A thunderstorm is a complex meteorological event, characterised by the presence of electrical discharges (lightning) followed by thunder. The conditions needed for a thunderstorm to develop are a sufficient level of humidity and the rapid upward movement of air, e.g. as a result of being warmed. Inside a storm cloud (cumulonimbus) there are often strong updrafts and downdrafts, which cause turbulence.
Hazardous weather events that can occur in a thunderstorm include lightning, heavy precipitation (downpour of rain, hail, graupel/snow pellets, and, in winter, snow) as well as wind gusts that can sometimes reach up to hurricane strength.
In spite of the most up-to-date tools and meth-ods, it is not currently possible to predict or give several hours’ warning of the strength, timing and location of severe thunderstorms.
Thunderstorm warnings are only issued for categories 3 and 4.
Lightning strikes and hail damage may occur. Very strong gusts of wind may topple trees and cause branches to break off. Heavy rain may lead to flooding and trigger landslides on steep slopes. Sudden flash flooding or debris flows may occur in streams.
Severe thunderstorm (flash thunderstorm; localised, warning time 0 to 1 hour): 90-120 km/h, 2-4 cm hailstones, 30-50 mm/h.
Lightning strikes and hail damage may occur. Very strong gusts of wind may topple trees and cause branches to break off. Heavy rain may lead to flooding and trigger landslides on steep slopes. Sudden flash flooding or debris flows may occur in streams.
Very severe thunderstorm (flash storm, local; warning time 0 to 1 hour): >120 km/h, >4 cm hailstones, >50 mm/h.