Tuesday, 20 May 2014

Severe Weather Awareness - Watch vs Warning

A spectacular image of the only EF-5 tornado (winds in excess of 320 km/h) ever recorded in Canada. The storm struck near Elie, Manitoba on the afternoon of July 22, 2007. (The Weather Network)
With the late but violent arrival of severe weather across the US, it was only going to be a matter of time before we had some here in Canada. Last week the first Canadian tornado of the season was reported in Ontario. Tornadoes can and do occur just about anywhere in North America, but the really violent super cell thunderstorms capable of a major tornado are primarily confined to the central portion of the continent into the US southeast. Unlike a hurricane or snowstorm, tornadoes are very small in scale affecting a few miles to perhaps a few hundred miles in the most extreme case. What they lack in size they make up for in destruction with some of the strongest winds measured on earth. Because it is nearly impossible to measure wind speed in such a severe environment, a scale was created back in 1971 by Ted Fujita from the University of Chicago. Doctor Tornado, as he was known, was a pioneer in the study of tornadoes and estimated wind speed based on the damage he observed after the tornado struck. While modified in recent years, the scale is still used today, the Enhanced Fujita scale measure the storms on a scale of EF-0 to EF-5. The tornado last week in Mildmay, Ontario was an EF-1
The deadly July 31, 1987 tornado that struck Edmonton, Alberta was a powerful EF-4 storm that killed 27 people, injured 300 and destroyed 300 homes.
It is important that Canadians understand that tornadoes can and do occur in Canada. Environment Canada just completed a study on tornadoes in this country. Each year on average 62 such storms develop. The can occur in any province but are most common in Saskatchewan with an average of 18 per year, followed by Ontario with 12.5. Quebec has an average of 4.7 tornadoes, the majority of which occur in and around the southern portion of the province. Since 1980, Canada has reported 1217 EF-0 tornadoes, with EF-1 (478), EF-2 (119), EF-3 (24), EF-4 (5) and the strongest deadliest storm the EF-5, only 1 in Eli, Manitoba on July 22, 2007. There have been other very notable storms over the years including the deadliest ever in Regina, SK in June 1912. Other deadly storms included Windsor, Ontario in April 1973, Edmonton, AB in July 1987, Barrie, ON in May 1985 and Red Deer in July 2000 just to name a few.


Environment Canada with the use of Doppler Radar, provides advance watches and warnings when tornadoes are possible. It is important as we head into the summer season for Canadians to understand the terminology they use when posting warnings. A severe thunderstorm watch occurs when conditions are favorable for severe thunderstorms. It is just a watch, so go about your daily routine but keep and eye to the sky or your Smart phone with an app from The Weather Network or other weather provider. If thunderstorms develop on radar and become severe with strong winds (over 90km/h), heavy flooding rains or hail (greater than 2cm in diameter) a severe thunderstorm warning will be issued. Warnings are typically issued for a large geographic region, but the storm may only affect a portion of that region. Be vigilant just the same. Remember to seek shelter when storms occur as lightning remains one of the deadliest components of thunderstorms in Canada. I will talk more about lightning safety in a future post.

If weather conditions become more favorable for stronger storms with rotation, a tornado watch will be posted. They are rare in Canada but do occur. The watch will be for a larger area. If a warning is required it means you should find shelter immediately. A tornado warning will be issued, typically for a more precise area, if a tornado is observed on the ground or detected on radar. Tornadoes typically are short lived in Canada and cover a very narrow path. They are extremely violent with intense winds and very low pressure. You must seek shelter indoors and at the lowest level of the dwelling, away from any windows or doors. If outside find the lowest place to lie flat and cover your head. Many people think a highway underpass is a safe alternative in a tornado, they are not.

Please remember that if any day this summer looks particularly dangerous for strong thunderstorms, I will post an update in advance as well as post any watches or warnings via my twitter feed during the severe weather. Most weather apps have features now that will notify you when watches or warning shave been posted for your region.

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