The southwest Ohio mini-supercell tornado outbreak of 11 July 2006. Part I: Mesoscale and radar analysis

During the evening of 11 July 2006, an outbreak of 12 tornadoes occurred over southwest Ohio, localized to the forecast area of the National Weather Service in Wilmington. This event was of particular interest for two reasons. First, because of the relatively low instability and weak deep-layer shear, severe thunderstorms were not forecast by the local office or by the Storm Prediction Center. Secondly, the thunderstorms' organizational mode took the form of minisupercells, with reflectivity and velocity signatures that fell well below the typical warning criteria for warm-season thunderstorms in the Midwest. For this reason, the first two tornado touchdowns went unwarned by local forecasters.

The environment over southwest Ohio that evening contrasted greatly from a 500-mb closed low minisupercell setting, but displayed striking similarities to one typical to a tornado outbreak associated with a landfalling tropical cyclone. Specifically, the environment showed strong low-level wind shear, a local wind maximum near 700-hPa, low surface dewpoint depressions, high relative humidity throughout the troposphere, and weak lapse rates and instability. In addition, radar signatures were comparable to tornadic minisupercells observed in landfalling tropical cyclones, with relatively weak radar and velocity signatures and low echo tops. This study uses the 11 July 2006 event to stress that forecasters not rely on pattern recognition to anticipate minisupercells, but that they understand which specific ingredients favor this type of storm. It is also recommended that minisupercell warning criteria be established at National Weather Service offices to prevent missed tornado warnings in the future.