The Multi-Scale Dynamical Processes Leading to the 29 June 2012 "Super Derecho"

This study traces backwards in time the progression of atmospheric adjustments that resulted in the 29-30 June 2012 “super” derecho. This impressive bow echo mesoscale convective system (MCS) left a swath of damage nearly 700 miles long affecting parts of nine states and Washington D.C., resulting in at least 22 directly storm-related deaths, and causing roughly $2.9 billion in damage. The 29 June derecho grew upscale in time very rapidly, beginning in northern Illinois and persisting for numerous hours before eventually propagating off of the Delaware, Maryland, and Virginia (DelMarVa) coasts. The processes responsible for organizing the extraordinary environment that supported the intense expansion and upscale growth of the 29 June storm can be traced back nearly two weeks’ time to the deserts of the southwestern U.S. and northwestern Mexico. The environment established early on 29 June involved the juxtaposition of a broad region of negative absolute vorticity along and slightly equatorward of the eventual derecho track. Superimposed upon the region of negative absolute vorticity was a mid-tropospheric cold pool which overlaid an anomalously hot, deep, well-mixed planetary boundary layer (PBL). The triggering of the incipient MCS on 29 June over northern Illinois then led to a track of expansive damage that largely follows the anticyclonic zone of reduced inertial stability from Indiana to the DelMarVa coastline.

The development of the unseasonably deep, hot, and dry PBL is traced back to its origins along the U.S. southwest Pacific coast and followed through its nearly two week journey to the Midwest/Mid-Atlantic states late June 2012. This remarkably hot/dry PBL and its perfect phasing with the broad expanse of negative absolute vorticity and mid-tropospheric cold pool is the result of four key synoptic and meso-alpha (α) scale adjustments prior to the subsequent derecho. We will present a HYSPLIT trajectory analysis of the 13 day period leading up to the 29-30 June super derecho, in addition to very high resolution (down to 1 km) Weather Research and Forecasting (WRF) model simulations of the developing and propagating derecho. We will also provide a validation of model-simulated wind gusts and reflectivity against remotely-sensed data, surface and rawinsonde observations, and severe weather reports from the event.