Two derecho events and MCS interactions during BAMEX

The first week of July 2003 was an active convective period with twelve coherent mesoscale convective systems (MCSs) progressing across the upper Midwest. Three of these MCSs met the derecho criteria and two are of interest here (4-5 July and 5-6 July 2003). Derecho development was assisted by interactions between transient upper-level shortwave disturbances (potential vorticity anomalies) that provided a source of lift, a favorable configuration of lower-level and upper-level jets, steep mid-level lapse rates, strong surface heating, deep layer shear in excess of 40 knots, and large values of surface-based CAPE.

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.