Thunderstorms form quickly when atmospheric conditions are right. With sufficient instability and moisture, a storm can develop in as little as ten minutes and drop golfball-sized hail in fifteen minutes. Here are a few signs that may help you recognize the threat, and could save lives and property.

In Advance:

Thunderstorms require moisture for fuel and heat for energy. Weather, such as, hot muggy days and warm uncomfortable nights indicate that thunderstorms may be forming. These storms can occur in the afternoon, evening, overnight or the next day. Be ready - listen to forecasts and keep an eye on the sky.
Severe weather the day before in the province to your west can indicate a system is approaching that has the potential to produce severe thunderstorms. Environment Canada records show that weather systems often produce severe weather each afternoon for several days in a row as they move across the country.

Before the Storm:

Cloud build-ups begin near or before noon but struggle to reach high altitudes.
South or southeast winds increase in strength and become gusty in the afternoon.
Cloud build-ups turn into thunderstorms in the mid or late afternoon of a hot, humid day. Some of these storms may become severe quickly.

As the Storm Forms:

Bursts of static on AM radio indicate lightning is nearby.
White towering thunderheads form rapidly to the west or southwest.
Thunderstorms stand straight up with well defined cauliflower edges on their sides. Some may have an "overshooting top" - a cauliflower-like bubble of cloud that penetrates the flat top of the storm. These can only be seen from a distance.

Increasing Danger:

Dark clouds with heavy rain and frequent lightning can be seen approaching from the west. At night, frequent lightning is your best clue that threatening weather is coming. These conditions are often associated with strong straight-line winds and large hail. Winds may arrive a considerable distance ahead of the rain.
A long horizontal roll of cloud or a shelf-like cloud can be seen moving toward you from a thunderstorm in the distance. These can be beautiful cloud formations, but are also a sign of strong winds - typically over 100 km/h. You have a few minutes - stow loose objects and prepare to take shelter.
Swirling dust is visible in the distance. Winds are approaching.
The main thunderstorm in the distance is accompanied by a series of growing thunderstorm towers (perhaps five or more) on its south side. These growing towers move into the main storm and become a part of it. This is a sign of a "supercell" thunderstorm - the kind that can produce tornadoes, large hail, winds and heavy rain.

Immediate danger:

Hail begins to fall and reaches "loonie" or golfball size. You are under a severe thunderstorm that may also contain a tornado. Take shelter now - don't stop to protect property.
You awake to rattling windows and an intense lightning display. Collect your family and take shelter - damaging winds may be on the way, or may have already arrived. Heavy rain or hail will follow shortly if it is not already present.
A strong thunderstorm is passing to your north - it seems to have missed you except for a little light rain. Winds are changing direction rapidly. Clouds above you, at several different levels, seem to be moving in different directions - lower ones from the southeast, higher from the west. This is where tornadoes are found. Keep an eye on the clouds, and listen for warnings. When tornadoes form, they are typically found in a rain-free area on the back or southwest side of a large thunderstorm and may be immediately preceded in time by a "clear slot" - a localized opening in the cloud which allows sunshine to reach the ground.
You are watching a thunderstorm and notice that some of the clouds beneath it appear to be rotating - the far side is moving right to left while the near side clouds are moving left to right. This storm could produce a tornado, large hail, or damaging winds.
A funnel cloud can be seen overhead. They are not dangerous as long as they remain aloft. Funnel clouds indicate that conditions may lead to tornadoes. Keep a close eye on them and be prepared to take shelter. Not all funnel clouds touch down.
Lightning can strike over 16 km from a thunderstorm - a "bolt from the blue". When lightning is visible or thunder can be heard, follow the 30-30 rule: If there is less than 30 seconds between lightning and thunder, find appropriate shelter immediately. Wait 30 minutes after the last thunder before resuming your activities.

When danger has passed, please report severe weather to Environment Canada at 1-800-239-0484. Your report may help warn someone else along the thunderstorm track.