The scale of devastation caused by the series of deadly tornadoes  that have hit the Midwest and southern U.S. this April and May has prompted some to wonder whether the twisters are yet another sign of climate change.

Experts, however, say it is too early to tell how these tornadoes fit into the overall pattern of severe weather events.

The National Oceanic Atmospheric Administration (NOAA) has clearly said that this year's very active tornado season in the U.S., although the nation's deadliest in 60 years, is not linked to climate change and is not a sign that such storms are getting bigger or more frequent.

The severity of the damage and the high number of deaths the tornadoes have caused have to do more with the population density and large number of mobile homes along the tornado routes than with global warming, NOAA research meteorologist Harold Brooks told the AFP news agency Monday.

'This could be well within the bounds of variability from year to year.'— Geoff Coulson, Environment Canada

Geoff Coulson, a warning preparedness meteorologist with Environment Canada, agrees and says the database of past severe weather events is not yet extensive enough to say whether we're seeing more thunderstorms and tornadoes, or to predict what the recent events might mean for the future.

"Linkages at this point to something like climate change — to say that's an underlying reason why this spring has been so active — are very premature," he said in an interview with CBCNews.ca. "This could be well within the bounds of variability from year to year."

More reporting, not more tornadoes

Tornadoes occur every year, and the type of damage they cause depends primarily on where they strike. Urban sprawl and greater population density in some of the areas that regularly experience tornadoes are more what account for the greater number of injuries and deaths today than in the past.

There is also more of a chance today that people will report severe weather events thanks to communication tools such as Twitter, Facebook and the internet generally. Weather centres are monitoring these sources and using them to get a more complete picture of how an event is affecting a specific area, Coulson said.

Tornado alley  

A region in the southern plains of the U.S. where a high number of tornadoes occur. 

The reason tornadoes are so common in the "alley" is that is where cool, dry air from Canada moves down the eastern slopes of the Rocky Mountains and warms as it flows east onto the plains. There, it comes up against moist, warm air moving in from the Gulf of Mexico. They meet along what's called a "dryline," and if the Gulf air is unstable and moist enough, the dryline pushes the moist air upwards and thunderstorms and tornadoes form.

The alley includes a swath of 12 states from North Dakota and Minnesota to northern Texas, Oklahoma and New Mexico. See map at bottom.

"We may have had a farmer back in the mid-80s who had part of his woodlot at the back of his property decimated by wind gusts or a tornado, but he figured it was at the back of his property, nobody got hurt.

"He just cut up the trees and life would go on," said Coulson. "Now, there is more of a tendency to say, 'I had 40 trees in my backyard ripped up, and here's some pictures.'

"So, it’s an increased likelihood of getting reports by a combination of there being more people to see things and experience them and also more ways for people to actually talk about or pass that information along."

In the U.S., the greatest number of tornadoes occurs in a region known as tornado alley, which encompasses the southern plains of the central U.S. and is generally understood to mean the area stretching northward from central Texas to northern Iowa and eastward from central Kansas and Nebraska to western Ohio.

Canada's own tornado corridor, which is much less active, is in southwest Ontario and runs from Windsor up through London and into the Barrie area.

Canada gets on average of 60 to 70 tornadoes a season, versus about 1,000 in the U.S.

Southern Ontario has the highest risk and gets about 12 tornadoes a season; the extreme southern part of the Prairies has the second-highest risk.

Canada's deadliest tornado hit Regina on June 30, 1912, killing 28 people, inuring 200 and leaving 2,500 homeless.

Warmer, wetter June could mean twisters

Coulson said that while Environment Canada is keeping an eye on the situation in the U.S., there is not necessarily any direct link with Canada's tornado season, which lasts from mid-April until early October.

At this point, all meteorologists can say is that the long-range forecast is calling for above-seasonal temperatures for southern Ontario in June and that warm and humid air masses coming up from the American Deep South could increase the risk of tornadoes.

si-tornado-sask220

A photograph of a tornado in Kelvington, Sask., on July 5, 1973, from the Natural Resources Canada website. Saskatchewan was the site of Canada's deadliest tornado on June 30, 1912, which killed 28 people. ((Mrs. Bauder, courtesy of Environment Canada Library) )

"If we do see more not only in the way of warmth but also in the way of humidity, which is kind of the fuel that drives these bigger thunderstorms, we could be in for a more active period here in Ontario into the month of June than what we've seen so far this season," Coulson said.

The first confirmed tornado of the season occurred April 27 near Fergus, Ont., northwest of Toronto.

Tornadoes are hard to predict because they are small-scale, localized phenomena that generally affect a small area and move quickly. Even with the help of tools such as a web of trained storm spotters who feed a network called Canwarn; Doppler radar; reports from provincial police; and lightning detectors, meteorologists can still only detect tornadoes about 20 minutes before they occur. 

That is why Environment Canada recommends people keep an eye out themselves for the following warning signs of a severe storm that might be accompanied by a tornado, damaging 90 km/h winds, hail or flooding rains:

  • Dark band of clouds moving across the northwest sky. 
  • Rapid wind shift — wind direction changes, picks up speed, feels cooler and stronger than earlier in the day.
  • Thunder. 

Tornado Alley

tornado-alley

Tornado Alley is a nickname for an area of the U.S. where a high number of tornados happen every year, approximately 40 per cent of all U.S. tornadoes. ((CBC, based on NOAA research))