In response to a public criticism that Environment Canada was slow to issue a public warning about the massive rainstorm that caused flooding in Thunder Bay last May, a spokesperson with the weather service says the system was difficult to track.

Geoff Coulson, a warning preparedness meteorologist with Environment Canada, said forecasters knew a large system was coming through, but what they didn't know — until 1 a.m. — was exactly where the storms would hold over.

"Some of these storms may be relatively slow moving, but to pinpoint with any kind of real advance notice exactly where they're going to … lock in and form and re-form, continues to be very difficult," Coulson said.

In a report to Thunder Bay city council earlier this week, independent meteorologist Graham Saunders called the timing of the severe thunderstorm warning on May 28 embarrassing.

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Geoff Coulson, a warning preparedness meteorologist with Environment Canada, says storms that stop dead in their tracks are challenging for forecasters. (Supplied)

Saunders said the warning only came after a new rainfall record had already been set.

Ensuring the warning is 'justified'

But Coulson noted storms that stop dead in their tracks are challenging for forecasters. They are very hard to forecast locally and continue to be one of the biggest challenges for weather services in Canada and the U.S.

For a severe thunderstorm warning to be issued for a specific locality, Environment Canada has to expect that storm will produce at least 50 mm of rain in one hour or less in that area, Coulson said.

"There's a verification process," he said.

"You want to make sure that the rainfall rates that we're looking at are going to meet the warning criteria, and so the forecasters are having discussion amongst themselves to ensure that the warning is, in fact, justified."

Coulson noted forecasters knew around 1 a.m. that thunderstorms were going to go into a holding pattern over Thunder Bay and a severe thunderstorm warning was released just before 1:30 a.m.

"These types of storms are difficult to predict with any type of advanced notice, and in many cases we're more reacting to these storms than actually predicting them."