The impact of a coastal baroclinic zone on the distribution of heavy precipitation associated with Hurricane Floyd

In September of 1999, heavy rainfall associated with Hurricane Floyd brought widespread flooding along the east coast of the United States from North Carolina northward through Maine. Rainfall amounts were as high as 500mm (15 inches) near Floyd's first landfall point near Wilmington, North Carolina. Residents in these regions had endured an extended period of dryness before the heavy rain from Floyd reversed it with severe flooding. Preliminary rain gauge data showed that a primary maximum occurred from around Wilmington, NC northward, while a secondary maximum occurred in a narrow band from northern New Jersey northeastward to western Connecticut where 325 to 425mm (10-13 inches) of rain fell.

The author, along with other observers, noticed a sharp drop in temperature shortly after the passage of the center of Floyd on western Long Island, New York. On a 00Z 17 Sept 1999 surface analysis, this sharp temperature drop was found to be associated with a progressive baroclinic zone that had already been established within the northern and western quadrants of the circulation of Floyd. Temperature gradient across the front was at least 6°C / 10km. On Doppler radar images, the baroclinic zone was recognizable as a sharp contrast between scattered echoes that were found on the warm side of the front and a swath of highly reflective echoes found immediately along the cold side of the front. The narrow band of heavy precipitation observed across New Jersey and western Connecticut lied within this swath where tropical air from the Atlantic never arrived.

Historically, a few infamous hurricanes had caused especially serious inland flooding as they tracked up the east coast. Tropical Storm Agnes of 1972, for one, was notorious in the devastating flooding it incurred in Pennsylvania and Virginia. Bosart and Dean (1991) investigated the surface features of the storm as it tracked northward along the mid-Atlantic states and found that the heaviest rain concentrated exclusively on the cold side of a baroclinic zone that formed in-situ on the northwestern quadrant of the circulation of Agnes. Hurricane Hazel of 1954 produced 200 to 350 mm (6-10 inches) of rain 150 km west of its path while less than 70 mm fell to the east. A lesser-known Tropical Storm Doria in 1971also resulted in flooding across central New Jersey and southeastern Pennsylvania. With Floyd, there was an approximate 6 to 1 ratio between the heaviest rainfall to the west of its path and the rainfall totals along it. Common to these hurricanes that caused especially heavy rain in the Northeast was that they were in the process of becoming extratropical. Hurricane Floyd of 1999 represented a case of extreme frontogenesis that occurred on the northwestern side of its circulation. The sharp contrasting features across the front were in the sub-mesoscale range. Doppler radar loops combined with surface observations showed clearly that the frontal boundary acted as an axis of surface convergence and vigorous low-level lifting. Its kinematic structure appeared to be similar to the over-running pattern associated with an idealized warm frontal boundary where precipitation is deposited in the cold sector edge of the front. Recognizing the formation of these mesoscale fronts associated with East Coast hurricanes will help forecasters to better forewarn locations of heavy rainfall enhanced by these mesoscale frontal features.