On Wednesday evening, 15 June 2011, showers and thunderstorms in the Chicago metro area prompted three tornado warnings by the National Weather Service and numerous reports of tornadoes and funnel clouds from law enforcement and the public. Local television media reported damage to the roof of a warehouse in the Calumet City and South Holland areas near Greenwood Road and 158th Street. Aerial survey footage suggested that the debris was deposited in a convergent path, with some of the debris tossed slightly upstream along the thunderstorm track, as would be expected from a tornado.
Radar signatures near the location of the reports showed a very small thunderstorm with extremely subtle rotation. Low level environmental winds also supported the idea that tornadoes could have developed Wednesday evening. Thus it is quite possible that a brief tornado or two did occur. Additional information on these storms will be provided as it becomes available.
Reflectivity image from the Midway Terminal Doppler Radar (TMDW) valid at 6:22 pm CDT on 15 June 2011. The image appears to show a very tiny supercell with a small hook echo on its southwest side.
Storm-Relative Velocity image from the Midway Terminal Doppler Radar (TMDW) valid at 6:22 pm CDT on 15 June 2011. Very weak, subtle rotation is indicated in the same area as the hook echo shown in the reflectivity image above.
The Effective Storm-Relative Helicity (SRH) field from the SPC Mesoanalysis page, valid at 6pm CDT on 15 June 2011, shows an area of enhanced support for rotating thunderstorm updrafts along the southwest shore of Lake Michigan just prior to the time that the tornadoes and funnel clouds were reported. Previous research has shown that larger values of low level SRH (greater than 100 m**2/s**2) suggest an increased threat of tornadoes with supercells. The values shown along the lake on the evening of June 15 were around 200 m**2/s**2.