The dramatic stage collapse that occurred at the Ottawa Bluesfest July 17, 2011 was the result of a well defined squall line that moved through the area. Forming a storm line classically described as a “bow echo” on radar displays, and known meteorologically as a “mesoscale convective system (MCS)”, the squall line is a tight line of convective thunderstorm cells that push out in a bow shape due to the strong outflowing winds that develop, particularly in the center of the line.
The storm line characteristically has a strong elevated rear inflow jet along its rear that create a low pressure differential which further enhances the jet flow. Once this flow gets to the leading edge of the convective updraft, it descends and flows outward from the squall in strong straight line winds. Bow echos are not rare – they are a very common product of summer heating across the north American continent, and can produce some very dramatic dark and fearsome skies. While tornadic activity can develop between the cells, its not a common feature on these storm lines. When the storm lines become large, long and very powerful (winds greater than 58 mph) they are referred to as Derechos.
EWR logged most of the composite reflectivity scan of the bow echo/squall line that passed over Ottawa and caused the stage collapse at the festival. The following radar reflectivity animation shows its dramatic passage across Ottawa. A few frames are missing in the animation; we were monitoring other areas when we noticed the bow echo and had to retrace for scans.
(Click on the image if it doesn’t self-animate)