Environment Canada answers the question: Where was Toronto’s severe thunderstorm warning?

TORONTO –  The first sign that it was going to be an unusual storm came around 1:30 p.m. Monday afternoon.

Southern Ontario and the GTA were forecast to get rain, but early Monday afternoon, it became clearer to Environment Canada that the warm and humid air mass in the area would produce high rainfall.

As is standard in these types of situations, a special weather statement was issued for Southern Ontario, including Caledon, Orangeville, Innisfil, Barrie, Orillia, Vaughan, Richmond Hill, Newmarket and Durham Region. Toronto and Southwestern Ontario were added almost an hour later.

It advised of “local heavy downpours giving 30 to 40 millimetres of rain in less than one hour” and told the public to monitor future forecasts and warnings as warnings may be required or extended.

Three hours later, Toronto was caught in a deluge of rain.

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Streets were flooded, cars became boats, passengers struggled out of a GO train stuck near the Don Valley Parkway and homeowners suddenly found themselves involuntary owners of indoor pools.

Although Environment Canada issued a thunderstorm warning for a storm that was moving south toward the city, no warnings were issued for the GTA. Not until after the storm had passed.

And that has left many Torontonians wondering: Where was our severe thunderstorm warning?

Any meteorologist will tell you that thunderstorms are difficult to predict. But the system that brought a record-breaking 126 mm of rain to Toronto Pearson — breaking the record set by Hurricane Hazel in 1954 — was well on its way.

“We did see a complex of thunderstorms near the Barrie area and that complex morphed or merged with another one coming from the west over Toronto around 5 o’clock,” said Peter Kimbell, a warning preparedness meteorologist with Environment Canada. “The merger with the second complex of thunderstorms coming from the west probably amplified it sufficiently and caught [the forecasters] off guard, just pure and simple.

“They were surprised by the magnitude of how bad it was. Should it ideally have been forecast? Of course. The answer is, yes.”

The difference between a special weather statement, a severe thunderstorm watch and a severe thunderstorm warning

Environment Canada issues a special weather statement when there is potential for an event that may approach the warning or watch criteria.

A severe thunderstorm watch is issued when there is a good chance of severe weather developing.

A severe thunderstorm warning has four elements: winds greater than 90 km/h; hail greater than 2 cm; flooding rains; or a tornado.

The threshold for flooding rains is 50 mm in a one-hour period. But Environment Canada didn’t believe that the rain criterion would be met. That’s why they only issued a special weather statement.

Kimbell recalls that some parts of the GTA saw a similar event like Monday’s storm on August 19, 2005. That storm was also one for the record books — except you won’t find it there.

“There was 175 mm in that event,” said Kimbell. “But it doesn’t appear in the Toronto Pearson archives because they didn’t get that much at Pearson. They got only 40-something…So it varies hugely across a little area.”

This was similar to what happened during Monday’s storm.

While Toronto’s Pearson International received 126 mm of rain, Toronto City received 97 mm; G Ross Lord Dam, near Dufferin and Finch received 68 mm; Buttonville Airport received only 20 mm; Oshawa and Hamilton received negligible rainfall.

“To try to pinpoint that level of detail with a couple hours of notice is enormously challenging,” Kimbell said. “The science is just not there yet. And we’re doing our best.”

Kimbell also said that Environment Canada comes under heavy criticism for issuing too many warnings.

“We put out a lot of warnings and watches over the summertime and one of the complaints we get is that we put out too many.”

Kimbell also noted that where the rain falls greatly influences if flooding is a serious potential.

“If this event had occurred in a rural area, that was relatively dry, it might not have been anywhere near the event that it was.”

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© Shaw Media, 2013