Lakes. These storms sometimes, but not always move into the city causing localized flooding resulting from downpours with high rainfall amounts, intense lightning but less often severe winds knocking down trees and powerlines. Severe weather and records In addition to snowstorms, ice storms, windstorms, heavy rainfall events associated with tropical storms or very severe thunderstorms, tornadoes are rare but do occur, particularly in the northern and western suburbs. That area sits on the edge of "tornado alley" where tornadoes occur with the most frequency per area in Canada. Downtown Toronto, on the other hand, generally avoids tornadic storms resulting from lake breeze inflows which assist in breaking up formation of tornadoes. Tornado warnings have been posted for the city on a few occasions in the early 21st century, however, no touchdowns have been confirmed within city limits since a weak tornado hit Scarborough in 1998. A pair of dangerous F2 tornados did touchdown in neighbouring Vaughan during a tornado outbreak on August 20, 2009. A powerful derecho affected the city on May 21, 2022. Toronto is susceptible to flash flooding due to its various ravines and valleys that can easily flood, as well as generally poor drainage due to its large urban area. Flooding can result from remnants of tropical storms, heavy rainfall combined with snowmelt, or summertime thunderstorms stalled out along lake breeze boundaries. Low-lying areas, such as the lower reaches of the Don River valley, are the most likely to flood. This often leads to the shutdown of the Don Valley Parkway and GO Transit's Richmond Hill line. A second mouth is being built for the Don River to mitigate flooding, creating Villiers Island. The tropical storm remnant of Hurricane Hazel caused 81 deaths in October 1954 due to flooding that swept homes along river banks into Lake Ontario. A sudden downburst during a strong thunderstorm was believed to have played a contributing factor in the Air France Flight 358 crash on August 2, 2005. Just a few weeks later on August 19, part of a tornado outbreak in Southern Ontario, record-breaking intense rainfall, the worst since Hazel, deluged north-central sections of the city within a couple of hours and destroying a section of Finch Avenue West in North York resulting in record insurance claims. Some rain gauges recorded of rain, over in just one hour. Another large rainstorm with intense, record rainfall amounts struck a wide swath