The Storm Prediction Center has issued a Tornado Watch valid until 2 AM for the Washington-Baltimore metro area (yellow shaded region below).
A new squall line is rapidly developing and approaching our region from the southwest - depicted on radar below:
Note how the squall line (red colors) is bowing outward through central Virginia. Bow echoes such as this can generate pockets of intense wind damage called downbursts. Regarding the tornado watch, the concern is not for strong, supercell-generated tornadoes, but rather isolated, weak tornadoes that often develop with these bowing line segments. Any small tornado that forms may be difficult to detect by weather radar, and they are often invisible during dark and/or when enshrouded by heavy rain.
Oddly, there is unlikely to be any lightning and thunder associated with this line of storms.
The likely time that this squall line will pass through our region is between 7:30-9 PM tonight.
A Flash Flood Warning is in place for counties to the north and west of DC-Baltimore. Several inches of rain are possible from echo training - when more than one storm cell passes repeatedly over the same location.