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Extreme storms

Phenomenon

Extreme storms

There are several types of winter and summer storms. According to Environment Canada, “winter storms are major weather phenomena called cyclones which gather energy from energy and humidity extremes between two masses of air that meet or collide. The greater the difference and temperature and humidity between the two masses of air, called fronts, the more energy is available to create storms.

Summer storms cause thunder, lightning, hail and tornadoes. These storms are caused by strong, rising currents of air, which occur more frequently in hot, humid weather. Tornados form when a mass of hot air collides with masses of cold air from the north.

Environment Canada Winter Hazards

Preparing and protecting yourself

Risks in Montreal

Deneigement - Crédit : Denis Labine - Ville de MontréalPhenomena such as hail, freezing rain, heavy snow, storms and violent winds are considered extreme storms, and are most likely to occur in Montreal. Unlike other types of risks, extreme storms are not a localized phenomenon and can affect large areas. Heavy rains in 1987, the ice storm in 1998 and frequent, heavy snowstorms in 2007-2008 are a few examples of weather phenomena that can disturb activity across Montréal and pose a threat to public health and safety.

In 2008, a series of winter storms hit the Montreal area, a city that is no stranger to heavy precipitation and strong winds. Following these storms, the Centre de sécurité civile implemented an extreme snowstorm special response plan.

Impacts on the public and infrastructures

Extreme storms can impact people’s health (injuries, mortality) and their safety (evacuation, shelter or emergency housing needed).

Among the potential impact caused by violent storms are:

  • Slowed traffic or blockages due to emergency vehicles
  • Problems accessing disaster scenes, victims, sensitive areas and vulnerable people
  • Delayed emergency response
  • Damage to infrastructures, including essential infrastructures
  • Necessay evacuations delayed by conditions (heavy snow, debris, etc.)
  • Electric outages
  • Slowed or stopped socio-economic activities

Types of storms

Snowstorms

Snowstorms can have many consequences including heavy snow, blowing snow and windchill. A map of snow on the ground is available on the Weather Network’s site.

Violent wind

Paysage de neige  - Crédit : Hélène BourdageViolent winds don’t happen exclusively during tropical storms or low-pressure fronts or systems. They can also be caused by thunderstorms, Chinook winds or local ground conditions.

Source : Environnement Canada

 

Heavy rain

Storms that occur during the summertime are caused by rapid elevation of hot, humid air, and can cause precipitation of great intensity called heavy or torrential rain. Heavy rain can cause flooding, sewer backups and other types of damage.

Freezing rain

Freezing rain, Crédit :Denis Labine, Ville de MontréalFreezing rain is liquid precipitation whose temperature is lower than 0°C and that freezes when in contact with the ground or other surfaces (trees, electrical wires, etc.) Freezing rain can make surfaces slippery for drivers and pedestrians, and can cause accidents as well as falling electrical wires.

 

Hail

Hail is precipitation in the form of ice granules whose diameter is equal to or greater than 5 mm. Hail forms in the centre of storms and hailstones can pick up water as they fall, becoming larger, heavier and more dangerous. Damage caused by ice storms can be greater, especially to cars and homes.

Source : Natural Resources Canada

Tornadoes

Tornadoes are columns of air that spiral around themselves at high speeds. They are small and can be very violent. They hit quickly, randomly, and often with no warning. The winds they cause can reach up to 100 metres per second. The damage caused by a tornado is due to its winds and the extremely rapid drop in atmospheric pressure.

Source : Natural Resources Canada, The Atlas of Canada.

Extreme snowstorm special response plan - snow section.

Indicators and actions

How OSCAM (Organisation de sécurité civile de l'agglomération de Montréal) implements mobilization levels for snowstorms is determined by the quantity of precipitation forecast and the effect of other aggravating factors. Montreal’s snow removal coordination cell ensures the coordination of snow removal activities and in response mode; it acts on behalf of OSCAM and coordinates activities with boroughs and towns.

Snow removal and cleanliness unitOSCAMActions
Level 1
Accumulations less than 30 cm expected
  • Snow removal and cleanliness unit on close watch
Normal
Accumulations less than 30 cm expected
 
Level 2
Accumulations between 30 and 59 cm expected
or
Accumulations of less than 30 cm expected, with exceedance of critical levels of at least two of the indicators of aggravating phenomena.
  • Implementation of snow coordination cell
Watch
Accumulations between 30 and 44 cm expected or
Accumulations of less than 30 cm expected, with exceedance of critical levels of at least two of the indicators of aggravating phenomena.
  • Clearing roads and sidewalks and spreading abrasives for 22 special zones identified by the STM
  • Stopping prohibited along six priority axes reserved for the STM, emergency vehicles and snow transportation
Alert
Accumulations between 45 and 59 cm expected
or
Accumulations between 30 and 44 cm expected, with exceedance of critical levels of at least two of the indicators of aggravating phenomena.
  • Clearing roads and sidewalks and spreading abrasives for 22 special zones identified by the STM
  • Stopping prohibited along six priority axes reserved for the STM, emergency vehicles and snow transportation
Level 3
Accumulations of 60 cm or more expected
or
Accumulations between 30 and 59 cm expected, with exceedance of critical levels of at least two of the indicators of aggravating phenomena.
  • Implementation of snow coordination cell
Response
Same indicators as coordination cell
  • Priority streets cleared
  • Ensure public transportation is available
  • Police officers ensure fluid traffic circulation and tow vehicles on priority streets.
  • Firefighters coordinate rescue operations (vehicles buried under the snow, etc.)
  • Send snow removal units urgent snow removal requests 

Aggravating factors during an extreme snowstorm

Indicators of aggravating phenomenaSeuils critiques
Previous accumulation of snow on the ground 15 cm and more during the past 24 hours
30 cm and more during the past 48 hours
Rapid accumulation of snow on the ground 9 cm/hour and more
Expected duration of snowfall Less than 6 hours
Strong wind 45 km/hour and more
Freezing rain 25 mm and more
Wind chill factor - 30°C and less
Time of snowfall Weekdays
Before rush hour

Priority snow removal operations

Certain sites which are defined as sensitive sites have priority for snow removal. These sites may include:

  • Fire stations
  • Police stations and neighbourhood stations
  • Emergency communication centres: 911, 811, 311
  • Streets around STM garages
  • Hospitals/CLSC
  • Drinking water treatment plants
  • Bridges and tunnels
  • Hydro-Quebec facilities
  • Airports
  • Shelters for displaced people
  • Day care centres
  • Long-term care facilities

Important Numbers

Health Info
8-1-1
Transports Québec
5-1-1
City information
3-1-1
Emergencies
9-1-1

Poison center

Toll-free
24 hours a day, 7 days a week 1-800-463-5060