Mesoscale Aspects of the 11 March 2006 Severe Weather Outbreak

March 11-13, 2006 was an extremely active period of severe weather across the nation's midsection. Several episodes of severe thunderstorms produced 76 tornadoes, 889 large hail reports, and 145 reports of damaging winds, resulting in 10 fatalities, 183 injuries, and over 80 million dollars in property damage. The severe weather on March 11 however was largely overshadowed by events of March 12-13 when the majority of the significant tornadoes occurred.

Severe weather on March 11th came in two distinct phases. During the afternoon and early evening hours, short-lived supercell thunderstorms produced numerous reports of large hail and several brief tornado touchdowns from central Missouri into west central Illinois. The character of the event changed dramatically during the evening. After a brief lull, the severe weather refocused further south as a series of supercell thunderstorms traversed southern Missouri and southwest Illinois. Only two of these supercells produced tornadoes, however they were cyclic, generating tornado families concurrently, including significant tornadoes. In fact the two storms separated by around 20 miles, both produced large strong tornadoes which simultaneously crossed Interstate 55 in southeast Missouri. The southernmost supercell was particularly long-lived with a lifespan of around 12 hours and origins in southeast Oklahoma. This storm was also the most destructive producing two strong long-tracked tornadoes and two fatalities. Despite its longevity, it is interesting that tornado production was largely confined to the later three hours of the supercell's existence.

This study will examine the evolving mesoscale environment which favored significant tornadoes across southeast Missouri and southwest Illinois during the evening. It will also take a close look at the long-lived supercell; factors contributing to its longevity, as well as the absence of significant tornado occurrence until the later part of its lifecycle. Lastly, radar data from several National Weather Service WSR-88Ds is shown to not only document the supercell structure, but also to highlight differences associated with radar viewing angle.