drowning death tied to the flooding, Florida officials said. The victim, a 67-year-old woman, drove or was swept into a drainage ditch Tuesday night, Escambia County spokeswoman Kathleen Castro said.
Florida Gov. Rick Scott said that crews had rescued 300 people.
'People have lost everything'
"It's unbelievable the amount of flooding we've had. We've have roads torn up, water clear through homes. I went to one home where the foundation was gone. ... You saw cars just thrown down a road, pushed down upside down," Scott said.
"People have lost everything, and they didn't buy flood insurance because they didn't think they were in a flood area."
Scott declared a state of emergency.
In Alabama, waters rushed in so fast that residents climbed on top of furniture and waited on rooftops to be rescued, said Reggie Chitwood, deputy director of emergency management in Baldwin County.
"The waters rose at a historical level. ... People had to scramble," Chitwood said.
Arkansas tornado was EF-4
The storms were part of the same system that has spawned tornadoes and other severe storms since Sunday, claiming at least 38 lives in Oklahoma, Iowa, Arkansas, Mississippi, Alabama, Tennessee and now Florida.
Tens of thousands remained without power in the South, where suspected tornadoes tore through homes and businesses late Monday.
Officials revealed new details Wednesday about the powerful tornado that struck the Arkansas towns of Mayflower and Vilonia.
The twister was rated an EF-4 on the Enhanced Fujita Tornado Damage scale, with an estimated peak wind speed between 180 and 190 mph, the National Weather Service said. It was on the ground for 41.3 miles on Sunday and was responsible for 15 deaths, officials said.
In addition to the Florida flooding death, police in Athens, Georgia, said a driver was killed Wednesday when storm winds toppled a tree onto a car.
Authorities in Mississippi confirmed another death from storms that struck Monday.
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