Hogan signed the order while receiving a briefing in Ellicott City from state and local emergency management personnel.
The aftermath of the floods in Main Street Ellicott City. Others said they gathered in the second storey of a building to anxiously watch the seething waters.
"Our administration is closely monitoring the situation and working in partnership with local officials, including Howard County Executive Allan Kittleman, to respond to this extreme weather as quickly and efficiently as possible".
Flash flooding covers the intersection of Rogers Avenue and Main Streets in Ellicott City, Md., May 27, 2018.
In a tweet Sunday evening, Maryland Governor Larry Hogan warned the flooding was "moving towards central Maryland", and urged residents to "avoid travel". There were not immediately any reports of injuries or fatalities.
Flash flooding and water rescues are being reported in Maryland as heavy rain soaks much of the state.
Jeff Halveson, professor of meteorology at the University of Maryland at Baltimore County and a contributor to the Capital Weather Gang, said the storm pattern had essentially stalled over Ellicott City, the same phenomenon that caused massive flooding in the City of Frederick two weeks ago.
Two years ago, 6.5 inches of rain fell on the town in just three hours.
At least five roads in Baltimore have been closed due to the flooding.
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Information still is preliminary, however, and authorities say they are still checking flooded buildings and streets.
Footage of Sunday's flash flooding showed water surging around cars and pickup trucks and coursing over the top of their wheels.
Rescue workers were out in force helping the stranded, braving flood waters that in some instances lifted automobiles to the first floor of some buildings.
Some Baltimore County roads also have been flooded.
"Strong storms bringing heavy rain & potential for flash floods are now moving across central Maryland".
Videos taken on Ellicott City's Main Street show boiling brown waters flowing down the street, carrying debris and trash. That's in the same area devastated by flash flooding in July 2016.
A flash flood emergency was issued in Maryland's Howard County at 4:40 p.m.
Governor Hogan said state and local rescue workers, as well as law enforcement, will be working throughout the night to help those in need in Ellicott City.