Severe weather terminology (Canada)
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This article describes Severe weather terminology used by the Environment Canada's Meteorological Service of Canada. This article focuses on terminology per se, such as tornado warning. Related weather scales and general weather terms are also addressed in this article. Some terms may be specific to certain regions.
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[edit] Types of bulletins
The Meteorological Service is responsible for issuing all Canadian weather alerts using a variety of different technologies. Watches, Warnings and Special Weather Statements alert Canadians to a number of weather conditions that are occurring or are expected to occur, that could impact the safety of persons or cause damage to property or the environment:
- Special Weather Statements are issued for weather events that are unusual, cause general inconvenience or public concern and cannot adequately be described in a weather forecast. They are written in a free style to describe weather systems, potential development such as thunderstorms or long term trends. They may reflect a warning in effect near the USA border.
- Watches provide a "heads-up" that conditions are favourable for a possible summer or winter storm. A watch in the summer may be issued up to six hours before the event and in the case of winter events at least 12 to 24 hours in advance.
- Warnings alert for severe weather occurring or developing. Activation of a specific warning takes into consideration local needs. Warnings for large scale event like snowstorms are issued ideally with least six and up to 24 hours lead time. Severe thunderstorm warnings, by their nature, will be issued less than one hour in advance.
Updated statements, watches and warnings are reissued or upgraded as needed.
[edit] Types of severe weather
Weather watches and warnings are issued for the following conditions:
Convective severe weather:
- Severe Thunderstorm including for:
- Tornado
- Funnel cloud
- Cold-Core Funnel
- Landspout
- Waterspout
Tropical:
Large scale:
- High Heat and Humidity or Humidex
- Rainfall
- Freezing rain and Drizzle
- Flash Freeze
- Wind including for:
- Les Suêtes wind
- Wreckhouse Wind
- Marine Wind
- Dust storm
- Blizzard
- Blowing snow
- Snowfall
- Snowsquall
- Winter Storm
- Windchill
- Arctic Outflow
- Frost
Other:
[edit] Issuing criteria
To issue the different type of bulletins, some criteria must be met.
[edit] Special weather statement
- Cold-core funnels, funnel clouds, landspouts and waterspouts. These tornadic type events are not associated with severe thunderstorms and are difficult to forecast but not usually severe.
- Hurricane Information Statements are issued when a tropical system threatens Canadian coastal waters or land. The statement includes public and marine impacts and warning summary, location and expected motion of the storm and technical discussion. This information is updated at least every six hours.
- Fog or Smoke: A Special Weather Statement may be issued to highlight widespread fog that is giving reduced visibilities less than one km or to explain hazy skies, reduced visibilities due to forest fire smoke or blowing dust in the winter.
- Air Quality: In partnership with the various provinces when standards are exceeded an Air Quality Advisory (British Columbia), Smog Watch and Smog Advisory (Ontario), Air Quality and Health Advisory (Atlantic Canada) or Smog Warning (Quebec) is issued by the provinces.
- Any other potentially hazardous situation in the mid to long term forecast.
- Severe Thunderstorms watch
When the following conditions could develop over a region: flooding rain, destructive winds with gusts greater that 90 km/h, hail of at least 20 mm in diameter or tornadoes .
- Tornado watch
When severe thunderstorms have developed and there is the possibility of one or more tornadoes developing within the areas and times specified in the watch.
- A tornado watch is issued if there is a reasonable probability landspouts or waterspout developing.
- Tropical Disturbances and hurricane
- A Tropical Storm Watch is issued when a tropical storm or tropical storm conditions pose a threat to coastal areas generally within 36 hours. A watch will generally cover a larger threat area than a warning, as the uncertainty on the track of the storm is greater.
- A Hurricane Watch is issued when a hurricane approaches the mainland and is considered a threat to coastal and inland regions.
- Winter Storm
- A Winter Storm Watch will alert for the potential of hazardous conditions up to 48 to 60 hours in advance.
[edit] Warning
- Severe Thunderstorms warning
Storm producing one or more of the following conditions: flooding rain, destructive winds with gusts greater that 90 km/h, hail of at least 20 mm in diameter or tornadoes. The storm’s expected motion and developments will be given in warnings.
- Tornado warning
- A Warning is issued when one or more tornadoes are occurring in the area specified or detected on Doppler radar. The expected motion, development and duration will be given in the warning.
