P1.3 Analysis of the 15 May 1998 Iowa/Minnesota derecho

Monday, 11 October 2010
Grand Mesa Ballroom ABC (Hyatt Regency Tech Center)
Wesley D. Terwey, University of South Alabama, Mobile, AL; and R. A. Wade

On 15 May 1998 multiple linear convective systems and isolated supercells traversed Iowa and Minnesota, the most significant of which was a derecho which caused impressive damage across the Minneapolis – St. Paul greater metropolitan area. While this strong derecho and other associated severe weather from this synoptic system produced over $1 billion in damage, little to no research has been done on this system due to it being overshadowed by the Great Lakes Derecho two weeks later on 30-31 May 1998. Multiple surges of convection formed in this situation. The last, along the dry line in Iowa, produced a number of strong tornadic supercells but will be not studied at this time. The first convective surge that produced widespread damaging winds of derecho criteria formed in eastern Nebraska / southwest Iowa around 1600 UTC in a highly sheared environment ahead of a surface low pressure associated with a negative tilt trough and 100+ knot jet max aloft. This first surge died a few hours later, but a surge behind it formed around 1800 UTC. This second surge exhibited an interesting double line structure as it progressed into southern Minnesota at 1930 UTC and a wind shift from southerly to east-southeasterly was noted in southern Minnesota and western Wisconsin after the passage of the weaker, more progressive line, but prior to the passage of the stronger and slower derecho behind it. The derecho-producing linear convective system exhibited a forward-leaning tilt in the upper levels with a pronounced descending rear inflow jet in the mid-to-lower levels which allowed the leading edge of the convective line at the surface to not be outrun by the upper levels of the convective system. Therefore, large hail being advected forward by the upper levels of the storm were able to fall into a region of strong winds at the surface associated with the descending rear inflow, resulting in wind-driven hail across southern Minnesota and the Twin Cities. Multiple distinct mesovortices and downbursts were also evident along the leading-edge of the system, particularly as it moved across the Minneapolis – St. Paul greater metro area. This presentation will introduce the synoptic, mesoscale, and thermodynamic aspects of the pre-storm environment, as well as present a radar examination of the kinematics and structure of this forgotten derecho-producing linear convective system.
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