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It is widely anticipated that in a warming climate, the number and intensity of intense precipitation events (IPEs) will increase over time (Janssen et al., 2014; Prein et al., 2016; Mallakpour & Villarini, 2017). Despite a somewhat muted warming signal within the Southeastern United States (SeUS) (Pan et al., 2013; Rogers, 2013; Meehl et al., 2015), this region is not exempt from this expected trend. Since 1950, the intensity and frequency of IPEs has increased in the SeUS. After controlling for climatic influences and oscillations, such as teleconnections, it appears that the increasing IPE trend in the SeUS is likely a result of climate change (Skeeter et al. 2018). With the likelihood of future SeUS IPEs continuing to rise, it is important to examine and understand the climatological controls, atmospheric patterns, and characteristics of IPEs in this region with an emphasis on identifying the synoptic features associated with IPEs.
A large volume of literature has been devoted to understanding the climatological characteristics, as well as temporal trends of IPEs in the SeUS. Keim (1996) classified the surface forcing mechanisms associated with IPEs across the SeUS as either frontal, tropical, or air mass events, and found that frontal events were the dominant forcing mechanism in the SeUS. Similarly, Kunkel et al. (2012) examined all five-year return events across the United States, and determined that for the SeUS, tropical systems comprised 51% of these anomalous events, while frontal systems were responsible for 34% of such events. This contrasts with the results for the entire contiguous United States. Frontal events accounted for the majority of intense events for the contiguous U.S., at 54%, but the SeUS has a greater influence from tropical systems. Powell and Keim (2015), found that across the SeUS IPEs have become more common, while diurnal temperature ranges across the region have narrowed as a result of higher extreme minimum and lower extreme maximum temperatures. Mallakpour and Villarini (2017) also found evidence of increasing frequency of heavy precipitation events for most regions of the United States, including much of the SeUS, utilizing a 95th percentile peaks over threshold. Further reinforcement of evidence for an IPE increase was found by Skeeter, et al. (2018), who found that since 1950, there have been sub-regionally variable increases in recurrence, strength, and surface weather types associated with IPEs. IPEs associated with moist tropical weather types more commonly encroached inland and northward from 1950-2016.
While the surface and synoptic characteristics, as well as the potential for increases in the recurrence and strength in IPEs have been studied, an implicit understanding of the upper atmosphere synoptic patterns that create IPEs is essential for accurate modeling and prediction (O'Gorman & Schneider, 2009; Nickl, et al. 2010; Sugiyama, Shiogama, & Emori, 2010; Pathirana, et al. 2014). Variations and trends in mid-tropospheric flow have long been used to explain trends in precipitation (Anzelmo & Soule, 1999; Diem, 2006; Gilles, et al. 2011; Pan, et al. 2017), and have also been used to explore other surface parameters, including temperature (Skeeter, 1990; Fall, et al. 2010), and particulate and pollutant variability (Camalier, et al. 2007; Sheridan, et al. 2007) among other variables. The Southeastern United States (SeUS) is a region subject to variable synoptic-scale influences on IPE throughout the entire year, such as the seasonal migration of the Bermuda high, or latitudinal location of the jet stream (Schumacher & Johnson, 2006). Understanding the upper-level patterns and seasonality that tend to accompany large-scale IPEs is given enhanced scrutiny in this region due to recent trend results.