Gravenhurst, ON F1 Tornado of August 20, 2009

Parry Sound - Muskoka

The F1 tornado at Gravenhurst, Ontario. Travelling for 15km along a path that was 900m wide, the tornado caused a swath of tree damage that was most extensive in town. There, power poles and large trees were snapped and uprooted, crashing onto and damaging houses and vehicles. The tornado occurred at about 6:30pm.

This was one of nineteen tornadoes that struck Southern and Central Ontario on August 20. Figure 1 depicts a line of severe thunderstorms entering southwestern Ontario from Michigan. These thunderstorms were responsible for the tornado outbreak on this day.

Figure 1. Doppler radar imagery at 3:38 pm EDT on August 20, 2009 (Source: Meteologix)

Figure 2 depicts the surface observations at 5:00 pm EDT, which shows a cold front moving through the Great Lakes and a warm front in central Ontario. The warm sector (southern Ontario), brought southerly winds moving across the open waters of Lake Ontario and Lake Erie. The advancing cold front and possible interactions with lake-breeze boundaries developed pre-frontal storms across southwestern Ontario, which moved east and brought one of the most significant tornado outbreak in Ontario’s history.

Figure 2. Surface analysis at 21Z on August 20, 2009 showing mean sea-level pressure (MSLP) contours, surface observations, fronts and pressure centres (WPC, 2017)

According to Environment and Climate Change Canada (2018), an F1 tornado touched down at 6:30 pm in Gravenhurst, ON. The tornado travelled for 14.3 km with a maximum width of 900 metres. The tornado caused no fatalities, injuries or property damage.


NWS Weather Prediction Center Surface Analysis Archive. (2017). Surface analysis 21Z Thu Aug 20 2009. Retrieved from:

Environment and Climate Change Canada Data. (2018). Canadian National Tornado Database: Verified Events (1980-2009) – Public. Retrieved from: