An early morning mid-Atlantic severe weather episode: short-lived tornadoes in a high-shear low-instability environment

On the morning of September 23, 2003, a line of thunderstorms moved rapidly northeast across the mid-Atlantic region and into southern New England. These storms produced at least four F1 tornados over eastern Pennsylvania and New Jersey, along with numerous other reports of wind damage. From an operational warning standpoint, this event was challenging for several reasons, including the unusual timing of the event, a lack of upstream history or other anticipatory factors, and a concurrent threat of heavy rain and flooding.

The pre-existing environment was a very moist tropical air mass left in the wake of Hurricane Isabel, which had tracked northwest across Virginia into western Pennsylvania a few days earlier. A line of convection developed overnight ahead of a cold front moving east across the central Appalachians, which did not produce severe weather until it encountered slightly more unstable air over the upper Chesapeake Bay and the Delaware River Valley region. Strong southerly flow at the surface off the relatively warm Atlantic Ocean is believed to have contributed to a slight de-stabilization of the air mass overnight.

The tornados all occurred within about 40 nm (65 km) of the National Weather Service WSR-88D radar at Fort Dix, New Jersey (KDIX), so their attendant mesoscale circulations were fairly well defined by the high-resolution (8-bit) velocity data from KDIX. The tornado circulations were short-lived, forming and moving rapidly, and they appeared to be of the non-descending type, with strongest rotations developing at the lowest radar elevation angle. Besides the tornados, there were numerous reports of wind damage, mainly with short-line bowing segments, and several reports of heavy rain and flooding. However, there were no reports of hail, and very little cloud-to-ground lighting from the storms.

The presentation will document this unusual weather event, using archived operational data sets from the National Weather Service AWIPS computer system. Focus will be first on the overnight evolution of the pre-storm environment (leading to an unexpected severe storm outbreak during the morning rush hour), and second on the radar depiction of severe weather signatures, particularly the high-resolution velocity data showing the low-level spin-up of tornadic rotation. Lessons learned for better operational warning procedures will also be included.