A comprehensive study of cool season tornadoes in the southeast United States
Severe convection which results in strong (F2 or greater) tornadoes has a cool-season maximum during the nocturnal hours along the Gulf Coast of the southeast United States (US). This study focused on understanding the physical mechanisms responsible for this phenomenon in order to increase the lead time and reduce the false alarm rate. Observational and gridded data, including radiosonde, surface, and pilot balloon data, were used to address questions such as: (1) What is the role of the Gulf of Mexico in destabilizing the planetary boundary layer prior to nocturnal convection? (2) What is the role of the nocturnal low-level jet in creating a favorable wind shear profile in the lowest 1 km? and (3) How does the structure of nocturnal tornado episodes differ from those which occur during the day?
Results indicate that there is a climatological tendency for a 0–1 km shear profile favorable for supercells to occur overnight along the Gulf coast, due to the low-level jet at 1000 m and the tendency for surface winds to back from southerly to southeasterly at night. When a system with strong forcing for ascent (e.g., vigorous 500 hPa trough, upper-level jet streak, and 850 hPa jet > 15 m s−1) and favorable thermodynamics (e.g., CAPE > 1000 J kg−1 and 850 hPa equivalent potential temperature > 335 K) moves across the southeast US, a nocturnal tornado episode near the Gulf coast can result.
Results of case studies emphasize the importance of surface boundaries in areas with favorable thermodynamic and shear profiles in serving as a focus for the intensification and organization of convection. The 22–23 February 1998 Florida tornado episode occurred ahead of a strong front where ample moist and unstable air was present and frontogenesis contributed to ascent as convective elements moved onshore. The null case of 29 November 2001 had many features necessary for a tornado episode: an upper-level jet entrance region, upstream trough, and low-level moisture and instability. However, due to factors such as the lack of surface-based CAPE and a unidirectional 0–6 km shear profile, the tornadic threat never materialized.