Flash flooding has caused major damage in Baltimore, sending cars floating down streets.

Key points: Authorities aren't aware of any fatalities or missing people

Authorities aren't aware of any fatalities or missing people In July 2016, Ellicott City received 17 centimetres of rain over a two-to-three-hour period

In July 2016, Ellicott City received 17 centimetres of rain over a two-to-three-hour period Only two weeks ago the state and county was awarded over $US1 million for flood prevention

After floods caused chaos in the Main Street of historic Ellicott City, Maryland, local officials said they were heartbroken to see the community so severely damaged again less than two years after a devastating flood killed two people and caused millions in damages.

As the flood waters receded late on Sunday (local time), officials were just beginning the grim task of assessing the destruction.

During an evening news conference, Howard County executive Allan Kittleman said authorities were not aware of any fatalities or missing people.

But first responders and rescue officials were still going through the muddied, damaged downtown, conducting safety checks and ensuring people evacuated.

Mr Kittleman said the damage was significant and appeared to him to be worse than the flooding two summers ago.

He said residents and business owners "are faced with the same daunting task again".

"We will be there for them as we were in 2016," he said.

A parked car is flooded in a lot near Main Street and Ellicott Mills Road, one of many to be swept up in the flooding. ( AP: Kenneth K. Lam/The Baltimore Sun )

Governor Larry Hogan also toured the area and promised "every bit of assistance we possibly can".

"They say this is a once every 1,000-year flood and we've had two of them in two years," Mr Hogan said.

The flooding swept away parked cars in Ellicott City, set along the west bank of Maryland's Patapsco River and about 20 kilometres west of Baltimore.

'Significantly higher' than fatal 2016 flood

Jessica Ur, a server at Pure Wine Cafe on the city's Main Street, told The Baltimore Sun that she watched as gushing waters swept three or four parked cars down the street.

"It's significantly higher than it was before," she told the newspaper, comparing the floodwaters to those of 2016.

Mike Muccilli, a meteorologist with the National Weather Service in Sterling, Virginia, said it's too early to make comparisons between the two floods.

But he said both were devastating.

The flooding caused major damage to buildings, streets and cars. ( AP: Libby Solomon/The Baltimore Sun )

In July 2016, Ellicott City received 17 centimetres of rain over a two-to-three-hour period.

On Sunday, the community received nearly 20.32 centimetres of rain over a six-hour period, but most of it fell during an intense three-hour period, Mr Muccilli said.

"In a normal heavy rain event, you wouldn't see this amount of flooding, where you see cars floating down the road," Mr Muccilli said.

"This was a true flash flood."

Some people reported hearing a blaring alarm during the flooding. Others said they gathered in the second storey of a building to anxiously watch the seething waters.

During the flood a handmade white flag was hung from an upper story of a Main Street building bearing the letters SOS.

"If you are trapped, we are coming," the Howard County Department of Fire and Rescue Services tweeted at one point.

Ellicott City has been rebuilding since the 2016 flooding damaged and destroyed businesses.

Local officials recently said that 96 per cent of the businesses were back in operation and more than 20 new businesses had again opened in the Main Street area.

Just two weeks ago, Mr Hogan announced that the Federal Emergency Management Agency had awarded the state and county more than $US1 million to pay for projects aimed at reducing the flood risk in areas around Main Street.

Some are already asking questions about whether enough was done after the last flood to prevent a similar catastrophe.

Mr Hogan said temporary improvements were in place and more things were in the works to reduce the community's vulnerabilities.

But he said big changes take time, and no one expected such a huge flood so soon after 2016.

AP