Severe weather terminology (Canada)
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.
- 1 Warning categories
- 2 Weather warnings
- 3 Special Weather Statements
- 4 Related weather scales as defined by Environment Canada
- 5 See also
- 6 External links
- 7 Notes and references
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 watches and warnings are issued when potentially hazardous weather is occurring or is forecast for the short term period.
Local-scale/Summer severe weather
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:
- 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.
- 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 are detected on Doppler radar. The expected motion, development and duration will be given in the warning.
Hurricanes and other tropical systems
- 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.
- Storm Surge Warning – Issued when a storm surge and/or high waves may result in significant flooding in coastal areas.
- 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.
- Blowing Snow Warning – Issued when high winds and snow reduce visibility to less than 1 km (0.62 mi) (<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 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 / Freezing Drizzle Warning – 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.
- 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.
- High Heat and Humidity Warning – Issued when temperatures at least 30 °C (86 °F), and Humidex values over 40 °C (104 °F) persist for at least one hour. Issued in Quebec only.
- Frost Warning – A Frost Warning is issued only when air temperatures are expected to fall to near freezing (0 °C (32 °F)) or below during the local growing season. A frost warning would normally be issued when the air temperature is forecasted to be +2 °C (36 °F) or below.
- Wind Chill Warning – Wind chill 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 the winds are expected to be at least 15 km/h (9.3 mph) and these extreme wind chills 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.
- Cold Wave Warning – 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.
- Flash Freeze Warning – Issued 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.
- 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.
- 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
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 include:
- Humidex Advisory – Issued when Humidex values exceed local criteria; most areas will issue an advisory if the Humidex is expected to be over 40 °C (104 °F).
- Air Quality Advisory – Issued in partnership with the various provinces when air quality 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.
- Fog or Smoke Advisory – Issued to highlight widespread fog that is reducing visibilities to less than 1 km (0.62 mi) or to explain hazy skies, reduced visibilities due to smoke from forest fires or blowing dust in the winter.
Hurricane Information Statements
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.
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
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. 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
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).
- List of severe weather phenomena
- Environment Canada
- Meteorological Service of Canada
- Severe weather terminology (United States)