Criteria Old

We use the following criteria when deciding the threat levels used in our forecasts:

Threat Level Description
No Convective Activity The probability of thunderstorms affecting any given location withi n the United Kingdom during the forecast period is considered to be less than 10%, and thus does not warrant a Threat Level.
As a result, forecasts are not usually issued with this level.
Watch The probability of thunderstorms affecting any given location within the enclosed area during the forecast period is considered to be between 10-40%.
As a result, isolated thunder, lightning and/or hail may occur, but it is not expected to be widespread, nor severe. The development of tornadoes is unlikely, although funnel clouds may occur, depending on the nature of the forecast.
Thunderstorms The probability of thunderstorms affecting any given location within the enclosed area during the forecast period is considered to be between 40-80%.
As a result, thunderstorms can be expected, perhaps widespread in places. Hail, torrential downpours and/or strong gusty winds may accompany the thunderstorms, and may cause minor structural damage and localised flooding.
Weak/moderate-intensity tornadoes are possible, depending on the nature of the forecast.
Severe The probability of thunderstorms affecting any given location within the enclosed area during the forecast period is considered to be between 80-95%.
As a result, widespread thunderstorms can be expected, some perhaps exhibiting severe or supercellular characteristics. Hail, perhaps medium in size, torrential downpours and/or strong gusty winds are likely to accompany such storms.
Moderate/strong-intensity tornadoes are possible, depending on the nature of the forecast. Structural damage and temporary disruption to transport and power supplies are also possible.
Extreme The probability of thunderstorms affecting any given location within the enclosed area during the forecast period is considered to be greater than 95%.
As a result, widespread severe thunderstorms can be expected, posing a threat to life and property. Hail, perhaps large in size, prolonged torrential downpours and/or severe winds are likely to accompany such storms.
Strong-intensity tornadoes are likely, depending on the nature of the forecast.
Severe structural damage is possible, with major disruption to transport and power supplies, which may last for several days. Extreme storms are extremely rare in the United Kingdom and are not usually forecast.