UPDATE: A Severe Thunderstorm Warning was posted at 5:01 for Howard County and southwestern Baltimore County and northwestern Anne Arundel County, as a storm cell drifts east from northern Virginia. It remains in effect until 5:45 p.m. Earlier warnings remain for parts of Prince Georges and Charles counties. That has now been extended into southern Arundel and the Chesapeake Bay and Baltimore harbor.

"AT 500 PM EDT...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED A
  SEVERE THUNDERSTORM CAPABLE OF PRODUCING DAMAGING WINDS IN EXCESS
  OF 60 MPH.  THIS STORM WAS LOCATED 17 MILES WEST OF COLUMBIA...OR
  NEAR DAMASCUS...AND MOVING EAST AT 30 MPH."

EARLIER: The National Weather Service on Thursday afternoon issued a Severe Thunderstorm Watch for all of Maryland east of Garrett County, in effect until 9 p.m. The Watch means conditions are ripe for severe storm development, but that the storms are not yet imminent. Severe thunderstorms can produce damaging winds, dangerous lightning and large hail. 

Here's more from the NWS forecast office in Sterling:

"ISOLATED SHOWERS AND THUNDERSTORMS WILL CONTINUE TO DEVELOP ACROSS
THE EASTERN SUBURBS OF WASHINGTON DC AND THE CITY OF BALTIMORE
THIS AFTERNOON. ANY SHOWERS AND THUNDERSTORMS THAT DEVELOP WILL
BE CAPABLE OF PRODUCING HEAVY DOWNPOURS...OCCASIONAL LIGHTNING
STRIKES AND WIND GUSTS TO AROUND 30 MPH.

"ACTIVITY WILL BECOME MORE WIDESPREAD BETWEEN 4 AND 6 PM."