An analysis is conducted of Tropical Storm/Depression Danny over the southeastern and middle atlantic states of the United States from both a traditional quasi-geostrophic and potential vorticity perspective. Hurricane Danny developed in the Gulf of Mexico and made landfall as a minimal hurricane on the southeastern Louisiana coast on 20 July, 1997, then continued to slowly drift towards Mobile Bay over the next 24 hours, producing rainfall amounts in excess of 80 cm. However, the more extraordinary aspects of Danny center around two key factors: 1) Danny was able to maintain its integrity as a midlevel vortex over land even as the surface pressure gradient deteriorated and 2) Danny reintensified to tropical storm strength over the piedmont of North Carolina even though it was not officially reclassified as a tropical storm until it had once again moved out over the open waters of the Gulf Stream on 25 July.
Preliminary results indicate that the environmental conditions contributing to the extraordinary evolution of Danny are as follows. A weak potential vorticity (PV) anomaly identified on the dynamic tropopause moves just to the north of Danny on 22 July, creating an area of enhanced divergence in the wind field over the mid-level vortex. The superposition of this divergence center over the center of the vortex led to a brief spin-up of the vortex by increasing the outflow aloft and enhancing the low-level moisture flux convergence. The system began to weaken once again until it began to interact with a strong PV anomaly and a baroclinic zone, just east of the Appalachian mountains. Initially, an area of enhanced ascent moved over the storm center, causing an increase in the prevalent convection. Eventually the superposition of the PV anomaly over the low-level center of Danny in combination with the enhanced baroclinicity led to a rapid spin-up of the surface cyclone, well before the cyclone reached the ocean by 25 July. Danny appears however to be a hybrid system at this point in time as evidenced by the potential temperature structure and upshear tilt, while satellite and radar imagery continued to indicate a vortex signature and the presence of eyewall-like convection.