Previous research has shown that there is a relatively high frequency of tornadoes in the overnight-to-early-morning hours during the cool season in the southeastern US, particularly in areas close to the Gulf of Mexico. For example, most strong and violent tornadoes (F2 or greater) in Florida occur during the cool season, and are associated with extratropical cyclones. Previous research has also documented the importance of the return flow of warm, moist tropical air across the Gulf region (e.g., after the passage of cold fronts through the Gulf) in the development of potential severe weather scenarios along the Gulf coast. The warm Loop Current in the Gulf also can increase fluxes of heat and moisture into this return flow air, which can lead to rapid air mass destabilization. It has also been shown, however, that forecasting the trajectories of return flow air is difficult, and that operational numerical prediction models are not able to accurately forecast the modification of the boundary layer (partially due to lack of data over the Gulf), which can be important in determining the severe weather potential over the Southeast.

Results of the climatological portion of this study indicate that regions of the Southeast in the immediate vicinity of the Gulf of Mexico, such as coastal Mississippi and Alabama and the Florida peninsula), exhibit much less dependence on diurnal heating for cool season tornado occurrence, with some areas even having a nocturnal maximum. Mountainous regions such as the Ozarks and the Appalachians exhibit a relatively high-amplitude diurnal signal of tornado occurrence in the cool season, with a strong peak of tornado occurrence associated with the maximum diurnal heating.

This presentation will report on the results of the climatological study, as well as continuing work on composites which are being created of cool season tornado events in the Southeast. Representative case studies will be used to illustrate issues relevant to this phenomenon which are being addressed during the course of this research.