UPDATE, 8:45 p.m.: The severe thunderstorm watch has been canceled for the region. However, intermittent, scattered showers and possibly the rumble of thunder are possible overnight as the slow-moving cold front nears the region.
UPDATE, 6:30 p.m.: Thunderstorms have weakened for the most part around the region and we’re left with mainly light to moderate showers which should gradually exit over the next couple hours from southwest to northeast.
UPDATE, 6:00 p.m.: The worst of the storms are well past D.C.’s northern suburbs. Scattered showers and storms are moving through northern Virginia through Alexandria into Dumfries and then stretching back to Culpeper. These are not severe but contain some locally heavy downpours. All of this activity will push east into southern Maryland and the northern neck of Virginia over the next 1-2 hours or so
UPDATE, 4:40 p.m.: A broken line of scattered strong to, at times, severe storms stretches from west of Baltimore to just north of the District and to the southwest into central Fauquier county. This line of storms is moving due east with strongest activity approaching Laurel and Columbia.
Thus far the strongest storms have concentrated north of D.C. In D.C. and points south, storms are likely to come through a bit later (between 5 and 7 p.m. west to east) and may be a bit more hit and miss.
UPDATE, 3:40 p.m.: A severe thunderstorm watch covers the entire metro region through 9 p.m.
Overview, from 3:10 p.m.: It’s hot (95 or higher for the 26th time this year, most on record to date), it’s humid (heat index 103 at 3 p.m.) and a cold front is gradually marching towards the region. The clash of the air masses will trigger scattered strong to severe thunderstorms this afternoon and evening from west to east.
Link: Mesoscale discussion about storm risk from NOAA SPC (technical)
Storms are likely to produce downpours, dangerous lightning and gusty winds (to 35-50 mph). Isolated storms could produce hail and damaging winds (58 mph or higher). The most likely timing for storms is between 4 and 8 p.m. around metro D.C. (early in that window in the western suburbs, later in the eastern suburbs), but in the far western parts of the region (western Loudoun and western Frederick counties), storms had already moved in before 3 p.m.