Abstract
Rainfall observations in the Pigeon River basin of the southern Appalachian Mountains over a 5-yr period (2009–14) are examined to investigate the synoptic patterns responsible for downstream flooding events as observed near Knoxville, Tennessee, and Asheville, North Carolina. The study is designed to address the hypothesis that atmospheric rivers (ARs) are primarily responsible for the highest accumulation periods observed by the gauge network and that these periods correspond to events having a societal hazard (flooding). The upper 2.5% (extreme) and middle 33% (normal) rainfall events flagged using the gauge network observations showed that half of the heaviest rainfall cases were associated with an AR. Of those extreme events having an AR influence, over 73% had a societal hazard defined as minor-to-major flooding at the USGS river gauge located in Newport, Tennessee, or flooding observations for locations near the Tennessee and North Carolina border reported in the Storm Data publication. Composites of extreme AR-influenced events revealed a synoptic pattern consisting of a highly amplified slow-moving positively tilted trough, suggestive of the anticyclonic Rossby wave breaking scenario that sometimes precedes hydrological events of high impact. Composites of extreme non-AR events indicated a large-scale weather pattern typical of a warm season scenario in which an anomalous low-level cyclone, cut off far from the primary upper-tropospheric jet, was located in the southeastern United States. AR events without a societal hazard represented a large fraction (75%–88%) of all ARs detected during the study period. Synoptic-scale weather patterns of these events were fast moving and had weak low-level atmospheric dynamics.
Investigation of Atmospheric Rivers Impacting the Pigeon River Basin of the Southern Appalachian Mountains