Increased Fall Precipitation In The Southeastern United States Driven By Higher-Intensity, Frontal Precipitation

During 1895-2018, fall precipitation increased by nearly 40% in the southeastern United States north of the Gulf of Mexico due to increased circulation around the western North Atlantic Subtropical High, which enhanced moisture transports into the region. We find here that these increases in southeastern U.S. fall precipitation manifested almost entirely as increases in precipitation intensity, not frequency. Further, the enhanced moisture transports increased precipitation totals far more on the highest-intensity precipitation days than on the lower-intensity days, leading to nearly all of the increase to be delivered on extreme (top-5% intensity) precipitation days. Eighty-seven percent of the fall precipitation increase was driven by non-tropical storms (mostly frontal), not tropical cyclones, though the proportion of precipitation falling as either storm type did not change. Further research is needed to evaluate whether these observed precipitation increases are likely to continue, stabilize, or reverse.Plain Language Summary Fall precipitation increased over the twentieth century in the southeastern United States due to increased wind-driven moisture transport from the Gulf of Mexico. Although fall is tropical cyclone (hurricane) season, it was non-tropical storms (mostly frontal) that dominated the precipitation increase. Further, the fall precipitation increase manifested as an increase in storm intensity, not frequency. The most intense 5% of non-tropical storms accounted for nearly three quarters of the increase in fall precipitation in the southeastern United States during 1895-2018. Climate models project that warming-induced humidity increases will also intensify storms, which would add to the wind-driven intensification detected thus far. If the observed wind-driven increase in intense precipitation continues and is added to by the effects of rising humidity, severe flooding from extreme frontal storms could become more common and severe in this region.