Assessment of the wind hazard due to tornado outbreaks in southern Ontario

A tornado outbreak is a severe weather event that involves the occurrence of multiple tornadoes in a short period of time in a region. Although widespread damage has been associated with many historical outbreaks, wind hazard assessments, and the methodology for making such assessments, are lacking in the literature. The current study addresses this by developing a probabilistic simulation-based approach and then applying it to southern Ontario, using a model for the outbreak characteristics based on the tornado database for the regions neighboring southern Ontario and the tornado characteristics obtained from historical occurrences of tornadoes in southern Ontario. A quantitative assessment of the outbreak hazard for an area representing an idealized city in southern Ontario was carried out and the results are compared with those obtained by ignoring the outbreak phenomenon. The comparison shows that ignoring outbreaks can lead to an order of magnitude decrease in hazard estimates when an accumulated damage area of 100km² or more is of concern.