The third largest tornado outbreak on record occurred on 24 October 2001 over the Northern Indiana National Weather Service County Warning Area (CWA) which covers northern Indiana, southwest lower Michigan and northwest Ohio. Ten distinct tornadoes touched down, including two rated F3 on the Fujita scale. Only the Super Outbreak of April 1974 and the Palm Sunday Outbreak of April 1965 rate higher in the number of tornadoes for this area.

Although the pattern was more reminiscent of early spring and a rare occurrence for late October, nearly ideal atmospheric conditions were in place for the development of severe weather. The synoptic pattern on 1200 UTC 24 October 2001 showed a surface low over northern Minnesota with a 500mb closed low over North Dakota. An intense cold front extended south from the low across Wisconsin, western Illinois and central Missouri down to a second low over the northeast corner of Oklahoma. 850mb plot indicated a 45kt low level jet over southwest Missouri. Analysis of the upper air sounding at Lincoln Illinois (ILX) revealed a moderately unstable and weakly capped atmosphere. Modifying the sounding for expected afternoon temperatures yielded up to 2000 j/kg of CAPE and a Lifted Index of -9. By 0000 UTC 25 October 2001, the 500mb low had deepened and become negatively tilted as it moved into eastern Minnesota. This resulted in height falls in excess of 100m over the CWA and the nose of a 100+kt jet pushing into the region. This further de-stabilized the atmosphere over the area during the afternoon hours as the surface cold front surged into area.

By 1900 UTC 24 October 2001, a squall line had formed along the cold front from northeast Illinois down through southwest Missouri. Analysis of surface observations showed a mesoscale low had formed along the cold front near Champaign Illinois, which intensified as it moved into western Indiana. This caused an increase and backing of the surface winds near the squall line which significantly increased the low level shear and storm relative helicity and provided a favorable environment for tornado development within the squall line. As the northern end of the squall line moved into northwest Indiana, a Line Echo Wave Pattern (LEWP) formed consisting of two strong bow echoes with strong cyclonic shear just north of the apex. The resulting tornadogenesis occurred just northwest of the apex of the bow echoes in the southeast portion of the comma head. One additional tornado occurred later over northwest Ohio as the LEWP merged with a minisupercell which had formed out ahead of the squall line. This paper will present a detailed analysis of radar data and near-storm environment data to explain the structure of the storms that produced the tornadoes and to determine the best method for identifying potential tornadic storms that are embedded within a squall line.