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

Weakly forced thunderstorms (WFTs), convection forming in the absence of a synoptic forcing mechanism and its associated shear regime, are the dominant convective mode during the warm season in the southeast United States. This study uses 15 yr (2001–15) of warm-season (May–September) composite reflectivity images from 30 WSR-88D sites in the southeastern United States to detect WFTs and pulse thunderstorms, defined as WFTs associated with a severe weather event. Thunderstorms were identified as regions of contiguous reflectivities greater than or equal to 40 dB Z using connected neighborhoods labeling. Ward’s clustering was then performed upon the duration, size, strength, initiation time, and solidity of the approximately 1 900 000 thunderstorms. Of the 10 clusters of morphologically similar storms, five groups, containing 885 496 thunderstorms, were designated as WFTs. In line with previous work, WFT development mirrors landscape features, such as the Appalachian Mountains and Mississippi Delta. However, the large sample size also reveals more subtle nuances to the spatial distribution, such as decreases over river valleys and increases along the Atlantic fall line. The most active pulse thunderstorm region, the Blue Ridge Mountains, was displaced from the overall WFT maximum: the Florida Peninsula and Gulf Coast. Most pulse thunderstorms were associated with larger moisture values, particularly in the midlevels, which supported larger and longer-lasting WFT complexes. Synoptically, two distinct modes of variability yielded WFT-favorable environments: the intrusion of the Bermuda high from the east and the expansion of high pressure over the southern Great Plains from the west.