This article describes severe weather terminology used by the Meteorological Service of Canada, a branch within Environment and Climate Change 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 are specific to certain regions.

Warning categories Severe weather bulletins are issued as a watch or a warning, depending on the risk or severity of the event.

Watches are issued when conditions are favourable for the development of severe weather, but the occurrence, location, and/or timing is still too uncertain to issue a warning. The target lead time for severe thunderstorm watches is typically six hours before the event, whereas watches for winter events have a target lead time of 12 to 24 hours in advance. Watches are intended to raise awareness of the public to the potential for hazardous weather conditions, and typically serve as a lead-up to a warning. Warnings are issued when severe weather is either imminent or occurring. Warnings for synoptic scale events such as snowstorms are issued with an ideal lead time of at least six, and up to 24 hours. Severe thunderstorm warnings and tornado warnings, by their nature, will typically be issued with very little lead time. Weather warnings are usually issued for regular forecast regions affected. Severe thunderstorm and tornado warnings, however, might be issued specifically for smaller warning or "sub-regions" within the regular forecast area where available. Advisories are issued 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, such as frost. Special Weather Statements are free form statements that are typically used to describe weather hazards that cannot be described by another watch, warning, or advisory; is not hazardous enough to warrant issuing a watch or warning; or to warn the public of a potentially hazardous weather event in the long term forecast. Alerts are typically updated every six hours, except for mesoscale/summer severe weather alerts which are updated when necessary.

Weather alerts (watches or warnings) are also accompanied by a colour, which is determined by the potential impacts of the event and its confidence, ranging from moderate to very high. Special weather statements are not accompanied by a colour. Yellow alert: This is the most common type of warning issued. Hazardous weather may cause damage, disruption, or health impacts, but impacts will be moderate, localized and/or short-term. Yellow alerts are also issued when there is the possibility of high or very high impacts, but forecast confidence is low to moderate. Orange alert: These are uncommon and indicate that severe weather is likely to cause significant damage, disruption, or health impacts. Impacts are major, widespread and/or may last a few days. Orange alerts are only issued when forecast confidence is high and there are likely to be high impacts. Red alert: Red alerts are the rarest, and indicate very dangerous and possibly life-threatening weather will cause extreme damage and disruption. Impacts are extensive, widespread, and prolonged. These are only issued for very high impact events with very high forecast confidence.

Low impact events are not given a colour, but they may result in the issuing of a Special Weather Statement if appropriate.

Warnings colour table

+ Alert colours Confidence Impacts Low Moderate High Extreme Low None None Yellow Yellow Moderate None None Yellow Orange High None Yellow Orange Orange Very High None Yellow Orange Red

Weather warnings

Weather watches and warnings are issued when potentially hazardous weather is occurring or is forecast for the short term period.

Note: Nunavik, Quebec only participates in two types of weather warnings: Extreme Cold Warnings and Wind Warnings. No other types of watches or warnings are issued in this region of the country.

Mesoscale/Summer severe weather Severe Thunderstorm Watch (SAME event code: SVA) – Issued when the potential exists for the development of severe thunderstorms, which are capable of producing one or more of the following: Large hail (2.5 cm or more in diameter) Damaging winds (Gusts or greater) Heavy rain (Alberta, Saskatchewan, Manitoba, Ontario, and Quebec: or more per hour. Interior dry sections of British Columbia: 15mm or more rain in one hour. Remaining sections of British Columbia, Yukon, Northwest Territories, Nunavut, New Brunswick, Prince Edward Island, Nova Scotia, Newfoundland and Labrador: per hour. Nunavik: 50 mm or more of rain in one hour) Severe Thunderstorm Warning (SVR) – 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 (2.5 cm or more in diameter) Damaging winds (Gusts or greater) Heavy rain (Alberta, Saskatchewan, Manitoba, Ontario, and Quebec: or more per hour. Interior dry sections of British Columbia: 15mm or more rain in one hour. Remaining sections of British Columbia, Yukon, Northwest Territories, Nunavut, New Brunswick, Prince Edward Island, Nova Scotia, Newfoundland and Labrador: per hour. Nunavik: 50 mm or more of rain in one 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 (TOA) – Issued when conditions are favourable for the development of severe thunderstorms with one or more tornadoes. Tornado Watches are also issued when cold core funnel clouds are possible and pose a threat to people on the ground. If cold core funnel clouds are not expected to touch down, a weather advisory will be issued instead. If there is a landspout on the ground, a Tornado Warning will be issued.

Tornado Warning (TOR) – Issued when one or more tornadoes are occurring in the area specified or rotation is detected on Doppler weather radar, or when someone spots a supercell tornado or a landspout on the ground. The exact location of the tornado or storm will be given in the statement.

Hurricanes and other tropical systems

Tropical Storm Watch (TRA) – 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 (TRW) – Issued when winds of are expected. Warnings are not issued more than 24 hours in advance. Hurricane Watch (HUA) – Issued when a hurricane approaches the mainland and is considered a threat to coastal and inland regions. Hurricane Warning (HUW) – Issued when winds greater than are expected to occur for coastal or inland regions. 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 Winter Storm Watch (WSA) – Issued when conditions are favourable for the development of hazardous winter conditions, including: a blizzard. a major snowfall of or more in a 24-hour period. a significant snowfall (snowfall warning criteria) combined with another winter hazard such as freezing rain, strong winds, blowing snow and/or extreme cold. Winter Storm Warning (WSW) – Issued when hazardous winter conditions are expected or occurring, including: a major snowfall of or more in a 24-hour period. a significant snowfall (snowfall warning criteria) combined with another winter hazard such as freezing rain, strong winds, blowing snow and/or extreme cold. Blizzard Warning (BZW) – Issued when winds of or more, are expected to cause widespread reductions in visibilities to less than , due to blowing snow, for at least four hours (except six hours north of the tree line). Snowfall Warning (WSW) – Issued when significant amounts of snow are expected to fall over a 12-hour period. These amounts vary across the country due to topographical and climatic considerations. Criteria ranges from in a 12-hour period, with local exceptions for southwestern British Columbia ( in a 6-hour period) and Haines Skagway roads in British Columbia/Yukon ( in a 24-hour period). Snow Squall Watch – There are two types of snow squalls of which alerts are issued for: Open-Water – Issued when, down wind of a large body of water, conditions are favourable for the development of snow squalls with one or more of the following conditions: Local, intense snowfall resulting in accumulations of or more in 12 hours or less. Reduced visibility less than , caused by snowfall, with or without blowing snow, for 3 hours or more. Frontal – Issued when conditions are favourable for the development of snow squalls that produce brief periods of very poor visibilities caused by heavy snow and blowing snow caused by the passage of a cold front. Snow Squall Warning (WSW) – There are two types of snow squalls of which alerts are issued for: Open-Water – Issued when, down wind of a large body of water, snow squalls are imminent or occurring with one or more of the following conditions: Local, intense snowfall resulting in accumulations of or more in 12 hours or less. Reduced visibility less than , caused by snowfall, with or without blowing snow, for 3 hours or more. Frontal – Issued when a brief period (less than one hour) of poor visibility of or less caused by heavy snow and blowing snow, accompanied by strong, gusty winds of or more, is expected or occurring with the passage of a cold front. Freezing Rain Warning (WSW) – Issued when freezing rain is expected to create a hazard to transportation and property, or when freezing rain is expected to persist for at least 2 hours (criteria is 4 hours in Atlantic Canada and Magdalen Islands). Flash Freeze Warning (FSW) – Issued when significant ice is expected to form due to a rapid drop in temperatures, causing freezing of residual water on roads, and sidewalks to quickly build up.

General warnings Wind Warning (HWW) – Varies based on climatology across the country. The national criteria, before exceptions, is when sustained winds of or greater, or wind gusts of or greater, are expected or occurring. Les Suêtes (Cape Breton) or Wreckhouse (Southwest Newfoundland) wind warnings are issued for local wind effects using the national criteria. Wind warning criteria for Southwestern Alberta, Dempster Highway (Yukon), Western Vancouver Island, and Newfoundland and Labrador is when sustained winds of or greater, or wind gusts of or greater, are expected or occurring. Wind warning criteria for North Vancouver Island, North Coast (British Columbia), Central Coast (British Columbia), and Haida Gwaii, is when sustained winds of or greater, or wind gusts of or greater, is expected or occurring.