An Overview of the May 10, 2003 Tornado Outbreak

The period from May 4-10, 2003 was one of the most active severe weather periods documented in United States history. A record 393 tornadoes occurred across portions of the central and eastern U.S. The culminating event was a tornado outbreak which affected the middle Mississippi Valley region on May 10th. Nine tornadoes were documented in northeast Missouri and west central Illinois within the county warning area for the National Weather Service Office in St. Louis. Of the nine tornadoes, four of these were classified as strong (F2 or F3) and long tracked with path lengths °Ý 10 miles. The Canton-Lima tornado had the longest documented track at 89 miles and a peak width of 300 yards. Despite damage estimated at 5-7 million dollars, there were no fatalities and only ten minor injuries. This great fortune was due to the fact that the tornadoes occurred over primarily rural areas and the tornado warnings had an average lead time of 23 minutes.

The tornadoes across northeast Missouri and west central Illinois were produced by three discrete cyclic supercells. The supercells formed over western Missouri along a pronounced dryline, with supercellular convective modes noted 40-50 minutes after the initial convective cells. Tornado production for each of the three supercells did not transpire until 1-2 hours after supercell characteristics were first observed. Each supercell produced multiple tornadoes, each successively stronger. While the large scale and mesoscale environment that day seemed supportive of tornadic supercells over a large portion of the Mississippi Valley, the majority of tornadoes were confined to the north of a retreating outflow boundary, where winds were locally backed and the lifting condensation level (LCL) heights were lower. Notable changes were observed in storm structure with each of the three highlighted supercells upon crossing the outflow boundary and prior to producing there strongest tornadoes. The three supercell thunderstorms morphed from classic "flying eagle" structures with low level appendages and hook echoes, to high precipitation structures with smaller overall vertical and horizontal dimensions. Storm splits and cells mergers were also noted, as well as an overall decrease in the highest radar reflectivity levels.