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When severe weather looms in Canada, there is a progression of statements, watches and warnings issued by forecasters to inform and protect the public. Here is a guide to those alerts.

Extreme weather warnings: How they work and whether the U.S. does it better


Scott Sutherland
Meteorologist/Science Writer

Saturday, April 11, 2015, 3:36 AM - Tornado season has arrived. Canada issued its first tornado watch of 2015 on Thursday, and several twisters have already touched down in the United States. While the Southern Prairies, Southern Ontario and Southern Quebec are the most tornado-prone regions, every province has seen damage from these extreme storms over the years.

When severe weather looms in Canada, there is a progression of statements, watches and warnings issued by forecasters to inform and protect the public. Here is a guide to those alerts.

Weather of Concern

When active weather is expected or occurring, but there's no current indication that conditions will be severe or extreme, Environment Canada will issue a Special Weather Statement.

This is a text statement, giving forecasters a chance to explain unusual conditions or weather of concern that a region may soon experience - heavy rainfall, thunderstorms, strong winds, high humidex values - anything that would certainly affect one's day, but are not expected to develop to the point where they could meet or exceed the criteria for severe weather.

Watching for Danger

Beyond weather of concern, there are times when the forecast indicates that conditions are favourable for severe weather, but it hasn't yet developed. In order to alert the public of this potential, while at the same time ensuring that undue alarm is avoided, a Watch is issued.

As the name implies, a watch means that forecasters are watching for severe weather conditions to develop. The public need not take any specific actions during a watch, but they should remain aware of the situation, and should be prepared to take action if the severe weather develops.

Imminent or Occurring

When it's no longer the case that there's simply the potential for severe weather, and severe weather is actually developing or occurring, forecasters issue a Warning.

Exactly when a warning is issued usually depends on what kind of weather is expected. Some winter storms can have warnings in place up to a day in advance. For severe thunderstorms and tornadoes, though, a warning is usually called after a watch has been in place for part of the day. However, since these conditions can pop up with little notice, it is possible for a warning to be issued spontaneously.

When Environment Canada alerts Canadians to risky conditions, The Weather Network distributes those warnings on this website, on television, on smartphones through our apps, by text messages and to other broadcasters through the Alert Ready public-safety messaging system.


A hypothetical example of weather statements, watches and warnings across Canada. Credit: Environment Canada

How Severe is 'Severe'?

What does it take for a watch or warning to be issued for severe weather?

Heavy Rainfall Warning

  • 50 mm of rain, or more, expected to fall within one hour (25 mm for coastal provinces and territories, 15 mm for dry interior of B.C.)

Severe Thunderstorm Watch
Conditions are favourable for a thunderstorm to develop with one or more of the following:

  • Wind gusts of at least 90 km/h
  • Hail of at least two centimetres in size
  • Heavy rainfall, as above

Severe Thunderstorm Warning

  • A storm with one or more of the above criteria is imminent or actually seen on radar or satellite imagery, or reported by a reliable weather spotter

Tornado Watch

  • Conditions are favourable for the development of severe thunderstorms with one or more tornadoes

Tornado Warning

  • A tornado has been reported; or when there is evidence based on radar, or from a reliable spotter that a tornado is imminent

Improve The System?

Turn your attention to how the U.S. National Weather Service reports severe weather and you'll see roughly the same system of alerts (although with the addition of the PDS Watch - Particularly Dangerous Situation). However, along with these alerts is a system of 'convective outlook' maps and reports, indicating not only the regions that could see severe thunderstorms - up to a week or more in advance - but also the chances of severe weather developing in those regions - marginal, slight, enhanced, moderate or high.

Why does the United States benefit from more information than Canada? It comes down to population and necessity.

Compared to Canada, the population of the United States is not only larger, but population densities are also more evenly spread out to all boarders and coastlines. As a result, NWS forecast offices and personnel, weather spotters, reporting stations and weather radar stations are more abundant, and also spread out more evenly throughout the country. 

Also, given the geography and latitude of the United States, the combination of air masses that clash over the country produce, on average, roughly ten times the number of severe thunderstorms and tornadoes that Canada experiences. Thus, it's necessary for the NWS to provide more coverage and more detailed products to keep on top of the abundance of extreme weather.

In contrast, with the Canadian population - thus weather reporting stations, radar stations and the majority of weather spotters - clustered in the southern part of the country, forecast coverage is more concentrated. While having products like the convective outlook forecast available here in Canada would certainly be beneficial, it's simply not needed here as often, and thus, the existing system of Environment Canada alerts is more than sufficient.

Sources: Environment Canada | Environment Canada | National Weather Service


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