A key difficulty in forecasting the occurrence of the second, more powerful and damaging QLCS was that it followed within 250 km of the initial progressive derecho. Many of the significant damaging wind and tornado reports with the second QLCS occurred north of the remnant gust front from the first QLCS, in an environment characterized by a strong statically-stable surface layer approximately 100-hPa deep. Above this layer, most-unstable CAPE (MUCAPE) values were approximately 2500 J/kg. Storm-relative helicity (SRH) was extraordinarily high north of the remnant thermal boundary, with values in the 0-1 km AGL layer ranging from 700-1000 m^2/s^2. Observations of these parameters were aided by the presence of 3 WSR-88D radars, a 0000 UTC NWS sounding from Davenport, Iowa (KDVN), and Aircraft Meteorological Data Relay (AMDAR) soundings from Southwest Airlines flights arriving and departing from Chicago-Midway International Airport (KMDW). Despite the impressive thermodynamics and kinematics of the environment ahead of the second QLCS, the question remains as to how tornadoes were able to form with such a stable surface layer.
This presentation investigates the potential role that planetary boundary layer (PBL) wave features may have played in allowing for tornadoes to form in the strongly-stable surface layer environment ahead of the second QLCS. Behaviors noted in this assessment include the likely initiation of the second QLCS by a bore propagating through the remnant cold pool of the first QLCS, a transition from pure bore propagation to a hybrid of bore and cold-pool propagation mechanisms as the convection associated with the second QLCS intensifies, the likely formation of solitary waves in parallel to and ahead of the second QLCS, and the development of solitary waves of depression perpendicular to the second QLCS near a pair of tornadoes in the southwest suburbs of Chicago. The WSR-88D radars at KDVN, Chicago/Romeoville, Illinois (KLOT), and Syracuse, Indiana (KIWX), the 0000 UTC KDVN sounding, the KMDW AMDAR soundings, the terminal Doppler weather radar (TDWR) at Chicago/Midway International Airport (TMDW), and several automatic surface observing stations (ASOSs) across Iowa, Illinois, and Indiana are used to detect and identify these features and verify their behavior with theoretical dispersion relations. The role of bore and bore/cold pool hybrid propagation on altering the storm-relative helicity available to the second QLCS and the role of the solitary waves of elevation on near-surface destabilization are discussed in the context of the evolution of the second QLCS and its associated mesovortices and tornadoes.