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Supplementary Information
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 Northwest Territories
- Tornado
The threat level on the Thunderstorm Outlook is determined using a combination of hazard thresholds, likelihood of occurrence and impacts based on known vulnerabilities and exposure. In each province and territory, the highest threat level is indicated on the associated risk matrix. If there are two polygons of equal threat level within one province or territory, the polygon that encompasses the highest impact will be the one indicated on the risk matrix.
The forecaster’s confidence in the individual hazards reaching the indicated value, as well as their confidence in the location of the thunderstorms and the overall development of the thunderstorms within the highest threat level polygon are also indicated in the Confidence section.