Severe Spring and Summer Weather in Southern Ontario - What you need to know!cumulonimbus.jpg

Southern Ontario's climate in the summer features a wide range of  fair and unsettled weather. Adverse weather conditions are often the result of the movement of two air masses - the dry air from Canada's west and the northwest and muggy air from the American south and southwest.

When the boundaries or fronts of these two very different air masses meet some spectacular thunderstorms may erupt.


Thunderstorms develop in an unstable atmosphere.  They form when warm, moist air near the ground rises and cools in the colder air above.  The moisture in the air condenses to form rain droplets, ice crystals and hail in dark thunder clouds called cumulonimbus clouds.

Thunderstorms frequently occur on a warm, humid summer afternoon, but may develop at any time of the day or night.  They usually form quickly and travel rapidly.  The storm which struck Southwestern Ontario on July 14, 1997 is a classic example, in mid-afternoon, the weather radar screens at Environment Canada showed no rain.  Twenty minutes later, a severe thunderstorm formed near Punkydoodles Corners- between Kitchener and Stratford.  In a few hours the storm dropped more than 200 millimetres (mm) of rain.  Winds of more than 115 kilometres an hour (km/h) uprooted trees and flipped over small aircraft as far east as Guelph.


Each year in Canada, lightning kills an average of 10 people, injures over 100 and causes more than 20 per cent of all forest fires. Protect yourself. picture of lightening at night

  • Close all windows and doors and keep away from them.
  • Don't go outside unless necessary.
  • Before storm hits, unplug all appliances including TV, computer and don't touch electrical items or the telephone.
  • Do not take a bath or use the faucets (the metal and water conduct it).
  • If you are outside, Get inside building or vehicle if possible. Avoid water and objects that conduct electricity (tractor, golf clubs, etc.)
  • Do not stay in open spaces or under tall objects (trees, poles).
  • If no shelter available, crouch down, feet close together with head tucked down.  




  • Take shelter in an interior room or a basement immediately.
  • Get low to the ground and cover your head with your arms.
  • Avoid taking cover in vehicles, near windows, on in a structure with a wide spanning roofLike an arena, gymnasium or shopping mall.
  • In and office or apartment, take shelter in an inner hallway or room, ideally in the basement or on the ground floor. Do not use the elevator.
  • Protect your head and watch out for flying debris.


 Signs that a tornado may be approaching: 

  • Severe thunderstorm and tall cloud.Picture of black clouds with tornadoes. It gets very dark.
  • Gusty winds
  • Rapid change in wind speed and direction 
  • Downpours of heavy rain and hail
  • Loud noise (difficult to decipher against the background, especially in urban areas)
  • Sky turns greenish colour .

    Watches and Warnings - What do they mean?
  • Severe Weather Watch: Conditions are favourable for a severe storm
  • Severe Weather Warning: Severe weather is imminent or occurring
  • Tornado Watch: Conditions are favourable for tornado development
  • Tornado Warning: A tornado has been spotted, take cover immediately

 A Tornado Watch is issued 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. Be prepared to take action if a warning is issued.

A Tornado 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. Find appropriate shelter.

In Canada, there are on average 80 tornadoes per year.  Of these 80, an average of 11 will occur in Ontario.  Although most tornadoes are found to have wind speeds less than 160 km/h and will cause relatively minor damage, it is necessary to be aware of the risks.  Severe tornadoes can have wind speeds greater than 300 km/h and will demolish structures easily. 

Southwestern Ontario is part of "Tornado Alley", which stretches through Southwestern Ontario from Windsor to Barrie. Prime tornado months are May to September with peaks occurring in June through July.  Tornadoes causing significant damage in Waterloo Region have been minimal, but the surrounding area has been severely affected (Examples are the tornadoes in Windsor in 1946 and 1974, Barrie in 1985 and Guelph in 2003).  Of Canada's 10 worst tornadoes, four have taken place in Ontario, three of which were in the Southwestern region. 

Tornadoes form during a thunderstorm when a funnel cloud descends, sometimes following heavy rain and hail and heavy rain.  The tornado may be clearly visible, but in some situations it may be ovscured by haze or wrapped in heavy rain. The sky will usually turn green, yellow or black and the tornado can sound like a freight train or jet engine.  Once a tornado forms it will usually move from southwest to northeast at anywhere from 20 to 90 km/h in a path that can range from 10s of metres wide and less than a kilometre in length to 100s of metres in width and many kilometres in length.  

