Redstone Lake, ON F0 Tornado of August 20, 2009


The F0 tornado at Redstone Lake, Ontario. This tornado touched down at 8:00 pm and travelled for 1.5 km across Redstone Lake. Trees were knocked down onto cottages and vehicles, and a few cottages received significant structural damage. One woman was injured when she was cut by flying glass.

The F0 rating of this tornado by Environment Canada is questionable given the extent of the damage around the lake. Tree damage alone appears to have easily been at the level of F1, with dozens of large and healthy trees snapped and shorn off at their tops. In addition, one well-built cottage had almost all of its roof torn off. Indeed, an F1 rating may have been more appropriate for this tornado. 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 F0 tornado touched down at 8:00 pm near Redstone Lake, ON. The tornado travelled for 1.42 km, but its width was not documented by ECCC. 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: