Storms in the time of drought—the June 28, 2012 derecho and beyond

The derecho that crossed several Midwestern and mid-Atlantic states in June 29–30, 2012 was one of the most destructive and deadly fast-moving severe thunderstorm complexes. The summer of 2012 was also one of the most severe drought in the central United States. Studying the climate forcing on progressive windstorms is difficult as data only permits reliable tracking for barely a decade. In this talk, a climatology of summer progressive convective storms occurring under weakly forced environments is presented. These storms are often coupled with mid-level short waves embedded in the zonal or northwesterly flow. A distinct type of short waves was found to initiate over the Rocky Mountains (pre-storm) through a combination of dynamical and thermal instabilities. These short waves are not to be mixed with synoptic/baroclinic waves as they do not appear at the upper levels and feature a barotropic structure in the lower-to-middle troposphere. Diagnostic analysis indicated that the diurnal evolutions between the mid-level flows over the Rockies and the Great Plains are nearly out-of-phase due to inertial oscillation. During the nighttime, the westerly flows over the Rockies intensify while flows at the same level over the Great Plains transition to easterly. These two flows then converge over the eastern Rockies forming cyclonic vorticity through vortex stretching. After sunrise, the convergence dissipates and the cyclonic vorticity is redistributed by horizontal vorticity advection, moving it downstream. When coupled with strong low-level jet (LLJ), the short waves can provide enough lift and instability to trigger organized convection, even though the smaller-scale waves and less sustainable moisture in summer prohibits the development of a rainstorm (see schematic diagram). In terms of forecasting, the difficulties of the NAM model in predicting convective rainfall over the northern plains are partly attributed to its deficiency in forecasting the aforementioned mid-level short waves. Forecasted short waves exhibit a slower propagation speed leading to biases in the precipitation. Skill scores of precipitation associated with these short waves are low, but can be improved after removing the short waves' position error that displaces the rainfall pattern.