The severe thunderstorms that tore through the Washington metro area on Sunday afternoon were fueled by a combination of historically hot and oppressively humid weather, and ignited by a cold front sliding east-southeastward. They were not the strongest thunderstorms ever to affect the region - after all, there were no confirmed tornadoes - but they happened to barrel right through the heart of some of the most populated areas of Loudoun and Montgomery counties, as well as the District itself.
Packing winds between 60-75 miles per hour, and in some spots closer to 90 mph, the storms' ferocity caught many off guard, resulting in three deaths.
Even President Obama was sent scrambling for cover as he tried to get in a sweltering afternoon game of golf at Andrews Air Force Base.
But what made the storms so fierce?
Radar and satellite imagery, as well as on-the-ground photos provide evidence for what set these storms apart from most of the afternoon downpours that typically rumble across the area during spring and summer.
One clue that is likely to jump out at any weather geek comes from the way the storms were oriented as they traveled from northwest to southeast ahead of the heat-wave busting cold front. From this radar image below, which shows the line of intense precipitation and their associated winds about to charge into Montgomery County, it's apparent that the storms formed a bow-like structure.
While not a classic, mature "bow echo", the storms' appearance on radar signaled the possibility that they would produce strong wind gusts. Bow echoes are often associated with very strong winds that blow in one direction, which are appropriately referred to as "straight line winds." According to the Baltimore/Washington office of the National Weather Service (NWS), the damage (at least in Montgomery County) was consistent with such straight-line winds, which storm surveyors estimated at 60 to 75 mph, with locally higher gusts (more on that in a sec).
A bow-like appearance results from strong mid-level winds that push some thunderstorm cells out ahead of a pack, stretching a line of storms into a telltale bow shape. Thunderstorm downdrafts in the storms that are "bowing out" can bring the strong winds from the mid-levels down to the surface, resulting in hurricane-force wind gusts.
This radar loop from the entire event shows what I mean, as the line of storms moves across the metro area.
The greatest damage was focused roughly near the tip of the bow, from west central Montgomery County to the National Mall
Doppler radar can detect the winds contained in storms as well as precipitation intensity, and the radar velocity measurements reveal that one of the storms in particular contained very strong winds. The colors in this loop of radar velocity images correspond to winds that are blowing either towards or away from the radar.
From this loop, you can track the core of strongest winds (first appearing as a darker blue region) embedded in the line as it marches from Oakton, Va towards Dulles Airport. Once the storms move past the radar site near the airport, the blues turn to orange and red, and that wind core continues across Montgomery County and D.C., as a general area of strong thunderstorm outflow winds pushes rain-cooled air ahead of the storms. This outflow was responsible for the strange "shelf clouds" that marked the storms' arrival for many.
At one point, radar detected weak rotation in part of the line of storms in Montgomery County, prompting a tornado warning. However, no tornado has been confirmed, and all of the damage the Weather Service has surveyed resulted from straight-line winds.
Here are some highlights from what the Weather Service found in their damage surveys of west central Montgomery County, and the Rockville, Derwood, Aspen Hill, and Glenmont areas.
Widespread damage to large hardwood and softwood branches and limbs was noted throughout the surveyed area. More sporadic but concentrated damage to entire trees also was observed...Consisting largely of uprooted or snapped pine trees. The most intense damage was found in the Potomac Chase Estates and along Travilah road... Where hardwood and softwood trees were snapped 30 to 50 feet off the ground. Fallen trees and limbs were to blame for bringing down utility lines and poles through much of the area.
Of particular interest was damage to a large container crane located at the Montgomery county transfer station in Derwood. The crane was installed on a track that was oriented from northwest to southeast such that the horizontal member of the crane faced toward the northwest. Winds orthogonal to the horizontal member of the crane likely produced sufficient bearing load force to push the unsecured crane along its track before causing it to topple down a hill adjacent to the transfer station.
In summary...All damage surveyed in Montgomery county was consistent with very strong thunderstorm outflow straight line wind. Widespread winds of 60 to 75 mph were assessed...With sporadic narrow focused swaths of 80 to 90 mph winds.
Thanks to CWG's Ian Livingston for his assistance with this post.