Multiscale Analysis of Dixie Alley Tornado and Null Events

Abstract

Some of the most destructive tornadoes throughout history have occurred in what is known as Dixie Alley within the Southeast U.S.  Previous studies for Florida defined a tornado event as ≥4 tornadoes within a 24-h period during December–May (avoiding tropical cyclone related events), while a null event was defined as a period when the NOAA Storm Prediction Center had tornado outlook probabilities ≥5% over any part of the respective study area, but <4 tornadoes occurred in 24 h.  This study presents a multiscale composite analysis (2002–2019) of 33 Alabama tornado and 65 null events; 46 Mississippi tornado and 92 null events; 24 Louisiana tornado and 98 null events; and 21 Georgia tornado and 32 null events.  Like the Florida cases, tornado events were primarily experienced across the northern and central parts of each state.  Using archived frontal analyses from the NOAA Weather Prediction Center, tornado events were associated with stationary boundaries more than any other synoptic or mesoscale feature (e.g., cold front; warm front).  Tornado events also occurred more frequently during La Niña and a positive Arctic Oscillation (AO) pattern.  A composite synoptic analysis showed jet-streak divergence, amplified anomalous midtropospheric troughing, and large positive precipitable water and potential temperature anomalies from Louisiana through Georgia during tornado events.  However, Louisiana tornado events featured more 850-hPa southerly winds compared to the classic southwesterly winds seen across its Dixie Alley counterparts.