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Rain

Rain is the most common form of liquid precipitation. It develops as a result of small, suspend-ed cloud droplets growing into larger drops of water, which then fall to earth because of their weight.

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Hazard category 5 (very severe hazard)
Possible impacts
  • Extreme rise in the water level of streams and lakes
  • Flooding in many areas
  • Failure of drainage and sewer systems and flood protection barriers
  • Flooding of underpasses, underground garages and cellars/basements
  • Massive disruption to bus and rail traffic (delays and cancellations)
  • Massive disruption to road traffic (longer journey times, risk of accidents, road closures) due to wet roads, poor visibility and flooding)
  • High likelihood of landslides on sloping terrain and mudslides
Description of measurement and prognostic values
  • Sustained rainfall north of the Alps and in the Alps: >120 mm/24h; >150 mm/48h; >170 mm/72h
  • Sustained rainfall south of the Alps and in Simplon: >160 mm/24h; >200 mm/48h; >240 mm/72 h
  • Sustained rainfall in Verbano, Valle Maggia: >250 mm/24h; >300 mm/48h; > 350 mm/72h

Hazard category 4 (severe hazard)
Possible impacts
  • Potential for rivers to overflow onto floodplains and flood zones as well as around lake shores
  • Heavy and rapid rise in the water level of streams and lakes
  • Potential for localized landslides on steeply sloping terrain
  • Potential for failure of drainage and sewer systems
  • Flooding of underpasses, underground garages and cellars/basements
  • Limitations to bus and rail traffic (delays and cancellations)
  • Limitations to road traffic (longer journey times, risk of accidents, road closures) due to wet roads, poor visibility and flooding)
Description of measurement and prognostic values
  • Sustained rainfall north of the Alps and in the Alps: 80-120 mm/24h; 110-150 mm/48h; 130-170 mm 72h
  • Sustained rainfall south of the Alps and in Simplon: 130-160 mm/24h; 160-200 mm/48h; 200-240 mm/72h
  • Sustained rainfall in Verbano, Valle Maggia: 200-250 mm/24h; 250-300 mm/48h; 300-350 mm/72h

Hazard category 3 (significant hazard)
Possible impacts
  • Rise in the water level of streams and normally dry ditches
  • Limitations to road traffic (longer jorney times, risk of accidents) due to poor visibility and flooding
  • Localized flooding of underpasses, underground garages and cellars/basements possible
  • Potential for localized landslides on steeply sloping terrain
Description of measurement and prognostic values
  • Sustained rainfall north of the Alps and in the Alps: 50-80 mm/24h; 80-110 mm/48h; 100-130 mm/72h
  • Sustained rainfall south of the Alps and in Simplon: 100-130 mm/24h; 130-160 mm/48h; 160-200 mm/72h
  • Sustained rainfall in Verbano, Valle Maggia: 150-200 mm/24h; 200-250 mm/48h; 250-300 mm/72h

Hazard category 2 (moderate hazard)
Possible impacts
  • Local swelling of streams possible in areas where the ground is already saturated
  • In exceptional cases local flooding, i.e. of underpasses, underground garages and cellars/basements
Description of measurement and prognostic values
  • Sustained rainfall north of the Alps and in the Alps: 30-50 mm/24h; 50-80/48h; 60-100 mm/72h
  • Sustained rainfall south of the Alps and in Simplon: 70-100 mm/24h; 100-130 mm/48h; 130-160 mm/72h
  • Sustained rainfall in Verbano, Valle Maggia: 100-150 mm/24h; 150-200 mm/48h; 190-250 mm/72h

Recommendations for action

What to do in the event of heavy rainfall

Notes

These warnings are primarily used for widespread precipitation (also potentially in combination with a thunderstorm in the summer).

Please note: the warning threshold values differ for north and south of the Alps within the same warning category!

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