In the last hour alone, some spots just west of the nation's capital have seen over 3 inches of rain, especially along the Potomac River.
Areas of concern include the Great Falls, Virginia, area and southeastern Montgomery County, Maryland. Extremely heavy rain continues to fall over much of the area, including the District of Columbia.
Storms are also affecting airports, with traffic heading to Reagan National stopped on the ground. Departure delays by more than one hour have been reported.
"Excessive runoff from heavy rainfall will cause flooding of small creeks and streams, urban areas, highways, streets and underpasses as well as other drainage areas and low lying spots," according to the weather service. More than a dozen high water rescues have already been reported in the area.
"Most flood deaths occur in vehicles," according to the National Weather Service, which advises against driving into "any area where water covers the road."
The service requests motorists to "move to higher ground as quickly as possible."