Videos and photos posted to social media and shared with Fox 5 showed the city's Main Street - the same street devastated by flash flooding in July 2016 - inundated with severe floods as water rushed through the area.
Ellicott City, Maryland, located about 12 miles west of Baltimore, resides in the valley of the Patapsco River, which flows into the Chesapeake Bay.
The aftermath of the floods in Main Street Ellicott City.
Here's what the flooding in Ellicott City looks like: In one dramatic video, a police officer instructs drivers to turn their cars around as the flooding intensified.
Portalli's Italian Restaurant employee Arianna Wilgar said the water had reached the establishment's second floor.
Ellicott City, Maryland, was hit with a devastating flash flood on Sunday.
Just two weeks ago, the Federal Emergency Management Agency gave the county $1 million to fix a culvert and replace a runoff pipe with a larger one. Major damage is reported and many cars have been swept away.More news: Hawaii's Kilauea volcano spews lava
Portolli's, an Italian restaurant that had been damaged in the 2016 Ellicott City flooding, was damaged again and forced to closed after being open for just six months. Affected Ellicott City residents may go to the Roger Carter Community Center or St. Peter's Episcopal Church, where Howard County emergency officials have set up shelters.
Authorities had no immediate report of any injuries or fatalities though information was still preliminary.
Flash flooding and water rescues are being reported in Maryland as heavy rain soaks much of the state.
Footage of Sunday's flash flooding showed the seething floodwaters engulfing cars and pickup trucks.
Howard County government said the flooding was on the same scale as a deadly 2016 storm.
Baltimore County Fire Department spokeswoman Elise Armacost said: "If you don't have to be outside, you shouldn't be".
Hogan also toured the area and promised "every bit of assistance we possibly can".
The National Weather Service issued a flash flood emergency at 4.40pm (9:40pm BST). Howard County Fire and EMS urged residents to evacuate downtown or move to higher ground while rescue teams swarmed the area.