Characteristics of circulations associated with the 29 June 1998 derecho in eastern Iowa

On 29 June 1998, supercell thunderstorms initiated early in the day in eastern Nebraska along a stationary surface frontal boundary, in an area of lower-tropospheric warm advection, and in the entrance region of an upper-tropospheric jet streak. The supercells moved into northwest Iowa and produced severe winds (> 25 ms-1), hail, and at least one tornado. The storms progressed through central Iowa and transitioned into a squall line as they moved into an environment characterized by moderate shear (0-6 km shear of 17 ms-1) and extreme CAPE (>5000 Jkg-1) . Characteristics of the squall line and the synoptic environment fit the classification of a progressive derecho as defined by Johns and Hirt (1987) and Johns et al. (1990).

The derecho produced damaging winds in central Iowa and intensified further in southeast Iowa. In addition to widespread wind damage, one fatality and numerous injuries were reported in southeast Iowa alone. Numerous circulations apparent in WSR-88D storm-relative velocity data were associated with paths of enhanced damage. One of these circulations produced winds measured as high as 55 ms-1. The storm tracked generally along a line from Des Moines to Iowa City to Burlington in Iowa, and continued past Macomb in western Illinois into central Illinois.

Basic information, including an assessment of ascending and descending development mode, shear strength, vortex life times, relationship to reflectivity structures, etc., on each of these circulations will be presented and compared relative to the resultant damage. Although a detailed post-storm survey was not conducted by the National Weather Service, an assessment of tornadic vs. non-tornadic vortices will be presented based on available data which includes extensive video documentation and trained spotter reports. Implications for warning decision-making will be discussed.