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Danger levels 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.

Danger level 5 (very severe danger)
Thunderstorm warnings are only issued for categories 3 and 4. In spite of the most up-to-date tools and methods, 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.
Danger level 4 (severe danger)
Effects
  • 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
Behaviour 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
Danger level 3 (significant danger)
Effects
  • 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
Behaviour

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
Danger level 2 (moderate danger)
A thunderstorm event that is usual for the time of year (for which, therefore, no special warnings are issued)

Notes:

“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.

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