Weather conditions - temperature, humidity, atmospheric instability, winds, etc - are favourable for the development of severe thunderstorms that could produce one or more tornadoes. In addition to the ponding and flooding concerns noted above, hail and wind from these storms can cause significant damage to property and pose a potential risk of injury.Ī storm with one or more of the above Watch criteria is imminent, or has actually been seen on radar and/or satellite imagery, or reported by a reliable weather spotter. These rainfall amounts are sufficient to cause ponding on roads, overland flooding as rainfall amounts exceed the ground's ability to absorb the water, and even flooding risks along swollen streams and rivers.Ĭonditions are favourable for a thunderstorm to develop, with the potential to produce one or more of the following: The forecast indicates the potential for 50 mm of rain, or more, expected to fall within one hour (25 mm for coastal provinces and territories, 15 mm for the dry B.C. What does it take for a watch or warning to be issued for severe weather? Warnings are indicated on Environment and Climate Change Canada's Public Weather Alerts map by regions highlighted in red. In this case, forecasters would immediately issue a severe thunderstorm warning, or possibly even a tornado warning. Since there are cases when severe conditions can pop up with little notice, however, it is also possible for a warning to be issued spontaneously.Īlthough it is fairly rare, we can see this when non-severe thunderstorms suddenly encounter conditions, which were not clearly evident beforehand, that cause them to intensify and even develop rotation.
Watches and warnings issued throughout southern Ontario could include a mix of alerts for severe thunderstorms and tornadoes. Specific storms in the line could be identified as potentially tornadic, prompting a watch, or they could suddenly develop rotation, causing forecasters to immediately issue a tornado warning for those in the storm's path. It can happen, though, that tornado watches and warnings are issued during a storm event, perhaps as a line of severe thunderstorms is tracking across a region. After that, they would simply be upgraded to warnings if the tornadic storms developed. Tornado watches can be issued, instead of severe thunderstorm watches, in the case where weather conditions specifically indicate the potential for the development of storms capable of producing tornadoes. Tornado watches and warnings are a somewhat special case. Regions under the watches, to the east, could have had potentially severe storms at that time, or they may have been under watch due to the potential for the severe storms to persist into those regions. At this time, severe thunderstorms would have been tracking across those regions in red. Watches (yellow) and warnings (red) are indicated across southern Saskatchewan on this sample Public Weather Alerts Map from Environment and Climate Change Canada. Thus, a severe thunderstorm watch could be in effect for much of the day, and then it is replaced by a severe thunderstorm warning as the storms pop up on radar. First a watch is issued, and this is followed by upgrading those watches to warnings as the storms or tornadoes develop. When conditions advance past simply having the potential for severe weather, and severe weather is actually developing or occurring, forecasters issue a Warning.Įxactly when a warning is issued usually depends on what kind of weather is expected.įor severe thunderstorms and tornadoes, the ideal goal is to follow a specific progression, of course. Credit: ECCCĮssentially, any conditions that would certainly affect someone's day, but are not expected to develop to the point where they would qualify as severe weather. This special weather statement was issued in southwestern Ontario, on June 20, 2018, due to the potential for hot and humid weather that was expected to give rise to conditions of poor air quality. 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, and/or high humidex values. When active weather is expected or occurring, but there's no current indication that conditions will be severe or extreme, Environment Canada issues a Special Weather Statement. With the approach of summer, forecasters will be on the lookout for more outbreaks of severe weather, and when extreme weather looms in Canada, there is a progression of statements, watches and warnings issued to inform and protect the public.
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