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Thunderstorms

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.

Hazard level 5 (very severe hazard)

Thunderstorm warnings are only issued for categories 3 and 4. 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. Three of the categories are therefore of little significance.

Hazard category 4 (severe hazard)
Impact
  • Flash flooding of streams
  • Toppling of trees
  • Possibility of localised landslides on steep slopes
  • Damage from hail and lightning strikes
  • Failure of drainage and sewer systems.
  • Flooding of underpasses, underground garages and cellars.
  • Disruption to road, rail and air traffic
  • Danger to vessels on lakes from very strong gusts of wind arising rapidly without warning
Recommended action Information on recommended actions can be found here: What to do during a thunderstorm
Description of measurement and prognostic values
  • Very severe thunderstorm (flash storm, local; warning time 0 to 2 hours): >120 km/h, >4 cm hailstones, >50 mm/h
Hazard category 3 (significant hazard)
Impact
  • Flash flooding of streams
  • Toppling of trees
  • Possibility of localised landslides on steep slopes
  • Damage from hail and lightning strikes
  • Failure of drainage and sewer systems.
  • Flooding of underpasses, underground garages and cellars.
  • Disruption to road, rail and air traffic
  • Danger to vessels on lakes from very strong gusts of wind arising rapidly without warning
Recommended action Information on recommended actions can be found here: What to do during a thunderstorm
Description of measurement and prognostic values
  • Severe thunderstorm (flash thunderstorm; localised, warning time 0 to 2 hours): 90-120 km/h, 2-4 cm (hailstones), 30-50 mm/h

Hazard category 2 (moderate hazard)

A thunderstorm event that is usual for the time of year (for which, therefore, no special warnings are issued)

Notes

A distinction is made between «severe thunderstorms (widespread)» and «flash thunderstorms»:

  • «Severe thunderstorms (widespread)» is the term used for a more extensive region, with a forecast of up to 12 hours, when the potential exists for widespread severe thunderstorms.
  • «Flash thunderstorm» is the term used to describe short-term warnings (0 to 2 hours) within a more localised region. This warning is only issued once the intensity of a thunderstorm cell has reached the corresponding category. At least one of the criteria of hail, wind or rain (see table) must be met in order for a particular thunderstorm warning to be issued.