Severe weather terminology (Canada)

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This article describes severe weather terminology used by the Meteorological Service of Canada a department of Environment Canada. The article primarily describes various weather warnings, and their criteria. Related weather scales and general weather terms are also addressed in this article. Some terms may be specific to certain regions.

Warning categories[edit]

Severe weather bulletins are issued as a watch or a warning, depending on the risk or severity of the event. Less severe events that could be a cause for concern will be issued as a special weather statement or Advisory.

  • Watches are issued when conditions are favourable for the development of severe weather. Watches are typically issued for local-scale events in which the timing and location of occurrence remains uncertain; such as severe thunderstorms or tornadoes. A watch is normally issued several hours in advance. A watch issued for severe summer storms is typically issued up to six hours in advance, whereas watches for winter events at least 12 to 24 hours in advance. Weather watches are issued for regular forecast regions affected.
  • Warnings are issued when severe weather is either imminent or occurring. Warnings for large scale events such as snowstorms are issued with an ideal lead time of at least six, and up to 24 hours. Severe thunderstorm warnings, by their nature, will be issued less than one hour in advance. Weather warnings are usually issued for regular forecast regions affected. Severe thunderstorm and tornado warnings however, may be issued specifically for smaller warning or "sub-regions" within the regular forecast area where available. Specific warning criteria varies by region, depending on geography, or other conditions in which a specific region may be vulnerable.
  • Special Weather Statements or Advisories are issued in a freestyle format for weather events that are unusual, cause general inconvenience or public concern and cannot adequately be described in a public weather forecast. They may reflect a warning in effect near the United States border. A Special Weather Statement may also be issued to indicate any potentially hazardous situation in the long term forecast.

Updated statements, watches and warnings are re-issued or upgraded if required.

Weather warnings[edit]

Weather watches and warnings are issued when potentially hazardous weather is occurring or is forecast for the short term period. Some of those bulletins have been changed on April 8th, 2014 and below is a list and definitions of the up-to-date watch and warning.[1]

Local-scale/Summer severe weather[edit]

Due to its local-scale nature, a watch is typically issued in advance for public forecast areas where conditions may be favourable for the development of severe weather. A warning is issued for areas where severe weather is imminent or occurring. Unlike other warnings, these are issued for smaller warning areas within its regular forecast region.

  • Severe Thunderstorm Watch – Issued when the potential exists for the development of severe thunderstorms, which are capable of producing one or more of the following:
    • Large hail (2 cm or more in diameter)
    • Damaging winds (Gusts 90 km/h (56 mph) or greater)
    • Heavy rain (Alberta to Southern Quebec: 50 mm (2.0 in) or more per hour. Pacific, Northern and Maritime provinces: 25 mm (0.98 in) per hour)
  • Severe Thunderstorm Warning – Issued when a severe thunderstorm is detected on radar or are observed by those in the immediate area. A warning is issued when one or more of the following has been detected or highly possible:
    • Large hail (2 cm or more in diameter)
    • Damaging winds (Gusts 90 km/h (56 mph) or greater)
    • Heavy rain (Alberta to Southern Quebec: 50 mm (2.0 in) or more per hour. Pacific, Northern and Maritime provinces: 25 mm (0.98 in) per hour)

Public bulletins will often mention the possibility of tornadoes; if a tornado is spotted or conditions are favourable enough for tornado development, the warning will be upgraded accordingly.[1]

  • Tornado Watch – Issued when conditions are favourable for the development of severe thunderstorms with one or more tornadoes. A tornado watch may also be issued if there are a certain number of funnel clouds (generally cold-core type) spotted over a certain area, or if there is a reasonable probability of landspouts or waterspouts developing.
  • Tornado Warning – Issued when one or more tornadoes are occurring in the area specified or rotation is detected on Doppler weather radar. The expected motion, development and duration will be given in the warning.

Hurricanes and other tropical systems[edit]

  • Tropical Storm Watch – 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.
  • Tropical Storm Warning – Issued when winds of 63 to 117 km/h (39 to 73 mph) are expected. Warnings are not issued more than 24 hours in advance.
  • Hurricane Watch – Issued when a hurricane approaches the mainland and is considered a threat to coastal and inland regions.
  • Hurricane Warning – Issued for coastal waters where winds greater than 117 km/h (73 mph) 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 a transition into a post-tropical system, the warning may only be issued a few hours in advance. Almost always accompanied by a Wind Warning.
  • Storm Surge Warning – Issued when a storm surge and/or high waves may result in significant flooding in coastal areas. Sometimes issued during strong Nor'easters or other non-tropical storms.

Winter weather[edit]

  • Winter Storm Watch – Issued when conditions are favourable for the development of hazardous conditions. These bulletins may be issued 48 to 60 hours in advance.
  • Winter Storm Warning – Issued when a combination of hazardous winter conditions are occurring or expected to develop no more than 12 to 14 hours in advance.
  • Blizzard Warning – Issued when winds of 40 km/h (25 mph) or more, are expected to cause widespread reductions in visibilities to less than 1 km (0.62 mi), due to blowing snow, for at least four to six hours and windchills are expected to be very low.
  • 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 5 cm (2.0 in) in 24 hours for parts of southwestern British Columbia to 15 cm (5.9 in), 20 cm (7.9 in) and even 25 cm (9.8 in) in 24 hours elsewhere.
  • Snowsquall Warning – Issued when 15 cm (5.9 in) 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. A warning may also be issued when blizzard-like conditions are expected to develop in the vicinity of a cold front for short periods.
  • Freezing Rain – Issued when hazardous walking and driving conditions are expected from freezing rain or drizzle. A warning may be issued if ice is over 2 mm (0.079 in) thick and has the potential to cause damage to trees and overhead electricity and telecommunications wires.

