Safety Reminder: Summer Can Bring Sudden Thunderstorms

June 13, 2023, Toronto, ON — Toronto and Region Conservation Authority (TRCA) reminds residents to be aware of flooding risk due to thunderstorms.

Thunderstorms can develop suddenly during the warmer months and are often accompanied by strong winds, hail, lightning, and heavy rain.

Flooding from a river system occurs when water in rivers and streams rise over their banks and spill into the surrounding land.

The rivers within TRCA’s jurisdiction are considered “flashy” due to urbanization, which means that large amounts of precipitation over a short period of time, often associated with thunderstorms, can cause water levels in river systems to rise with little warning – even in as little as a few minutes.

flooding of the Humber River in Etobicoke following the July 2013 storm event
Flooding of the Humber River in Etobicoke following the July 8, 2013 storm event.

Significant rainfall amounts are sometimes associated with thunderstorms or cells within a tropical system, or a hurricane remnant. The Atlantic Ocean hurricane season typically occurs every year between June 1 and November 30.

Extreme rainfall amounts from tropical or hurricane remnant systems have been known to travel to areas of Southern Ontario – mostly notably Hurricane Hazel in 1954.

Residents should be aware that flooding can occur very quickly with passing thunderstorms.

Flooding Safety Tips for Residents this Summer

  • Stay away from riverbanks and avoid recreational activities in watercourses (such as kayaking, fishing, or swimming) both during and immediately after storm events – especially thunderstorms.
  • Do not drive through, stand, or walk in any moving water. Avoid low-lying areas such as road underpasses and walkways.
do not attempt to drive through moving water
Do not attempt to drive through moving water during a flood event.
  • Stay informed about weather hazards such as thunderstorms through local radio, television, mobile, or other public alerting systems.
  • If you are instructed by emergency officials to evacuate due to flooding, do so immediately.
  • If you are in danger, call 911 immediately.

Facts about TRCA’s Role in Flood Monitoring

  • TRCA continuously monitors the weather and watershed conditions in its jurisdiction for potential flooding, through the Flood Forecasting and Warning Program.
  • If significant rainfall is forecasted for TRCA’s jurisdiction, a flood risk assessment is completed by an on-call Flood Duty Officer (FDO), as part of the daily operations of the Flood Forecasting and Warning Program.
  • TRCA may issue a flood message when there is certainty that a storm event with heavy rainfall could cause river flooding. TRCA issues flood messages to designated stakeholders, and to members of the public who self-subscribe.
flood event in City of Vaughan
A flooded trail and pedestrian bridge at Glen Shields Park in the City of Vaughan, July 28, 2014.
  • During severe storm events, FDOs will monitor the real-time weather and watershed conditions in TRCA’s jurisdiction, using tools such as radar and weather forecast models, as well as TRCA’s Flood Monitoring Gauging Network.
  • Although not within the delegated responsibilities of Conservation Authorities, urban flooding, or flooding in urban areas caused by water exceeding municipal drainage and stormwater system capacities, may also occur as a result of a thunderstorm that brings heavy rains in a short amount of time.

Previous Flooding Events

  • July 8, 2020: A sudden downpour associated with thunderstorm activity caused flooding in several parts of the City of Toronto. Within a half-hour, approximately 65 mm of rainfall poured over the city’s west end, causing flooding in low-lying areas along Black Creek and the Humber River.
  • July 8, 2013: Parts of the Greater Toronto Area were hit by an extreme rainfall event that caused widespread power outages and disrupted evening traffic, stranding many commuters and flooding a GO Train near the Don River. The maximum total rainfall recorded for this event was 138 mm, near Toronto Pearson International Airport. Many areas along Etobicoke Creek, Mimico Creek, the Humber River, and Don River were impacted by flooding due to the storm.
aerial photo showing aftermath of July 2013 storm event
This aerial image shows the flooding of the Lower Humber River in the aftermath of the July 8, 2013 storm event.
  • August 19, 2005: A thunderstorm caused extensive flooding when Toronto and surrounding areas received as much as 150 mm of rain over a few hours. In the Black Creek sub-watershed, where the creek flows under Finch Avenue West, the flood washed out a culvert, collapsing the road.

Learn more about summer weather hazards and safety tips HERE.

For more information on river flooding risk due to thunderstorms, and who to call when you see flooding, visit