The thunderstorm forecast is issued daily from April 1 to September 30.

Monday, June 21st:  

Severe thunderstorms are forecasted for parts of southwestern Ontario this morning, due to an area low-pressure, and large amounts of storm energy. As well as this, there will be a decent amount of helicity (the rotation in the updraft of a storm) in this region, which will add an increased tornado risk. Large hail measuring between 2-5 cm in width, heavy downpours, and wind gusts up to 110 km/h are all likely as well. 

Isolated-severe thunderstorms are possible in numerous areas of Ontario, and Quebec today. With an unstable atmosphere due to a low-pressure system, these storms will be in an area with enough instability to create supercells, and potentially even an isolated Tornado. Large hail measuring up to 2-3 cm, isolated heavy downpours, and wind gusts up to 100 km/h are all likely with these thunderstorms.


Non-severe thunderstorms are forecasted for numerous areas of Canada on Monday. Small hail, wind gusts up to 90 km/h, and isolated flooding may occur with these storms. 

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To learn more about the terms used in the thunderstorm outlook, visit our glossary.

 

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In Canada, there are two types of severe thunderstorm alerts. Severe thunderstorm watches and severe thunderstorm warnings.

A watch is issued when: conditions are favourable for the development of severe thunderstorms with one wind gusts of 90 km/h or greater, which could cause structural wind damage; hail of two centimetres (cm) or larger in diameter; or heavy rainfall, as per rainfall criteria, excluding those for winter and during thaw.

A warning is issued when: there is evidence-based on radar, satellite pictures, or from a reliable spotter that any one or more of the following three weather conditions is imminent or occurring: wind gusts of 90 km/h or greater, which could cause structural wind damage; hail of two centimetres (cm) or larger in diameter; or heavy rainfall, as per rainfall criteria, excluding those for winter and during thaw.