Protecting Yourself 

If the forecast calls for severe thunderstorms, you should monitor the weather closely and keep up to date by frequently checking the weather on your television, radio, or internet.  Updates to the conditions will be provided by Environment Canada, and appropriate watches and warnings will be issued accordingly.  However, in the case of tornadoes, there may be little warning before a tornado develops and begins to cause damage, therefore it is also important to personally watch the conditions outside for yourself.  A tornado warning will only be issued once one has been spotted or detected on radar which may give little time for those closest to the tornado to react.

Another great way to protect yourself against severe weather events (specifically tornadoes) is to purchase a Weatheradio receiver.  The Weatheradio service is provided by the Meteorological Service of Canada and gives continuous weather coverage for your area, including immediate watches and warnings.  You can also check live radar images to see severe weather approaching.

Once a tornado has been confirmed in your area, you should move yourself and your family to the lowest area possible and in the most well built structure.  If you are at home, move to the basement preferably, or to interior rooms such as a bathroom.  Move to an area with no windows and if this is not possible, then position yourself away from the windows because tornadoes can often break windows and cause severe injury from flying broken glass.  Position yourself on the floor and protect your head with your arms.

If you find yourself outside during a tornado, move to the nearest strong structure.  Avoid barns, gymnasiums or large open structures because the roofs are more likely to collapse.  If you are unable to make it to a structure, find the lowest area possible (ditch), position yourself flat on the ground and protect your head with your arms.  Be mindful of the possibility of floods due to heavy rain (if you are in a ditch).  Do not try and outrun the tornado in your car, and do not take shelter in a car, as tornados can overturn the car or lift it off the ground. 

Fujita Scale 

The Fujita Scale is used to rate the severity of damaging wind gusts and  tornadoes as a measure of the damage they cause.  It classifies these destrictive winds using the following scale.

F0 - winds up to 115 km/hr; Light damage.
Peels surface off some roofs; some damage to gutters or siding; branches broken off trees; shallow-rooted trees pushed over.  

F1 - winds between 120 and 170 km/h; Moderate damage.
Roofs severely stripped; mobile homes overturned or badly damaged; loss of exterior doors; windows and other glass broken.    

F2 - winds between 180 and 240 km/hr; Considerable damage.
Roofs torn off well-constructed houses; foundations of frame homes shifted; mobile homes completely destroyed; large trees snapped or uprooted; light-object missiles generated; cars lifted off ground.  

F3 - winds between 250 to 320 km/hr; Severe damage.
Entire stories of well-constructed houses destroyed; severe damage to large buildings such as shopping malls; trains overturned; trees debarked; heavy cars lifted off the ground and thrown; structures with weak foundations blown away some distance.  

F4 - winds between 330 and 415 km/hr; Devastating damage.
Well-constructed houses and whole frame houses completely leveled; cars thrown and small missiles generated.  

F5 - winds >420 km/hr; Incredible damage.
Strong frame houses leveled off foundations and swept away; automobile-sized missiles fly through the air in excess of 100 m (109 yd); high-rise buildings have significant structural deformation; incredible phenomena will occur. Tornadoes of this magnitude account for less than one per cent of all tornadoes and have never been officially recorded in Canada.

In Ontario about 90 per cent of the tornadoes fall within the F0 and F1 ratings.

After a Tornado 

After a tornado, move very carefully around your home and the surrounding area as there may be damaged structures, downed power lines, fallen trees and debris.  If you smell gas then leave your home immediately.  Check your home for electrical damage and any signs that may indicate the risk of fire.  Also, be wary of tap water in case sewer or water lines have been damaged.  Remember to inspect the damage to your home and its contents and take pictures if at all possible prior to cleaning or repairing your home.  This will provide accurate indication of the damage to your home for insurance purposes.  If you are unable to pay for the necessary repairs to your home you may seek government financial assistance from the Ministry of Municipal Affairs and Housing's "Ontario Disaster Relief Assistance Program".  Also, if you are suffering emotional effects from the experience, Victims Services of Waterloo Region are available to discuss your concerns.

Learn More about Tornadoes and Severe Summer Weather

Watch the birth of a tornado:

Monitor live radar images for Southwestern Ontario:

View recent lightening Strikes in Ontario:

Watches and Warnings- Definitions:

Current Watches and Warnings

Keeping safe in severe summer weather:

Severe Summer Weather and Camping in Ontario

Tornado Fact Sheet:

Test your Tornado knowledge












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