Mark Ratzer, Andy Boxell, and Eric Lenning
National Weather Service, Chicago IL
On 19 August 2009, a mature Mesoscale Convective System moved into the forecast area of the NWS office in Chicago after producing considerable wind damage across portions of central and northern Illinois. As this MCS continued east and began to dissipate somewhat, additional thunderstorms redeveloped behind the MCS and quickly became supercellular and surface-based, despite the increasing stability of the airmass. These thunderstorms ended up producing an EF1 tornado in Illinois and an EF2 tornado in the town of Chesterton, Indiana. There was an additional EF1 tornado in Illinois associated with the comma head of the MCS itself. The first part of this case study explores the relevant synoptic and mesoscale-α environments before and during the severe weather outbreak, including both observational data and numerical model forecasts. It then explores the mesoscale-β and storm-scale environments during the outbreak, with a particular focus on storm-scale changes and interactions before and during tornadogenesis.
A seasonably robust short wave trough and associated surface low were moving across the Midwest on the morning of August 19, 2009. As a warm front surged north across central Illinois and Indiana, convection developed in the remnants of a mesoscale convective vortex moving out of Kansas and Missouri. Analysis of rawinsonde, VAD winds from the WSR-88D network, AMDAR aircraft soundings, and wind profiler data suggests that supercellular convection likely developed in response to increasing shear from a compact but strong shortwave trough and associated jet streak moving across the area coincident with the MCV. Analysis of the near-storm environment showed increasingly stable air across the Chicago WFO CWA, with surface-based CAPE values ranging from around 1000 J/K across the far southern CWA to less than 100 J/K across the northern CWA. Despite the increasing stability, convection remained organized as it moved across the CWA, likely due to unseasonably strong shear values across the area.
Two EF-1 tornadoes were confirmed with the initial wave of convection associated with the MCS. The first occurred within the well-developed bookend vortex, likely the result of extremely high values of environmental vorticity being stretched in the vertical by a strong convective updraft. The second EF-1 tornado, which occurred behind the main MCS, was possibly influenced by gravity wave interaction as suggested by the presence of several non-severe cells exhibiting deviant storm motion ahead of the main MCS. The final tornado, rated EF-2, occurred as a newly developed storm quickly became supercelluar and surface-based across northwestern Indiana. Radar and mesoscale analysis indicates that tornadogenesis occurred as this storm moved into an area of high surface moisture and backed surface winds and simultaneously interacted with a northward moving convective cell which also suggested the presence of gravity wave activity.
This study concludes with a brief look at resulting storm damage from the three tornadic storms.