The Extratropical Transition and Precipitation Distribution of Hurricane Floyd (1999)

Abstract

Several recent landfalling tropical cyclones (e.g., Dennis, Floyd, and Irene 1999) have highlighted a need for a refinement in the forecasting paradigms and techniques in the area of quantitative precipitation forecasting. Floyd proved to be a particularly challenging forecast problem as it was accompanied by catastrophic flooding over large regions of the East Coast, in spite of its relatively quick northward movement. The extent and intensity of the precipitation distribution was strongly modulated by the storm's interaction with a midlatitude trough. In an attempt to better understand and quantify the relevant dynamics during this interaction, potential vorticity (PV) and quasigeostrophic perspectives are utilized. As Floyd approached the East Coast, precipitation shifted to the left of the storm track due to the presence of a deep midlatitude trough in the Ohio valley. The juxtaposition of a cold-core PV anomaly associated with the midlatitude trough and a warm-core PV anomaly associated with Floyd created a strong and tropospheric-deep baroclinic zone along the eastern seaboard. This baroclinic zone provided a region favorable for deep isentropic ascent as the circulation of Floyd approached, resulting in prolific precipitation production. The latent heat release associated with this precipitation in turn acted to enhance outflow ridging north of Floyd, which was underpredicted by current numerical models. The enhanced outflow ridge resulted in enhanced jet-streak dynamics and a restructuring of the tilt of the midlatitude trough in a manner favorable for excessive precipitation production. Furthermore, the uplifting of the dynamic tropopause in southwesterly flow ahead of Floyd in response to ascent and differential diabatic heating resulted in a tropopause fold, a feature usually associated with upper-level fronts and differential subsidence in northwesterly flow.