The National Weather Service has issue a Flash Flood Watch for Carroll County, Maryland and surrounding areas:
FLASH FLOOD WATCH REMAINS IN EFFECT THROUGH LATE TONIGHT…
The Flash Flood Watch continues for

* Portions of Maryland, The District of Columbia, Virginia, and
West Virginia, including the following areas, in Maryland,
Anne Arundel, Calvert, Carroll, Central and Eastern Allegany,
Central and Southeast Howard, Central and Southeast
Montgomery, Charles, Extreme Western Allegany, Frederick MD,
Northern Baltimore, Northwest Harford, Northwest Howard,
Northwest Montgomery, Prince Georges, Southeast Harford,
Southern Baltimore, St. Marys, and Washington. The District of
Columbia. In Virginia, Albemarle, Arlington/Falls
Church/Alexandria, Augusta, Central Virginia Blue Ridge,
Clarke, Culpeper, Eastern Highland, Eastern Loudoun, Fairfax,
Frederick VA, Greene, King George, Madison, Nelson, Northern
Fauquier, Northern Virginia Blue Ridge, Orange, Page, Prince
William/Manassas/Manassas Park, Rappahannock, Rockingham,
Shenandoah, Southern Fauquier, Spotsylvania, Stafford, Warren,
Western Highland, and Western Loudoun. In West Virginia,
Berkeley, Eastern Grant, Eastern Mineral, Eastern Pendleton,
Hampshire, Hardy, Jefferson, Morgan, Western Grant, Western
Mineral, and Western Pendleton.

* Through late tonight

* Numerous showers and thunderstorms are expected this afternoon
and continue through tonight. Between 1 and 3 inches of rain can
be expected this afternoon, with locally higher amounts. Due to
recent rainfall, the ground will not be able to absorb this
rainfall.

* Rapid rises of creeks and streams will be likely due to
repeated rounds of torrential rainfall. Low lying areas will
be particularly susceptible. Runoff from the heavy rainfall
likely will accumulate on roads making them impassible. Flash
flooding may develop quickly. Be prepared to act if
floodwaters threaten.

A Flash Flood Watch means that conditions may develop that lead
to flash flooding. Flash flooding is a very dangerous situation.
You should monitor later forecasts and be prepared to take action
should Flash Flood Warnings be issued.