Tornadic outbreak of April 20th 2004 with low CAPE

On 20th April, 2004 tornadic supercells developed along an advancing warm front in an environment where CAPE was relatively small compared to typical tornadic supercell conditions. Large and long-tracked tornadoes affected Illinois and Indiana with fatalities and damage reported throughout the region. The location of the severe storms was poorly forecasted by meteorologists. Tornadic storms in small CAPE settings with notable shear are associated with the CAPE concentrated at low levels in the atmosphere. In small CAPE tornado cases around 50% or more of the CAPE is located below 500 mb with the maximum buoyancy found at this level. The low-CAPE values for this event were collocated with the available shear. This paper will study why the severe storms developed in the low CAPE / high shear environment in preference to the higher CAPE environment located in Western Illinois and Iowa.