In the southeastern United States tropical cyclones are some of the most severe rain events (Schumacher and Johnson, 2006). While tropical cyclones occur less frequently than other rain-producing events, they cause the most damage because they cover a large geographic area and often cause widespread flooding (Greenough et al., 2001; Mousavi et al., 2011; Schumacher and Johnson, 2006). On average, tropical cyclones occurring in the southeast bring 240.4 mm of rain in a 24 h period (Schumacher and Johnson, 2006). The severity of flooding following tropical cyclone events is a function of tropical storm frequency; landfall location; precipitation intensity; and, for coastal areas, mean sea level (Irish and Resio, 2013). In addition to flooding, these storms cause further damage from their strong winds (Greenough et al., 2001; Mousavi et al., 2011), and they frequently can cause tornadoes and landslides (Greenough et al., 2001; National Science Board (NSB), 2007).

Coastal communities in the United States, especially along the east coast and the Gulf Coast, are most at risk of the flooding, strong winds, and heavy precipitation associated with tropical cyclones (Irish et al., 2014). Approximately half of the United States population lives within only ∼ 80 km of the coast (NSB, 2007), and, on average, areas that are prone to tropical cyclones are 5 times more heavily populated than the rest of the nation (Frey et al., 2010). About 70 million people live in hurricane-prone areas (Greenough et al., 2001). Recent increases in coastal populations and development in coastal areas are posing an increasing risk to coastal infrastructure and human life (Greenough et al., 2001; Irish et al., 2014). Based off of 2010 estimates, 39 % of US homes are located in coastal counties, an 8 % increase since 2000 (NOAA, 2013). The monetary losses from hurricanes are increasing; in 2006 dollars, average annual losses were USD 1.3 billion from 1949 to 1989, USD 10.1 billion from 1990 to 1995, and USD 35.8 billion from 2002 to 2007 (NSB, 2007). Flooding from high storm surges during hurricanes has caused approximately 14 600 deaths over the last century; about 50–100 deaths occur per hurricane event (Greenough et al., 2001). In addition to deaths caused by flooding, hurricanes can cause a variety of health impacts, including illnesses that result from ecological changes (changes in the abundance and distribution of disease-carrying insects, rodents, mold, and fungi), damage to healthcare infrastructure and reduced access to healthcare services, damage to water and sewage systems, overcrowded conditions in shelters, and psychological effects from the trauma faced by victims (Greenough et al., 2001).

Several studies have looked at the influence of tropical cyclones on river flooding in small catchments. Kostaschuk et al. (2001) investigated tropical-cyclone-induced flooding in the Rewa River system in Viti Levu, Fiji. They observed that rainstorms caused a higher number of floods but that floods caused by tropical cyclones were much larger (Kostaschuk et al., 2001). Waylen (1991) conducted a partial duration series flood analysis for the Santa Fe River in Florida and found similar results. Tropical-cyclone-induced floods were found to occur less often than floods from other rain-producing events. However, they tended to have larger magnitudes and longer durations (Waylen, 1991). Specifically, they found that tropical cyclone floods were ∼ 3 times larger and ∼ 2 times longer than other floods (Waylen, 1991).

Greenhouse gases in the atmosphere not only increase atmospheric temperature but also can lead to increased sea surface temperatures (Irish et al., 2014). The warmer the sea surface temperature, the greater the intensity of tropical cyclones. Thus, global warming may intensify tropical cyclones, such that storms may tend to have higher storm surge levels (Frey et al., 2010; Irish et al., 2014; Mousavi et al., 2011). The Intergovernmental Panel on Climate Change (IPCC) predicts that global sea surface temperatures will increase 1.1–6.4 ∘C over the next century (Irish and Resio, 2013; Mousavi et al., 2011). Sea surface temperatures need to be at or above ∼ 26.7 ∘C for tropical cyclones to form (Steenhof and Gough, 2008). The current hurricane season extends from June to November; however, longer seasons (i.e., storms occurring before June and/or after November) have been occurring in recent years (Dwyer et al., 2015). While research on this topic is not conclusive, there is some indication that global climate change may lead to a change in the Atlantic hurricane season (Dwyer et al., 2015). There is an 8 % increase in a tropical cyclone's central pressure for each 1 ∘C increase in tropical sea surface temperature (Irish and Resio, 2013; Irish et al., 2014; Mousavi et al., 2011). Further, there is a 3.7 % increase in a tropical cyclone's wind speed for each 1 ∘C increase in tropical sea surface temperature (Irish et al., 2014). Climate models also suggest that precipitation rates from tropical cyclones may increase by 20 % by 2100 (Geophysical Fluid Dynamics Laboratory (GFDL), 2016; Knutson et al., 2010).

