4. Forecast Applications

Convection » Severe Thunderstorms Introduction

Photo of a supercell thunderstorm

Severe thunderstorms are thunderstorms that produce at least one of the following:

  • In the U.S.:
    • Damaging winds: 93 kph (50 kts, 58 mph) or greater
    • Large hail: 2.5 cm (1 inch) diameter or greater
    • A tornado
  • In Canada:
    • Damaging winds: 90 kph (48 kts, 56 mph) or greater
    • Large hail: 2 cm (¾ inch) diameter or greater
    • Heavy rain: 50 mm (2 in) or more per hour (Alberta to Southern Quebec), 25 mm (1 in) or more per hour (Pacific, Northern, and Maritime provinces)*
    • A tornado
* Note: In the U.S. heavy rain thresholds are used as criteria for Flash Flood events (watches and warnings) and, thus, are not included in the definition for severe thunderstorms.

The environments that produce severe thunderstorms are marked by either high CAPE, strong shear, or both. The vertical distribution of the shear (e.g., shear depth, strength) influences the organization and evolution of severe thunderstorms. Examination of certain skew-T parameters is essential but not sufficient for forecasting severe thunderstorms. The skew-T parameters should always be used in combination with other information (surface and upper air analyses, NWP output, etc.) to make a full assessment of severe weather potential.