Rethinking flood risk: Tropical cyclones, orography, and the upper tail of flood peaks for the Appalachian region of the United States

Tropical cyclones over land do not simply decay; they reorganize and interact with orography, producing extreme rainfall and flooding far from the center of the storm. In the Appalachian region, orographic enhancement results in areas with extreme rainfall amounts, leading to precipitation and flood peak distributions that do not exhibit evidence of an upper bound based on available observations and statistical analyses, although the existence of a physical upper limit cannot be ruled out. Here we use an empirical approach revolving around Hurricane Helene and long-term records to support the notion that design practices anchored in upper bounds (e.g., Probable Maximum Precipitation) are increasingly misaligned with observed extremes and projected risks. We argue for probabilistic frameworks that explicitly account for extremely low annual exceedance probabilities, mechanism-dependent tail behavior, and uncertainty beyond historical records. By integrating statistics reflective of the lack of an upper bound, high-resolution modeling, and by accounting for different processes in flood frequency analyses, the meteorological and hydrologic community can better inform resilient infrastructure and risk communication in an era of accelerating climate extremes.