Detailed surface features associated with Tropical Storm Floyd (1999) at landfall

Hurricane Floyd brought devastating flooding since its first point of landfall near Wilmington, North Carolina northward through the New England coastal areas. Many of these areas were in the midst of a worsening drought situation before Floyd's devastating rainfall reversed that with severe flooding in a matter of hours. A more seriously affected area was from central New Jersey northeastward through Rockland, Westchester, and Putnam Counties of New York. Many localities received over 13 inches (360mm) of rain within 32 hours. The heaviest rain fell in the afternoon of 16 September 1999 when the center of Tropical Storm Floyd moved north-northeastward along the New Jersey coast. At around 00 UTC 17 September 1999, Floyd made landfall on western Long Island. As Floyd approaches the coast, eyewitnesses on western Long Island reported gradual calming of the wind which was followed by the onset of northwesterly winds increasing to 30 knots (15 m/s), a brief burst of heavy rain, and a sharp (7°C) drop of temperature. All of these happened within a 30-minute time frame.

Official surface observational sites on western Long Island were plentiful. Detailed surface analyses using these data were constructed to examine the surface features associated with Floyd. Preliminary surface analyses indicate that the sharp temperature drop reported by eyewitnesses on Long Island in the wake of Floyd was associated with a sharp baroclinic zone oriented southwest-northeast from the western edge of Floyd. Temperature gradient across this zone was in the order of 7°C/10km. This zone separated the strong east to southeast winds with scattered heavy showers in the warm sector (23°C), and north to northeast winds with heavy rain in the cold sector (16°C). The heaviest rain that produced the devastating flooding occurred right along the cold sector edge of the baroclinic zone. Animation of radar images confirmed that the highest reflectivities persisted along this leading edge. Combined with the surface analyses, the radar animation suggests that abrupt or vigorous ascent accompanied the baroclinic zone where surface convergence was ensured. It is hypothesized that rain-producing mechanisms associated with the ascent along the baroclinic zone was much enhanced as the circulation of Floyd approached, resulting in the heaviest rain to occur in the areas observed. In the conference, a series of surface analyses illustrating the evolution of the baroclinic zone, and its association with Floyd will be presented.