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Why post-tropical storms can be worse than the original storm


February 13, 2019   by Jason Contant


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Inland flood damage arising from post-tropical storms can be an even greater risk than wind damage, a meteorologist from Aon said last week at the CatIQ Connect conference in downtown Toronto.

“That’s concerning because here in Canada, as in the U.S., the majority of your standard homeowner policies do not cover overland flood damage,” said Steven Bowen, director (meteorologist) with Aon, during the panel discussion Lessons Learned on Hurricanes. The panel reviewed the lessons learned from recent landfalling hurricanes, and discussed changes in future risks, including the threat of these storms to Canada.

The offshore supply vessel Atlantic Osprey enters the St. John’s Narrows in the fog and rain generated by post-tropical storm Maria. THE CANADIAN PRESS/Paul Daly

About 50% of all storms in the Atlantic basin that make landfall go through some form of extra-tropical transition, said another panelist, Christopher Melhauser, assistant vice president of research and development and catastrophe risk management at reinsurer SCOR. There are also “enhanced waves… and this will cause more energy to be available at landfall with storm surge to cause some type of damage.”

In Atlantic Canada, a “population clustering” offers the opportunity for losses when post-tropical impacts from hurricanes hit the region, Bowen said. He noted that people “continue to move into the most vulnerable locations,” particularly in the United States. “Even in Atlantic Canada – although not quite as extensive as say, New York or Boston – you can certainly see there is a notable population clustering in Atlantic Canada that does offer opportunity for losses when you do get some of these stronger hurricanes or post-tropical impacts from these hurricanes making their way up into the region.”

In fact, post-tropical storms, which tend to affect Canada, can sometimes be stronger than the original tropical storm, said panelist Bob Robichaud, warning preparedness meteorologist with Environment and Climate Change Canada.

“The term ‘post-tropical’ does not necessarily mean a lesser storm, and that’s a challenge to get the public [and media] to understand,” Robichaud said. “If that tropical storm was fairly weak [and] if the perfect ingredients are in place, we can end up with a post-tropical storm that is stronger than the originating tropical storm.”

The path of a tropical storm is also a little easier to predict, whereas with a post-tropical storm, “not only is it accelerating, but the rate of acceleration is usually increasing as well, so trying to predict the timing becomes very difficult,” Robichaud said. In addition, intensity changes are fairly gradual with a tropical storm, but with a post-tropical storm, “you can have a decrease in intensity followed by an increase in intensity once it reaches the post-tropical phase.”

A tropical storm’s structure stays more or less the same, but during the post-tropical transition, the entire structure is changing from one storm to another. “In the case of a purely tropical storm, you have fairly symmetric wind and rain patterns relative to the centre, to the track, whereas in the post-tropical storm, you can have a complete dissection of your rain and wind, so the predictability of that is very difficult as well,” said Robichaud.