The storms yesterday afternoon were highly localized and packed a punch but what caused them? Something called an outflow boundary.
- Storm clouds gather over Arlington as the rain begins to creep across the area.
So what is an outflow boundary anyway?
Outflow boundaries, also called gust fronts, are produced when a thunderstorm or even cluster of thunderstorms, produces an outflow of cool air that originates from the downdraft of a storm. That cool air, from the downdraft, hits the earth and then spreads out in all directions a lot like water does if you pour it straight onto the ground.
These outflow boundaries can be identified a couple of ways either as a fine curved line on radar or as arcs of low clouds on satellite imagery. Impressively, these boundaries can linger for up to 24 hours after the original thunderstorm dissipates and can continue to travel even further.
Sometimes you may even hear us reference this on the air saying something like, “while there is no cold front to trigger storms there are still left over boundaries from yesterday’s activity that may produce more storms.” It is worth noting that while many times these boundaries create new storms, they don’t always. However, in yesterday’s case it did.
At around 3 p.m. myself and the other meteorologists in the office clearly identified on Live Supper Doppler an outflow boundary that was produced by a storm cell just north of Baltimore. I even noted on WTOP that we would have to watch this area for possible future development of thunderstorms.
At this same time air temperatures had warmed to 90 degrees in D.C. and in the upper 80s to 90 degrees in many of the surrounding suburbs. This meant that should a storm form along this boundary, it would have plenty of “fuel” to become rather potent. The slow moving and highly localized thunderstorms that did ultimately develop along this boundary started in northern Montgomery County and Prince George's County before drifting south and dissipating over D.C.
Portions of D.C. were hit especially hard with heavy rain, including the area near Children’s Hospital, which recorded 2.36 inches of rainfall. Flash flooding also occurred along Rhode Island Avenue in Bloomingdale causing some roads to be closed.