Severe thunderstorms are thunderstorms with one or more of the following conditions:
Wind gusts of 90 km/h or greater
Hail of 2 centimetres or larger in diameter
Heavy rainfall when 50 mm or more is expected within one hour over the Prairies, or 25 mm or more in one hour over the
The thunderstorm outlook uses a 4-level scale to represent the potential weather expected with thunderstorms within the shaded areas. The weather associated with each colour is:
Thunderstorm Threat Legend
Rain (in 1 hour)
Gust less than 90
Less than 20mm (nickel)
Less than 50 mm
Gust 90 to 100
20 mm (nickel) to 35 mm (walnut)
50 to 75 mm
Gust 100 to 120
35 mm (toonie) to 50 mm (hen egg)
75 to 100 mm
Gust over 120
Greater than 50 mm (hen egg) diameter
More than 100 mm
The thunderstorm outlook graphic also includes iconography to highlight the most probable hazards posed by thunderstorms on a given day. It is important to note that the text that accompanies the thunderstorm outlook graphics offers further details regarding exactly what kind of hazards are expected.
Categorical Probability of Thunderstorms
30% or greater probability of non-severe or near severe thunderstorms.
5-30% probability of severe thunderstorms of either limited organization and longevity, or very low coverage and marginal intensity.
30-60% probability of organized severe thunderstorms with varying levels of intensity.
60% or greater probability of widespread severe weather with tornadoes and/or numerous severe thunderstorms, some of which may be extreme.