The convective development of the two derechos was intertwined with two less severe MCSs. The earlier 4-5 July derecho resulted in the formation of a quasi-stationary surface boundary in Iowa and Illinois along the southern edge of its significant cold pool. Radar imagery showed this derecho remained robust as it crossed Lake Michigan despite cool water temperatures, weakening only as it progressed further eastward of favorable jet support and low-level moisture. A less severe MCS subsequently collapsed in the derecho's cold pool. Gravity waves and an outflow boundary moved eastward following the MCS's convective demise and interacted with the quasi-stationary boundary, inducing a second MCS near the southern tip of Lake Michigan. At the same time, the 5-6 July derecho began progressing eastward following a similar path across Iowa as the first derecho. The cold outflow in the wake of this second derecho strengthened the cold pool and quasi-stationary boundary left behind by the first derecho. This derecho moved eastward, merging with the gravity wave-induced MCS. The combined system weakened as it crossed the Great Lakes, moving eastward of the most plentiful moisture and jet support. Fortuitously, these derechos along with the other weaker MCSs, occurred during the Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX), which made for the availability of enhanced observational datasets.
A detailed assessment of the similarities and differences between the derecho events will be presented using model forecasts, satellite imagery, radar composites, trajectory analyses and surface observations. Additionally, a synoptic and mesoscale analysis will be presented of derecho/MCS interaction on 4-6 July 2003. Supplementary observations from P-3 aircraft as well as dropsondes released during BAMEX augment the other data. These all contribute to an understanding of the synoptic and mesoscale environments present in the upper Midwest during that period, permitting the derechos and other MCSs to be examined in great detail.