Brown water rushed through Ellicott City's historic Main Street, toppling buildings and upending cars, as the nearby Patapsco River swelled to a record-breaking level.
Authorities said they had no immediate reports of fatalities or injuries Sunday.
Maryland Gov. Larry Hogan earlier declared a state of emergency in the flood-stricken area. Swift water units from as far away as Northern Virginia also were summoned. Amid the flooding, the Howard fire department warned people trapped on the city's Main Street to climb to the second floors of buildings as they awaited rescue and said the damage could rival devastating flood that occurred there in 2016. Two residents died in the flood waters. He said he thought the flooding could ultimately be worse than what was seen in Ellicott City in 2016.
During the July 2016 storm, 6 inches of rain fell in about an hour in Ellicott City.
Some residents of Ellicott City told The Baltimore Sun the flooding appeared to be worse than the 2016 flooding that killed two people and destroyed local businesses. A Flash Flood Emergency remains in place.
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The areas of Ellicott City and Catonsville, outside Baltimore, were experiencing "catastrophic flash flooding", with another forecasted surge of water "imminent", according to the National Weather Service.
"I can tell you my heart is broken thinking about what the people have gone through here and the people's lives who were devastated two years ago and rebuilt".
"If you don't have to be outside, you shouldn't be", Baltimore County Fire Department spokeswoman Elise Armacost said in a statement obtained by The Sun.
The National Weather Service says a Flash Flood Warning remains in effect for Anne Arundel, Howard, and Baltimore, and Carroll Counties, and a Flood Warning is in effect for Prince George's and Calvert Counties. During that flooding, houses along Main Street shook as residents filmed the devastation out their windows. The county's emergency operations center is open and responding to the emergency. The Community Action Council of Howard County is asking for water, cleaning supplies and flashlights.