Storm Scale Meteorological Processes in the 30 June 2014 Double Derecho Event

An unusual evolution of a pair of quasi-linear convective systems was observed during the evening hours of 30 June 2014. The first QLCS manifested as a progressive derecho, with numerous high-end wind damage reports across Iowa and northern Illinois. As the first QLCS began to weaken, a second QLCS formed over central and eastern Iowa. This one also exhibited derecho characteristics as it propagated quickly across northern Illinois and northern Indiana before weakening in northwestern Ohio.

While the development of a secondary QLCS certainly is not unusual, the severity of the second QLCS in this case was uncommon. The second QLCS was particularly damaging, with numerous instances of 35-45 m/s (80-100 MPH) estimated straight-line wind gusts and at least 29 tornadoes, all rated EF0-EF1. The passage of the second QLCS through a relatively dense surface observational network and good radar coverage allowed for several noteworthy observations. These observations include:

1) The bore-driven nature of the second QLCS for its entire lifecycle, with numerous clear bore-apparent surface observation traces and system forward motion substantially greater than that supported by its associated buoyancy deficit or mean tropospheric flow;

2) Apparent wave interactions with the QLCS, including one particular interaction that was spatially and temporally associated with mesovortex genesis and quickly followed by formation of an EF1 tornado;

3) A remarkable detection of a tornado within 10 km of the Chicago-Romeoville NEXRAD radar (KLOT), which detected a maximum wind speed of approximately 66 m/s (128 kt or 147 MPH) at an elevation of approximately 230-235 m (755-775 ft.) AGL; and

4) Growth and intensification of a mesovortex upon interacting with a remnant thermal boundary from the first derecho-producing QLCS. This mesovortex veered to propagate along the boundary, eventually split into two subvortices, and produced a remarkable flurry of at least 14 confirmed tornadoes, along with areas of widespread wind damage estimated to be caused by winds of at least 45-50 m/s (100-110 MPH).

This presentation provides a detailed review of these observations and examines the role of mesoscale and especially storm-scale interactions that contributed to the severity of the second QLCS, and particularly its tornadic nature. It also discusses how these observations raise additional questions to be addressed through further research into this case and others.