A case study of three severe Tornadic storms in Alberta, Canada

Central Alberta in Canada is highly susceptible to severe thunderstorms which spawn tornadoes. There are about 10 tornadoes reported each summer, most of which are of F0-F1 intensity on the Fujita scale. During the past 20 years there have been three tornadic thunderstorms in central Alberta with intensities of F3 or greater. These events were: the Edmonton tornado of 31 July 1987 (27 deaths and 250 million dollars of property damage), the Holden tornado of 29 July 1993, and the Pine Lake tornado of 14 July 2000 (12 fatalities, more than 130 injuries and over 13 million dollars damage) A synoptic analysis of these three cases was made to examine the validity of the Smith and Yau conceptual model of severe thunderstorm outbreaks in central Alberta. In each case a capping inversion allowed the build up potential instability resulting in large values of convective available potential energy. Also these storms had strong wind shear and large precipitable water. The storms differed in the triggering mechanism to break the cap and also in temperature advection. For the Pine Lake storm there was mid-upper level cooling whereas, for the Edmonton and Holden cases there was low level warming but little or no mid-upper level cooling. The Pine Lake and Holden storms had straight tracks, while the Edmonton storm made an abrupt change in direction and speed. Thus, extrapolation of tornadic thunderstorm movement would not be a viable nowcasting technique in this case.