 The derecho that crossed into Ontario and Quebec was the result of a warm and very humid airmass that had plagued much of Central and Eastern Canada and the vast majority of the continental U.S.Temperatures in Toronto, Ottawa, Windsor and Montreal were approaching 35 °C (95 °F) for a few days including the day of the storms, while Heat Index values were exceeding well over 40 °C (104 °F) in some areas.Temperatures hit 100 °F (38 °C) at LaGuardia Airport in New York on July 18 and in St. Louis, Missouri on July 19, before storms pummeled those metropolitan areas.