General warnings[edit]

  • Wind Warning – Issued when wind speeds are expected to or currently blowing steadily at 60 to 65 km/h (37 to 40 mph) or more, or winds gusting to 90 km/h (56 mph) or more. Les Suêtes (Cape Breton) and Wreckhouse (Southwestern Newfoundland) Winds Warning are for local effect winds. This warning could be issued if necessary during a tropical cyclone, in addition to the regular Hurricane and/or Tropical Storm Watches or Warnings.
  • 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. As with the Wind Warning above, this warning may be issued along with tropical cyclone advisories.
  • Heat Warning – Issued when temperatures at least 30 °C (86 °F), and Humidex values over 40 °C (104 °F) persist for at least one hour. Definition may be different in some regions.
  • Extreme cold Warning – Criteria varies across the country, ranging from −55 °C (−67 °F) in some Arctic regions to −30 °C (−22 °F) in southwestern Ontario. Warnings will be issued when windchill, with winds of at least 15 km/h (9.3 mph), or actual temperature of the same values are expected to persist for at least three hours.
  • Arctic Outflow Warning – An Arctic Outflow Warning is based on a combination of wind speed and temperatures which produce wind chills of at least −20 °C (−4 °F) 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.
  • Dust Storm – Issued only in the Prairie Provinces when blowing dust caused by high winds is expected to reduce visibility to 1 km (0.62 mi) or less for one hour or more.
  • Storm surge warning – Issued for abnormally high water levels and high waves caused by storms, which have the potential to cause coastal flooding. This usually occurs when astronomical tides are at their maximum.
  • Weather warning – A generic weather warning may be issued for extreme weather events for which there is no suitable warning type, because they rarely occur. For example, for 50 km/h winds following an ice storm which could cause structural wind damage, even if the wind warning criteria of 90 km/h is not expected to be reached.

Marine warnings[edit]

  • Strong Wind (Small Craft) Warning – Issued if winds of 20 to 33 kn (37 to 61 km/h) are forecast.
  • Gale Warning – Issued if winds of 34 to 47 kn (63 to 87 km/h) are forecast.
  • Storm Warning – Issued if winds of 48 to 63 kn (89 to 117 km/h) are forecast.
  • Hurricane Force Wind Warning – Issued for winds of 64 kn (119 km/h) or greater.
  • Squall Warning – Issued for forecast or observed wind gusts of 34 kn (63 km/h) or greater that are associated with a line, or an organized area, of thunderstorms.
  • Freezing Spray Warning – Freezing spray occurs when a combination of low temperatures and strong winds cause sea spray to freeze on a ship's superstructure or on other structures either in the sea or near the water's edge. A weather warning is issued whenever moderate or heavy ship icing is expected.
  • Waterspout Warning/Alert – Issued commonly for Atlantic provinces (usually M-IS). Issued when a waterspout is detected on radar or is observed by trained spotters. The warning is commonly issued to warn persons on water. A waterspout warning can be sometimes issued for an area on land, if it is supected that the funnel will travel inland.

Special Weather Statements[edit]

Advisories[edit]

Advisories are issued as part of a special weather statement in a similar format to that of an official warning. Unlike warnings, however, these types of bulletins describe exceptional weather events that are generally not considered hazardous, but may be a potential concern to the public (for example, High Humidex that may affect certain age groups). Advisories may also be issued for potentially hazardous weather in the long term forecast. Commonly issued advisories as of April 8, 2014 include:[1]

  • Blowing Snow Advisory: Low visibilities in blowing snow expected for a significant duration. (this alert is not issued for Nunavik, or areas north of the treeline)
  • Fog Advisory: Low visibilities in fog expected for a significant duration. Newfoundland and Labrador, New Brunswick, Nova Scotia and Prince Edward Island have different criteria from the rest of Canada (this alert is not issued for Nunavik).
  • Freezing Drizzle Advisory: Freezing Drizzle is expected for a significant duration. (this alert is not issued for Nunavik)
  • Frost Advisory: Frost is expected (this alert is not issued for Nunavik).
  • Weather Advisory: A meesage that can be used for any situation for which there is no other alert that effectively describes the conditions expected.

Hurricane Information Statements[edit]

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.

Thunderstorm Potential[edit]

Issued in Ontario only, these statements are issued twice daily from May through September. These statements give an in-depth description of general thunderstorm potential for the next 48 hours, whether severe or non-severe.

Related weather scales as defined by Environment Canada[edit]

Enhanced Fujita Scale[edit]

Main article: Enhanced Fujita Scale

The Enhanced Fujita Scale (EF) is a scale for rating tornado intensity based on the damage on human-built structures and vegetation. While the United States adopted the Enhanced Fujita Scale in 2007, Environment Canada continued to use the original Fujita Scale to assess tornado intensity until April 18, 2013, when the agency adopted the Enhanced Fujita Scale.[2] Tornadoes exceeding F2/EF2 intensity are rare in Canada, though several tornadoes, such as the Edmonton Tornado in 1987, have been as strong as F4. The only F5/EF5 tornado recorded in Canada to date was the Elie, Manitoba tornado in 2007.

Saffir-Simpson Hurricane Scale[edit]

The Saffir-Simpson Hurricane Scale is used by the Canadian Hurricane Centre for hurricanes affecting the East Coast of Canada. The Scale ranges from Category 1, the weakest, to Category 5, the strongest with sustained winds exceeding 250 km/h (160 mph).

See also[edit]

External links[edit]

Notes and references[edit]

  1. ^ a b c Meteorological Service of Canada. "Public Alerting Criteria". Retrieved May 23, 2014. 
  2. ^ Assessing tornado damage: EF-scale vs. F-scale