The 29 June 2012 Derecho: Analysis of a 3 km WRF-ARW Forecast and Comparisons to the 8 May 2009 Derecho Event

On the morning of 29 June 2012, a line of convective storms developed in northwest Indiana and quickly evolved into a severe, long-lived bow echo system, producing surface wind gusts of 70 to 90 MPH over the next 12 hours along a nearly continuous swath from northern Illinois, through Indiana,Ohio, West Virginia, Virginia, and Maryland, leading to unprecedented power outages, and resulting in at least 26 deaths. This event was forecast in realtime by several high resolution, convection permitting models, including a 3 km version of the WRF-ARW, initialized at 12 UTC that morning. Beyond reproducing the observed bow-shaped reflectivity pattern and eventual asymmetric MCS structure, perhaps more impressive was the ability of the model to explicitly predict the observed swath of potentially damaging winds, producing widespread surface winds greater than 25 ms-1 (50 kt) and maximum surface winds greater than 40 ms-1 (80 kt).

For this talk, an analysis of this 3 km forecast will be presented, emphasizing the structure and evolution of the simulated system (e.g., cold pool, rear-inflow jet, mesoscale vortices, etc.) as well as characterizing the environmental conditions that lead to its development. Comparisons will then be made to the structural characteristics and environment for the 8 May 2009 Super Derecho event. Of particular interest, while the 8 May 2009 event was characterized by an intense, warm core mesovortex extending from the surface to the upper troposphere, the 29 June 2012 case possessed a more classic asymmetric MCS structure, with a strong cold pool and a northern-end bookend vortex remaining primarily at mid levels. It is noteworthy that this distinction was accurately captured by the high resolution forecasts. Hypotheses as to possible environmental characteristics leading to these structural differences will be presented along with a discussion of the overall predictability of these events, based on ensemble results comparing various physics and initial analysis options.