Toronto flood highlights the challenges of severe weather forecasting

Scott Sutherland

The record-breaking rainfall that caused all the flooding in Toronto on Monday certainly came as a surprise to many of us, and that included the weather forecasters that are responsible for issuing the watches and warnings to keep the public informed of severe weather.

"It was a bit of a surprise yesterday," Environment Canada meteorologist Peter Kimbell said in an interview with the Canadian Press. "The [severe thunderstorm] warning was actually not issued until the storm had begun."

With this 'miss' on their record, and with subsequent weather statements, watches and warnings being issued, but with nothing coming out of it to rival Monday's chaos, it might be tempting for some to speak ill of weather forecasters (and there may have already been some unkind words said). However, speaking from my own personal experience — as a weather forecaster for 10 years — it is an incredibly challenging job, and as the climate warms, the job is only going to get tougher.

[ Related: Toronto storm sets rainfall record amid flash flooding, traffic chaos ]

On the day in question, the Environment Canada forecast office had issued a special weather statement, which talked about the potential for severe thunderstorms, and the possibility of rainfall amounts of 50 mm or more in one hour. They do this for every day that there's a possibility for severe thunderstorms, and you can find them on the Special Weather Statements website. So, they were aware of the possibility on Monday, and they had informed the public of that possibility.

As for the lack of a severe thunderstorm warning before the storm hit, that's pretty standard. A warning isn't issued until they know that a severe storm has formed, and any areas that are ahead of the storm after it forms can usually count on about a 20 minute lead-time. The only reason the Greater Toronto Area didn't have that much lead time is because the storms apparently only reached severe proportions just as they reached the city.

Given the urgency of a severe thunderstorm warning, issuing them before the storm reaches severe proportions runs the risk of 'crying wolf', and we all know how that story ended.

For Tuesday and Wednesday, although the forecast office may have been a bit 'robust' with where they issued the weather watches concerning Wednesday's weather, it's difficult to know exactly where these kinds of storms are going to pop up. So 'casting the net' a little wider than usual, and then calling off the watches once they had a better indication of where the storms were going to hit, was a good safety precaution. As for the warnings, they were spot-on, issuing them as soon as the storms came across Lake Huron and following them along as they tracked across the province.

However, with Monday's storms dumping the greatest amount of rainfall Toronto has ever seen since official records began, that is simply part of the event that couldn't be foreseen.

Partly this is a technical issue. The computer models that are used for weather forecasting are very powerful and sophisticated, but by necessity, they work on a scale that's much larger than the storms that caused all the problems on Monday. To run the models on a smaller scale would require far more tax dollars from us, to install many more weather stations across the country, to give us the weather data needed for such a task. Not only that, but until we master quantum computing, running one of these models with that much data would take far too long. By the time the model gave a result, the weather would already have passed us by. So, although the models they currently use are a great help in producing a forecast, and a forecaster's experience can fill in the gaps, there are definite limitations to what can be forecast.

The rest of it is simply that we can't forecast these kinds of once-in-a-decade or once-in-a-century events. By definition, they're defined after the fact (and there's no official word yet if this was such an event), but although a forecaster can certainly give some indication of what we could expect, the magnitude of some events is just completely unpredictable.

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When it really comes down to it, the reason why everything turned out so badly on Monday was due to the flooding, which is very likely as a result of outdated storm- and waste-water infrastructure (although that's a story for another day).

An alarming part of all this is that these kinds of once-in-a-decade and once-in-a-century events are expected to become more frequent. With the planet warming due to increasing greenhouse gases in the atmosphere, we may be seeing the once-in-a-decade events every year or two, and the once-in-a-century events will happen every 20 to 50 years instead. Personally, I'm dreading the day when scientists need to start using an official 'once-in-a-millennium' label for some storm.

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