Numerous studies have indicated that global climate warming may intensify tropical cyclones and is very likely to result in sea level rise (Bronstert et al., 2002; Frey et al., 2010; Greenough et al., 2001; Irish and Resio, 2013; Irish et al., 2014; Kostaschuk et al., 2001; Mousavi et al., 2011; Ouellet et al., 2012). Major hurricanes, those that are category 3 or higher on the Saffir–Simpson scale, are the most likely to intensify (Frey et al., 2010; Mousavi et al., 2011). However, there is some debate about changes in tropical cyclone frequency. Some research predicts that tropical cyclone frequency will increase (e.g., Greenough et al., 2001; Ouellet et al., 2012), while other research suggests that tropical cyclones are likely to intensify with global climate warming but occur less frequently (e.g., Irish and Resio, 2013; Kostaschuk et al., 2001).

Several studies about the effects of climate change on tropical cyclone intensity have been conducted for the Corpus Christi, TX area (Frey et al., 2010; Mousavi et al., 2011). Frey et al. (2010) determined how severe historical hurricanes would be if they were to occur in the current climate, and those predicted for the 2030s and 2080s. They found that, in all three climate scenarios, storm surge flood depth, area of flood inundation, population affected, and economic damages would all increase compared to the historical levels (Frey et al., 2010). In a follow-up study by Mousavi et al. (2011), sea level rise and tropical cyclone intensification, due to global warming, are likely to equally contribute to increased flood depths.

While there has been much focus on the impact of tropical cyclones on coastal flooding, there has been little research on how these high-intensity precipitation events affect the hydrology of streams just inland of coastal areas. Further, few studies have focused on how inland flooding is likely to be altered with an extended hurricane season in the near future due to likely global climate change. This study investigates the potential increase in flooding risk with an extension of the hurricane season on four rivers in the southeastern United States. The goal is to help determine how flooding potential may change in the near future in order to elucidate the impact such changes may have on communities in the southeastern United States.

This research is focused on the southeastern United States, where tropical cyclone events occur quite frequently and where severe flooding following these events can have profound impacts on the prosperity of communities. Specifically, four river basins (Neches, Pearl, Mobile, and Roanoke) were selected for analysis (Fig. 1; Table 1). These four basins were chosen to be in areas that experience tropical cyclones and a high number of severe hurricanes. Currently, tropical cyclones impacting these four basins rarely cause flooding. As is shown later in this paper, this is primarily due to the overlap of the current hurricane season with the low-discharge seasons on these four rivers. However, an extension of the hurricane season, such that it encroaches upon the high-discharge seasons on these rivers, could likely lead to increases in flooding following tropical cyclones that impact these basins.

Gaging stations along these rivers were chosen to be inland of coastal areas so that tidal fluctuation and storm surge would not be factors when analyzing discharge, and far enough downstream to include as much of the basins as possible. These four basins were selected to represent a range of sizes and geographic locations that exist throughout the southeastern United States. United States Geological Survey (USGS) gages were used where data were available for the period extending from 1998 to 2014. In many cases (Pearl, Mobile, and Roanoke) USGS stream gages for these basins either did not have daily discharge data or did not have a long enough history of daily discharge data, or if sufficient daily discharge data were available, the location of the gaging station was either too close to the coast where there were tidal fluctuations or too far upstream in the catchment such that only a small fraction of the catchment was flowing to the gaging station. In these situations, Dartmouth Flood Observatory (DFO) satellite river gages were used (Brakenridge et al., 2016).

