...The combination of a slow moving frontal zone with a tropical air mass in place will increase the risk of flash flooding this afternoon and evening... * WHAT...Flash flooding caused by excessive rainfall continues to be possible. * WHERE...Washington DC, portions of Maryland, including the following areas, Anne Arundel, Calvert, Carroll, Cecil, Central and Southeast Howard, Central and Southeast Montgomery, Charles, Frederick, Northern Baltimore, Northwest Harford, Northwest Howard, Northwest Montgomery, Prince Georges, Southeast Harford, Southern Baltimore, St. Marys and Washington, Virginia, including the following areas, Albemarle, Arlington/Falls Church/Alexandria, Central and Southeast Prince William/Manassas/Manassas Park, Clarke, Culpeper, Eastern Loudoun, Fairfax, Frederick, Greene, King George, Madison, Northern Fauquier, Northern Virginia Blue Ridge, Northwest Prince William, Orange, Page, Rappahannock, Shenandoah, Southern Fauquier, Spotsylvania, Stafford, Warren and Western Loudoun, and eastern West Virginia, including the following areas, Berkeley, Jefferson and Morgan. * WHEN...From 4 PM EDT this afternoon through this evening. * IMPACTS...Excessive runoff may result in flooding of rivers, creeks, streams, and other low-lying and flood-prone locations. Flooding may occur in poor drainage and urban areas. * ADDITIONAL DETAILS... - Multiple rounds of heavy rainfall are possible this afternoon and evening, some of which may occur over areas which recently experienced flooding rains. Slow storm motions and repeat thunderstorm activity will occur within a tropical air mass. Hourly rainfall rates could push into the 2 to 3 inch range in the strongest storms, with an inch in 20 minutes possible. - Please visit www.weather.gov/safety/flood for flood safety and preparedness information.