Geographical Variability in Storm Modes that Produce Significant Tornadoes in the United States

Recently, there has been a resurgence of studies investigating tornado climatologies in the United States, many of which compare tornado occurrence, environmental characteristics, and storm modes in predetermined regions. The primary objective of this study is to determine if there actually are statistically significant differences in storm modes and structures for certain areas or states relative to the mean climatological distribution by analyzing over 900 significant tornadoes (rated EF2 or higher on the Enhanced-Fujita scale) identified in Storm Data between 2007 and 2011 (excluding those associated with tropical cyclones). Each tornadic storm is analyzed using the Gibson-Ridge level-II radar software package (GRLevel2) to group each storm's convective mode into 1) discrete supercells, 2) mesoscale or quasi-linear convective systems (MCS/QLCSs), or 3) other (e.g., multicell clusters). Discrete supercells and MCS/QLCSs are further subdivided into a) non-mergers and b) mergers based on whether they were isolated cells or merged with another cell or line within one hour of tornadogenesis.

Preliminary research analyzing 360 storms between 2007 and 2010 suggests that significant tornadoes produced by QLCS/MCSs were significantly more likely to occur in Mississippi, Alabama, and Georgia relative to other Southeast or Plains states. This study adds in significant tornadoes from all states and an active year in 2011 previously not included to help solidify or nullify the significance of MCS/QLCS storms in the aforementioned states using statistics for tornado days in addition to tornado occurrence. In addition, tornadoes associated with cell mergers that account for approximately 20% of significant tornadoes will be further analyzed to investigate the time frame of merger completion to tornadogenesis. Finally, interannual, seasonal and diurnal variations associated with each storm type and variability in storm structures (CL, HP, LP, or mini for supercells and TS, PS, LS, TS-AS, QS-BB, or other QLCSs for MCSs) will also be investigated along to help forecasters and emergency managers that serve the general public better understand tornadic threats for their respective regions.