Most tropical cyclones impacting these four basins occur during September, or the middle of the low-discharge season (Figs. 2 and 3). The current hurricane season coincides primarily with the low-discharge seasons of the four basins. Thus, tropical cyclones rarely cause flood events on these rivers, even though they bring high amounts of precipitation, because they occur primarily during the low-discharge season. This is in contrast to tropical cyclones in Southeast Asia, for example, which are frequent during the monsoon season, causing widespread inland flooding (Darby et al., 2013). Some May tropical cyclones have already occurred in the Roanoke Basin during 2007, 2009, and 2012, and NOAA's HURDAT2 dataset contains other May and December tropical cyclones occurring in the Atlantic Ocean. This suggests that, while tropical cyclones rarely led to inland flooding from 1998 to 2014 in the four basins, a future extension of the hurricane season, such that it encroaches upon the high-discharge season in these rivers, has the potential to considerably enhance flooding risks.

Adding the months of May and December increases the number of days during the year that fall within the hurricane season by 34 %. For just a 34 % increase in the length of the hurricane season, there was, on average, a 63 % increase in the number of days at risk of a tropical-cyclone-induced flood along these southeast rivers (Table 2). When averaged over the 17-year period analyzed in this study, the number of days at risk of tropical-cyclone-induced flooding increases from 1.9 to 3.1 days yr-1. While 3 day yr-1 may not seem substantial, it not only represents a 63 % increase, but it is also a conservative number, as it excludes predicted enhancements in the intensity and/or frequency of future tropical cyclones (Bronstert et al., 2002; Frey et al., 2010; Greenough et al., 2001; Irish and Resio, 2013; Irish et al., 2014; Kostaschuk et al., 2001; Mousavi et al., 2011; Ouellet et al., 2012). Further, this research does not consider synergistic effects due to the potential interplay between May and/or December tropical cyclones and midlatitude cyclones, which could increase precipitation and flooding risk even further.

The timing of the hurricane season in relation to the high- and low-discharge seasons is crucial to understanding flooding risk following tropical cyclones on these rivers. The Mobile and Roanoke rivers showed the greatest increase in flooding risk (68 and 180 % respectively) in an extended May–December hurricane season as compared to the Neches and Pearl rivers (Table 2). The Pearl River showed the least increase in flooding risk following the average tropical cyclone (28 %) in an extended May–December hurricane season. While the Neches, Mobile, and Roanoke rivers tend to have slightly higher discharges in May than in June, discharges in the Pearl River are slightly lower in May than June. Thus, this study reveals not only that flooding risk following tropical cyclones is expected to increase if the hurricane season is extended due to global climate warming but also that this increase will not be uniform across the southeastern United States. Rivers with high-discharge seasons in May and December, such as the Mobile and Roanoke rivers, are likely to be most affected by a lengthened hurricane season.

The main limitation of this study is its use of average statistics. Future work could extend this study to look at increase in flood risk not only due to the average tropical cyclone but also due the full range of tropical cyclones that a basin is likely to experience (the tropical cyclone with the maximum increase in discharge, the tropical cyclone with the minimum increase in discharge, etc.). For instance, given that tropical cyclones are likely to intensify (Bronstert et al., 2002; Frey et al., 2010; Greenough et al., 2001; Irish and Resio, 2013; Irish et al., 2014; Kostaschuk et al., 2001; Mousavi et al., 2011; Ouellet et al., 2012), flooding risk in an extended hurricane season likely could exceed the results presented in this paper, although May and December tropical cyclones likely could be weaker than mid-season storms. Further, more explicit modeling of future tropical cyclone dynamics using a stochastic approach, rather than average statistics, could potentially produce a more robust understanding of the effects of future climate dynamics on flood susceptibility. Because the high-discharge season varies from basin to basin, extending this study to other basins along the east and Gulf coasts would allow for a fuller understanding of which areas in the southeastern United States are likely to be more at risk of flooding following tropical cyclones due to an extension of the hurricane season in response to global climate warming.