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Rain

Rain is the most common form of liquid precipitation from clouds, and 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.

Hazard category 5 (very severe hazard)
Impact
  • Flooding, even in areas which are not usually prone to flooding.
  • Extreme swelling of streams, waterfalls or normally dry ditches, even in areas which are normally at low risk.
  • Failure of drainage and sewer systems and flood protection barriers.
  • Flooding of underpasses, underground garages and cellars/basements.
  • Massive disruption to road and rail traffic.
  • High likelihood of landslides on sloping terrain and mudslides.

Recommended action

Information on recommended action can be found here: What to do during heavy rainfall

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/72h
  • Sustained rainfall in Verbano, Valle Maggia: >250 mm/24h; >300 mm/48h; > 350 mm/72h
Hazard category 4 (severe hazard)
Impact
  • Potential for rivers to overflow onto floodplains and flood zones as well as around lake shores.
  • Heavily swollen streams, waterfalls and normally dry ditches, and flooding of cave systems.
  • Potential for localised landslides on steeply sloping terrain.
  • Failure of drainage and sewer systems.
  • Flooding of underpasses, underground garages and cellars/basements;
  • Disruption to road and rail traffic.
Recommended action Information on recommended action can be found here: What to do during heavy rainfall
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/72
Hazard category 3 (significant hazard)
Impact
  • Swelling of streams, waterfalls and normally dry ditches, and flooding of ex-posed cave systems.
  • Disruption to road traffic on exposed stretches.
Recommended action Information on recommended action can be found here: What to do during heavy rainfall
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)
Impact Local swelling of streams or local flooding possible in areas where the ground is already saturated.
Recommended action Keep an eye on local weather developments, and act appropriately, according to the information you receive on the conditions.
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

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!