One video taken captured a rush of water breaking down from the surface and penetrating the concrete ceiling at the Virginia Square metro station in Arlington, Virginia.
Many streets and sidewalks flooded and pedestrians had to leap over flows of water at curbs in the US capital.
Areas of concern include the Great Falls, Virginia, area and southeastern Montgomery County, Maryland. No deaths, injuries, or missing people are reported at this time, said Alexandria Fire Department Senior Public Information Officer Raytevia Evans.
In Potomac, Maryland, the water tore a large gap in Belfast Road, local media reported.
Roads continue to be closed in Frederick City and County.
By early afternoon, thousands of area residents remained without power, according to a Washington Post report.
The forecast for the Harrisburg region called for the major rain and storm threat to lessen after 11 a.m.
The heavy rainfall could bring several inches of rain and could cause flooding concerns in areas where drainage is poor and near small streams. CSX and Amtrak have also put high-water speed restrictions in place. As the rains stopped, amounts totaled higher than 4 inches across the D.C. metro area.