Damage surveys in the age of Google Street View and polarimetric radar—the August 20th 2009 southern Ontario tornado outbreak

On August 20 2009 between 3:30 and 10:00 PM EDT, 19 tornadoes were confirmed in Southern Ontario within 250 km north of a west-east line through the city of Toronto. This is the largest number of tornadoes in a single outbreak ever recorded in Canada. Four of these storms attained F2 intensity. One death was reported, and damage claims were in the tens of millions of dollars. The mapping of the outbreak and some of the unusual aspects of the event will be discussed. Although the morphology of the event, evolving from supercellular mode to accelerating line segments, was not atypical, a remarkable feature was the abnormally large number of supercells and their longevity. Several of the most significant supercells tracked close to Toronto.

Particular emphasis will be on a unique assessment of three supercells which affected the densely populated suburbs north of Toronto where two tornadoes touched down causing F2-rated damage. Numerous eyewitness accounts provided conflicting information on which supercells spawned the tornadoes. The sequence of tornado touchdowns was resolved using a detailed analysis linking the eyewitness accounts, YouTube videos, Google Street View, and radar outputs (Toronto and Buffalo reflectivity and Doppler data, and Toronto dual-polarimetric data). The results were both unusual and surprising, in that they debunked several key eyewitness accounts. Many of these modern-day tools, such as Street View, can be helpful for post-event damage surveys.

The RhoHV scans from dual polarimetric radar are able to detect tornadic debris signatures by measuring irregularly shaped objects. The debris fields of the two tornadoes were indeed detected on some of the RhoHV scans, albeit weakly on one tornado. The timing and locations of the RhoHV debris fields were compared to the independent analysis and found to match very closely, to within a few hundred metres. This helped confirm the accuracy of the independent analysis.

Dual polarimetric radar also shows some promise in discriminating between tornadic and non-tornadic supercells, namely in their ability to reveal some of the microphysical differences between them. This was especially true of the Aug 20th tornadic outbreak using the Toronto dual polarimetric radar, although it will not be explored in this paper. This underscores the importance of the independent analysis' results of the tornadic supercells in north Toronto.