Occasionally , thunderstorms can be severe .
Occasionally , thunderstorms produce large hailstones as seen in Ottery St Mary , Devon in October 2008 , where drifts reached 1.8 metres ( 5 ft 11 in ) .
Occasionally , thunderstorms can be severe and produce large hailstones as seen in Ottery St Mary , Devon in October 2008 , where drifts reached 1.8 metres ( 5 ft 11 in ) .