Evolution of the 30 June 2014 Double Derecho Event in Northern Illinois & Northwest Indiana

In a seven hour period on the evening of 30 June 2014, two separate quasi-linear convective systems (QLCSs) exhibited derecho characteristics as they tracked across northern Illinois. As the first derecho gradually waned and moved northeast, the second derecho intensified behind it and progressed southeast. Both lines of storms resulted in widespread wind damage, gusts of 26-35 m/s (58-80 mph) or higher, sporadic reports of large hail, and areas of flooding. The second derecho also was responsible for 29 documented tornadoes across northern Illinois and northern Indiana. While the late spring and early summer period is favored for QLCS events in the Corn Belt states, the rapid succession of these two organized convective features with the latter being more pronounced made this event especially noteworthy.

This presentation will explore the synoptic and mesoscale processes that contributed to this unusual event. It also will describe how the atmospheric evolution after the first derecho enabled the second one to be such a prolific tornado producer. Key factors include a limited cold pool imprint from the first derecho, strengthening of a surface theta-e boundary, more pronounced deep layer shear, and enhanced storm relative helicity in advance of the second storm complex. The stronger dynamics in the presence of only slightly elevated instability favored storm-scale structures and interactions more conducive for tornado development.