Tornado outbreaks associated with landfalling hurricanes in the North Atlantic Basin: 1954–2004

Abstract

Tornadoes are a notable potential hazard associated with landfalling hurricanes. The purpose of this paper is to discriminate hurricanes that produce numerous tornadoes (tornado outbreaks) from those that do not (nonoutbreaks). The data consists of all hurricane landfalls that affected the United States from the North Atlantic basin from 1954 to 2004 and the United States tornado record over the same period. Because of the more than twofold increase in the number of reported tornadoes over these 51 years, a simple least-squares linear regression (''the expected number of tornadoes'') was fit to the annual number of tornado reports to represent a baseline for comparison. The hurricanes were sorted into three categories. The first category, outbreak hurricanes, was determined by hurricanes associated with the number of tornado reports exceeding a threshold of 1.5% of the annual expected number of tornadoes and at least 8 F1 and greater tornadoes during the time of landfall (from outer rainbands reaching shore to dissipation of the system). Eighteen hurricane landfalls were classified as outbreak hurricanes. Second, 37 hurricanes having less than 0.5% of the annual expected number of tornadoes were classified as nonoutbreak landfalls. Finally, 28 hurricanes that were neither outbreak nor nonoutbreak hurricanes were classified as midclass hurricane landfalls. Stronger hurricanes are more likely to produce tornado outbreaks than weaker hurricanes. While 78% of outbreak hurricanes were category 2 or greater at landfall, only 32% of nonoutbreak hurricanes were category 2 or greater at landfall. Hurricanes that made landfall along the southern coast of the United States and recurved northeastward were more likely to produce tornadoes than those that made landfall along the east coast or those that made landfall along the southern coast but did not recurve. Recurvature was associated with a 500-hPa trough in the jet stream, which also contributed to increased deep-layer shear through the hurricane, favoring mesocyclogenesis, and increased the low-level shear, favoring tornadogenesis. The origin of the hurricane, date of landfall, and El Niño-Southern Oscillation phase do not appear to be factors in outbreak hurricane creation. The results of this study help clarify inconsistencies in the previous literature regarding tornado occurrences in landfalling hurricanes.

A statistical analysis of the association between tropical cyclone intensity change and tornado frequency

“…The 6-h periods associated with TCs weakening at greater rates also generally have larger mean and median TCTOR frequency metrics. Numerous studies (e.g., Hill et al 1966;Novlan and Gray 1974;Gentry 1983;McCaul 1991;Verbout et al 2007;Moore and Dixon 2011;Rhodes and Senkbeil 2014) have reported that strong TCs tend to produce a larger numbers of TCTORs than weak TCs. Figure 7a supports these many studies.…”

“…Anomalously high soil moisture can provide an environment with increased instability that favors inland TC strengthening (Kellner et al 2012) and TCTORs. Interaction between a TC and the midlatitude westerlies and upper-level troughs provides vertical wind shear, helicity, and dynamic lift via upperlevel divergence that are important for TCTORs (McCaul 1991;Verbout et al 2007;Moore and Dixon 2014). Although environmental vertical wind shear most often weakens TCs, interaction with an upper-level trough embedded within the westerlies has been shown to lead to both weakening and strengthening (Hanley et al 2001;Hart and Evans 2001;Jones et al 2003;Wang and Wu 2004;Reasor and Eastin 2012).…”

“…Belanger et al (2009), for example, used multiple linear regression to model the number of TCTORs a TC produces based on TC size, TC intensity, TC trajectory, and mid-level specific humidity. Onderlinde and Fuelberg (2014) more recently employed statistical techniques to develop a composite parameter based on TC intensity, TCTOR spatial and temporal characteristics, and an array of thermodynamic and dynamic environmental parameters to forecast the likelihood that one or more TCTORs will occur in a period of 6 h. Both of these models used TC intensity as a predictor because multiple studies have reported that TCs that are strong at landfall tend to produce a larger number of TCTORs than those that are weak (e.g., Hill et al 1966;Novlan and Gray 1974;Gentry 1983;McCaul 1991;Verbout et al 2007; Moore and Dixon 2011;Rhodes and Senkbeil 2014).…”

“…While some TCs produce few to no TCTORs, others may produce a substantial number over a period of several days (Verbout et al 2007;Schultz and Cecil 2009;Edwards 2012; Dixon 2011, 2013). TCs may even produce a substantial number of TCTORs over a relatively short period of time-TCs Ivan (2004) and Rita (2005) produced more than 30 TCTORs in ≤6 h (Moore and Dixon 2013).…”

Spatial Distributions of Tropical Cyclone Tornadoes by Intensity and Size Characteristics

“…Previous studies suggest that TCs produce more tornadoes when they track into higher latitudes and encounter westerly wind shear [6,28,29]. The increased proportion of strong F/EF 2-3 tornadoes, along with the greater median path lengths, path widths, and DPIs, in the 32 • N-34 • N and >34 • N regions suggest that larger proportions of the tornadoes that TCs produce as they track into higher latitudes are stronger, larger, and potentially damaging (Figure 2; Table 1).…”