Tornadic Near-Storm Environments of the Southeastern United States

The Southeastern United States has recently become a region of primary interest in the study of tornadic near-storm environments; this interest is rooted in part in the abundance of tornado events with environments that differ from the "textbook" Great Plains tornadic environment, with a shift of the distribution of events toward higher vector shear magnitude and lower convective available potential energy (CAPE). Here, we examine 3,670 tornado events and 14,973 tornado warnings occurring in the Southeastern U.S. between 2003 and 2015, [over half of which have both mixed-layer CAPE (MLCAPE) values less than 1,000 J/kg and 0-6-km vector shear magnitude (SHR6) values greater than 35 kt] in the context of the characteristics of their near-storm environments. We also build a climatology of these events and warnings in terms of time and date of occurrence, intensity, casualties, and warning skill, and make use of self-organizing maps (SOMs) to expand our understanding to encompass the spatial patterns of these environments.

We find that:

• Southeastern tornadoes in general are characterized by a shift in the distribution toward higher SHR6 and 0-1-km storm-relative helicity (SRH1), with MLCAPE and mixed-layer lifting condensation level (MLLCL) heights shifting toward lower values compared to tornadoes in the continental U.S. (CONUS) as a whole. The Significant Tornado Parameter (STP), which incorporates all four of these atmospheric parameters, generally peaks in the Southeast for tornadoes occurring in the springtime and overnight.
• (E)F2+ tornadoes are relatively common in the Southeast, with (E)F1 tornadoes more common than (E)F0 tornadoes for the high-shear, low-CAPE (HSLC) subset. Comparing with a 1950-1976 dataset, the fraction of tornadoes rated (E)F0-1 has increased by about 0.15 for all times of day.
• In the Southeast, probability of detection (POD; the percentage of all tornadoes that had positive lead time) is generally higher/better and false alarm ratio (FAR; the percentage of all tornado warnings that did not encompass any tornado reports) is generally higher/worse than for the CONUS. These numbers remain consistent even when only considering the HSLC subset of Southeastern tornadoes, suggesting that warning skill (as measured by POD and FAR) for the HSLC environments is comparable to that for all Southeastern tornadoes.
• The Southeast features nearly double the percentage of deadly tornadoes (i.e., the percentage of all tornadoes for which at least one death was reported) compared to the CONUS, with peaks in deadly tornado percentages just after sunset in the winter and just before sunset in the spring. Over half of all (E)F4+ tornadoes in the Southeast result in deaths, but these tornadoes have a perfect (100%) POD, suggesting that the deadliest storms are indeed being successfully warned; it is likely that these relatively high rates of death could be improved with FAR reductions, longer warning lead times, or improved communication of said warnings.

The figure depicts mean values of (a) MLCAPE, (b) SHR6, (c) MLLCL, (d) SRH1, and (e) STP for all Southeast tornado events between 2003 and 2015. The (E)F-scale ranges, from left to right, from (E)F0 through (E)F5. Seasons from left to right consist of spring (MAM), summer (JJA), fall (SON), and winter (DJF). Times of day are split into three categories: day (from local sunrise until two hours before local sunset), early evening transition (from two hours before until two hours after local sunset), and night (from two hours after local sunset until local sunrise).