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 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.
According to Environment and Climate Change Canada (2018), an F2 tornado touched down at 6:32 pm in Maple, ON. The tornado travelled for 2.71 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: https://www.wpc.ncep.noaa.gov/archives/web_pages/sfc/sfc_archive.php
Environment and Climate Change Canada Data. (2018). Canadian National Tornado Database: Verified Events (1980-2009) – Public. Retrieved from: http://donnees.ec.gc.ca/data/weather/products/canadian-national-tornado-database-verified-events-1980-2009-public/