Each year, around 15,000 severe weather warnings are issued across the country by the Meteorological Service of Canada.
A warning indicates severe weather is either occurring or will occur and is usually issued 6 to 24 hours in advance.
Some severe weather, like thunderstorms and tornadoes, can occur rapidly with less than a half-hour’s notice.
Environment Canada has established thresholds for determining when weather watches and warnings should be issued and not all weather events will be preceded by a watch.
To help explain some of the mystery, here is the criteria used by Environment Canada for common weather events in Saskatchewan during the summer.
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Severe Thunderstorm
Watch
When conditions are favourable for the development of severe thunderstorms with one or more of the following conditions:
– wind gusts of 90 km/h or greater;
– hail of 2 cm or larger in diameter; or
– heavy rainfall (per rainfall criteria)
Warning
When there is evidence based on radar, satellite pictures or from a reliable spotter that one of the three conditions in a watch is imminent or occurring
Tornado
Watch
Issued when conditions are favourable for the development of severe thunderstorms with one or more tornadoes.
Warning
Issued when a tornado has been reported, if there is evidence of a tornado based on radar or a reliable spotter reports a tornado is imminent.
Rainfall Warning
When 50 mm or more of rain is expected within 24 hours or when 75 mm or more of rain is expected within 48 hours.
Heavy Downpour Warning
When 50mm or more of rain is expected with one hour.
Wind Warning
Sustained wind of 70 km/h or more and/or gusts of 90 km/h or more.
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