An examination of three Derecho events during the first week of July 2003 concurrent with BAMEX

The first week of July 2003 saw ten coherent mesoscale convective systems (MCSs) propagate across the upper Midwest. Three of these met the criteria set forth for a derecho and are of interest here. Pre-existing baroclinic zones were the focus for moisture pooling while warm advection and the associated isentropic upglide over the baroclinic zones allowed for continued moisture infusion. Steep mid-level lapse rates, along with daily diabatic heating, allowed for destabilization of high CAPE atmospheres. Shortwave disturbances (potential vorticity anomalies) helped to strengthen the upper-level flow, which aided in the generation of the derechos once organized MCSs developed. Deep layer shear in excess of 40 knots favored derecho evolution. The intensification of the nocturnal low-level jet further enabled high equivalent potential temperature/CAPE (surface based) air to accelerate poleward where it was intercepted by the eastward traveling MCSs. As a result of a combination of these factors, the upper Midwest saw extensive wind damage during this period associated with the derechos.

Complex interactions occurred on both the synoptic and mesoscale with each event. This allowed each derecho to develop and evolve in a unique fashion as different dynamic and thermodynamic factors coexisted. In addition, other features such as the Great Lakes influenced the bow echoes, leading to intensification in one case and weakening in another. Fortuitously, these three derechos 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 and surface observations. Supplementary observations from P-3 aircraft as well as dropsondes released in the vicinity of the derechos augment the other data. These all contribute to an understanding of the synoptic and mesoscale environments present in the upper Midwest during this week and permit each distinctive derecho to be examined in great detail. Further analysis will be posted as it becomes available at http://www.atmos.albany.edu/student/nmetz/ATL06.html.