What the May 4 windstorm can teach the industry about catastrophe modelling

Rare events like the May 4 windstorm that primarily affected southern Ontario often cause the most damage, but are also the most difficult to generalize in catastrophe modelling predictions, according to a Canadian weather forecasting and research firm.

“While the combination of factors on May 4 was rare, it is important for insurers, and those modelling future catastrophes, to recognize that it’s often rare events like these that cause the most damage – and they are also the events that are difficult to generalize in model predictions,” Scott Kehler, president and chief scientist of Winnipeg-based Weatherlogics Inc. told Canadian Underwriter Wednesday.

A fallen tree lays lays across a road after a windstorm in Toronto on Saturday, May 5, 2018. Environment Canada said wind gusted at close to 120 kilometres per hour on Friday in the wake of a cold front that moved across Ontario and Quebec.THE CANADIAN PRESS/Chris Young

Typically, summer-time damaging wind events come from relatively localized severe thunderstorms, so most severe wind modelling assumes that such storms are the damage mechanism, Kehler explained. “However, in this case, the winds were actually from weak storms, which models would assume are rather benign,” he said. “In addition, a large amount of the wind damage didn’t come from storms at all, but rather a passing trough of low pressure, which again isn’t usually considered to be a mechanism which would cause damage like we saw on May 4, 2018.”

The storm cost insurers $380 million in Ontario and $30 million in Quebec, the most expensive event since the 2013 flooding in Toronto (nearly $1 billion in damage), Catastrophe Indices and Quantification Inc. (CatIQ) said on June 1. Strong wind gusts – including a maximum measured gust of 126 km/h and record-setting gusts in some locations – resulted in downed trees and power lines, and three fatalities.

Weatherlogics released Wednesday a case study on the storm, which found that, initially, thunderstorms produced by a cold front helped to force these extreme winds to the surface on a localized basis. Later, a trough of low pressure allowed these winds to reach the surface on a more widespread basis. “Strong winds were already occurring aloft, so these two mechanisms didn’t produce the winds, but rather gave them a way to get pulled down to ground level,” Kehler said.

In the past, there have been cases where weak thunderstorms or troughs of low pressure produced strong winds. “However, I’m not sure if these two factors have ever occurred in the same event and caused as much damage as we saw on May 4,” Kehler said, adding that it would require additional in-depth research to determine if something like this has happened before because it’s such a rare combination of events. “The fact that many wind speed records were set on May 4, including some that approached all-time records, suggest it’s unlikely any past events reached this magnitude.”