Criteria Key

We use threat levels to highlight the probability of both thunderstorm coverage and severity. The threat levels cater for thunderstorms of different varieties that can occur in the United Kingdom, and allow us to forecast more accurately events when an isolated severe thunderstorm develops, for example.

COVERAGE

Threat Level
Probability of lightning at any given location
No Convective Activity
< 5 %
Slight (S)
5 - 35 %
Moderate (M)
36 - 70 %
High (H)
> 70 %

SEVERITY

If 2 or more of the following criteria are met, these severity threat levels may be issued.

Threat Level
Criteria
Severe (SV)
  • 25mm or more precipitation in 3 hours or less
  • Hail greater than 2.0cm in diameter
  • Wind gusts greater than 55mph
  • Tornadoes up to T3 intensity
Extremely Severe (EX)
  • 50mm or more precipitation in 3 hours or less
  • Hail greater than 5.0cm in diameter
  • Wind gusts greater than 75mph
  • Tornadoes of T4 intensity or greater