Derecho and MCS Interactions During BAMEX

During the first week of July 2003, three mesoscale convective systems (MCSs) that formed in the upper Midwest met derecho criteria (2-3 July, 4-5 July and 5-6 July). These systems formed in a cyclonic environment in the lee of the Rockies during a period of anomalously fast westerly flow aloft. Derecho development was further assisted by interactions between transient upper-level shortwave disturbances, pre-existing low-level boundaries, a favorable configuration of lower and upper-level jets, steep mid-level lapse rates, strong surface heating, robust deep layer shear, and large values of surface-based CAPE.

Each derecho alone represented a significant convective event. However, even more noteworthy was the manner in which the three derechos and two less severe MCSs interacted with, and had a considerable impact on, each other. The 2-3 July derecho dissipated in Illinois after traversing the upper Midwest. It left behind a filament of low-level vorticity stretching into Colorado. The southern portion of the 4-5 July derecho formed near this vorticity filament. As this second derecho moved eastward, a quasi-stationary surface boundary formed in Iowa/Illinois along the southern edge of its cold pool. This derecho weakened as it progressed eastward of the low-level jet and moisture axis in Ohio. A less severe MCS subsequently formed and collapsed in the derecho's cold pool over Iowa. Gravity waves and an outflow boundary traveled eastward following the MCS's demise, eventually moving into proximity of the quasi-stationary boundary near Lake Michigan, where a second non-derecho MCS formed. Concurrently, the initial convection of the 5-6 July derecho formed in southeast Montana, and became robust after moving over another low-level vorticity filament left behind by the second derecho near the dissipating quasi-stationary boundary. The cold outflow in the wake of the third derecho created a new cold pool and reinvigorated the aforementioned quasi-stationary boundary. This derecho moved eastward, merging with the gravity wave-induced MCS. The combined system weakened while crossing the Great Lakes, outrunning its moisture and jet support.

Fortuitously, these convective systems occurred during BAMEX, which made for the availability of enhanced observational datasets. A detailed synoptic and mesoscale analysis of each derecho event, along with derecho/MCS interactions during the 2-6 July 2003 period, will be presented. While the favorable synoptic pattern for repeated derecho development is of interest, the focus will be on storm-environmental interactions on the mesoscale that occurred in the formation and progression of the derechos and MCSs.