- Tropical Disturbances and hurricane
- A Tropical Storm Warning is issued when winds of 63 to 117 kilometers per hour are expected. Warnings are not issued more than 24 hours in advance.
- A Hurricane Warning is issued for coastal waters where winds greater than 117 km/h are expected. It may also include areas where storm surge or exceptionally high waves are predicted, even though winds may be less than hurricane force. Warnings are not issued more than 24 hours in advance. If the path is erratic or if the hurricane undergoes post tropical transition, the warning may only be issued a few hours in advance.
- A Storm Surge Warning is issued when a storm surge and/or high waves may result in significant flooding in coastal areas.
- High Heat and Humidity / Humidex
- Environment Canada Humidex Advisory or Warning is issued when temperatures are expected to reach or exceed 30°C and the humidex values are expected to reach or exceed 40.
- Large scale systems
- Rainfall warning: Local rainfall thresholds vary considerably across Canada and reflect a potential for regional flooding. Some seasonal considerations are made for ground that is frozen or sodden.
- Freezing Rain/Drizzle warning: A Freezing Rain Warning is issued when you can expect slippery walking and driving conditions and possible damage to trees and overhead wires due to rain freezing on contact to form a coating of ice usually for more than 2 mm thickness.
- Flash Freeze warning : When conditions are likely to rapidly cause the temperature to drop within two to three hours, from above the freezing mark to below the freezing mark, and the roads are wet roads from prior rain or wet snow during the time of rapid cooling.
- Wind warning: when blowing steadily at 60-65 km/h or more, or winds gusting to 90 km/h or more. Les Suêtes (Cape Breton) and Wreckhouse (Southwestern Newfoundland) Winds Warning are for local effect winds.
- Marine Wind warnings: For mariners use
- Strong Wind Warning - Issued if winds of 20 to 33 knots are forecast
- Gale Warning - Issued if winds of 34 to 47 knots are forecast
- Storm Warning - Issued if winds of 48 to 63 knots are forecast
- Hurricane Force Wind Warning - Issued for winds of 64 knots or greater
- Dust Storm: issued only in the Prairie Provinces when blowing dust caused by high winds is expected to reduce visibility to one kilometre or less for one hour or more.
- Blizzard Warning: issued when winds of 40 km/h or more, are expected to cause widespread reductions in visibilities to less than one kilometre, due to blowing snow, for at least four to six hours and windchill is expected to be high.
- Blowing Snow warning: Visibility less than 1 km (<5/8 mile) but not necessarily meet the blizzard definition for duration and temperature.
- Snowfall warning: issued when hazardous amounts are expected to fall over a 12- or 24-hour period. These amounts vary across the country due to topographical and climatic considerations. They range from five cm in 24 hours for parts of southwestern BC to 15, 20 and even 25 cm in 24 hours elsewhere.
- Snowsquall warning for:
- Lake effect snowsquall: A warning will be issued when 15 cm or more of snow is expected to fall within 12 hours, or the visibility is likely to be near zero for at least four hours because of falling and blowing snow. These are for areas much smaller than a snowstorm would cover.
- Frontal snowsquall: A warning may be issued when blizzard-like conditions are expected to develop in the vicinity of a cold front for short periods.
- Winter Storm Warning will alert that a combination of hazardous winter conditions are occurring or expected to develop no more than 12 to 14 hours in advance.
- Others
- Wind Chill: Wind chill warning criteria vary across the country, ranging from -55 in some Arctic regions to -30 in southwestern Ontario. Warnings will be issued when the winds are expected to be at least 15 km/h and these extreme wind chills will persist for at least three hours.
- Cold Wave: When temperatures are expected to fall dramatically within 24 hours from above normal or near seasonal temperatures to very cold temperatures a Cold Wave Warning is issued in some parts of the country.
- Arctic Outflow: An Arctic Outflow Warning is based on a combination of wind speed and temperatures which produce wind chills of at least -20 for at least six hours during the winter when very cold Arctic air breaks from the interior mainland of British Columbia and spills out through mountain gaps and fjords.
- Frost: A Frost Warning is issued only when air temperatures are expected to fall to near freezing or below during the local growing season. A frost warning would normally be issued when forecast air temperature is +2°C or below.
