A Climatology of Weakly Forced and Pulse Thunderstorms in the Southeast United States

Weakly forced thunderstorms are a nearly daily feature of the southeastern U.S. during the warm season. However, despite their frequency, forecasters still face difficulty anticipating which of these storms will ultimately produce severe weather. This study relies on WSR-88D observations to investigate basic information gaps in disorganized convection, including the first-known climatology of weakly forced thunderstorms. Thunderstorm events are identified from composite reflectivity images from 30 NEXRAD sites across the Southeast United States between the 2001 and 2015 warm seasons (May-September). Cells with composite reflectivities greater than or equal to 40 dBZ were grouped into contiguous thunderstorm objects yielding approximately 35 million unique clusters; however, only a small fraction of these objects were judged to correspond to sustained convective cells. A principal component analysis (PCA) was performed on basic spatial, temporal, and intensity measures of each object which yielded three principal components roughly corresponding to the storms’ size/intensity, morphological consistency, and diurnal tendency. Objects residing in the morphologically consistent and diurnally dependent region of the transformed data space, roughly 850,000 storms, were categorized as weakly forced. As expected, these storms are spatially concentrated in areas of complex terrain and coastlines with a general decrease in frequency with more poleward latitudes. However, marked inter-annual anomalies in geographic concentrations in frequency are observed. Similar variations are noted for pulse thunderstorms – the subset of weakly forced thunderstorms associated with severe weather events. Thermodynamic and kinematic environmental parameters and synoptic composites are compared between pulse-thunderstorm and nonsevere weakly-forced-thunderstorm days. The results of this study will help forecasters better detect the subtleties between benign weakly forced thunderstorm environments and potentially severe, yet synoptically quiescent, regimes.