A Statistical and Synoptic Climatology of Tropical Cyclone Tornado Outbreaks
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Tropical cyclones are capable of producing tornadoes prior to landfall as their outer rainbands move ashore through several days after landfall as they track inland. These tornadoes do not occur uniformly throughout a tropical cyclones' lifespan but, rather, occur in spatio-temporal clusters. This study sought to identify synoptic patterns associated with these spatio-temporal clusters, or tropical cyclone tornado outbreaks. Tropical cyclone tornado outbreaks are defined as three or more tornadoes in a six hour period centered about a primary synoptic hour and were identified using the Tropical Cyclone Tornado Database produced by Roger Edwards of the Storm Prediction Center. One-hundred and thirty-three outbreaks were identified over the 1995–2010 period in association with East and Gulf Coast landfalling tropical cyclones. Statistical descriptions of the outbreaks are provided to highlight their typical severity and their temporal and spatial characteristics. Synoptic plots depicting 500 hPa geopotential height contours and 500 hPa wind vectors that temporally correspond to each identified outbreak were obtained from the National Centers for Environmental Prediction-North American Regional Reanalysis project to represent the outbreaks' synoptic environment. Using the synoptic climatology paradigm of pattern recognition, this study categorized 97% of the identified outbreaks into one of five synoptic patterns; the synoptic environments of four outbreaks did not reasonably fit the criteria of any of the five patterns. Outbreaks categorized as Pattern 1, 2, or 3 are produced by tropical cyclones that are beginning to interact with, or are embedded within, the midlatitude westerlies whereas those categorized as Pattern 4 or 5 are produced by tropical cyclones that are isolated from the westerly flow. Various degrees of synoptic scale forcing are likely to be present in Pattern 1–4 outbreaks. The results of this study suggest that tropical cyclone tornadoes can occur in spatio-temporal clusters of relatively high tornadic activity over relatively short time periods and that those tropical cyclones interacting with or located within the midlatitude westerlies are most likely to produce outbreaks.