Arnstein, ON F1 Tornado of August 20, 2009

Burk's Falls - Bayfield Inlet

The F1 tornado at Arnstein, Ontario. Witnesses observed this tornado form as a waterspout over Seagull Lake on the south side of Arnstein, then watched as it moved onto shore at approximately 5:30pm. Its path over land was short, only 1.3km in length, but damage to forest was extensive. It touched down and tracked along Seagull Lake Road, leaving a trail of downed trees and snapped hydro poles, as well as a shed that was shifted off of its foundation and minor damage to a house. The tornado then moved off into the forest, carving a path along which whole tracts of trees uprooted, snapped and sheared off at the tops.

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 5:30 pm near Arnstein, ON. The tornado travelled for 1.34 km, but the width was not documented by ECCC. The tornado caused no injuries, fatalities 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: