The Role of a Squall-line and Boundary Interaction in the Development of the Long-lived 21-22 July 2003 Tornadic Supercell across Eastern New York and Western New England

A mesoscale convective vortex (MCV) moved across northern Pennsylvania into southeast New York on 21 July 2003.  The MCV originated from an area of convection over eastern Iowa around 0300 UTC. The system strengthened and acquired a large hook-shaped satellite signature as it moved across the Midwest reaching central New York by 0130 UTC on 22 July 2003.  A line of strong convection extending south from the vortex center crossed eastern New York, where previous convection had created a pre-existing outflow boundary.  Widespread severe weather ensued, including a long-lived supercell responsible for a series of tornadoes with a maximum F2 intensity. The convection developed in a highly favorable dynamic environment aloft. Eastern New York and western New England were located in the right-rear quadrant of the 250 mb upper level jet, which was marked as an area of strong, concentrated divergence.  A strengthening southerly 850 mb low level jet intensified to 50 kt, as it crossed the Hudson Valley.  Winds increased and veered from the surface through the mid troposphere.  This created a shear profile favorable for severe weather, including bow echoes and supercells.  Instability was very large with Surface Based Convective Available Potential Energy values exceeding 2000 J kg^-1. Surface dewpoints in excess of 20°C were in place prior to the tornadic convection. Observational data, including surface, upper-air and satellite coupled with radar data will be used to examine the role the interaction between the pre-existing outflow boundary and the squall line played in the evolution of the long-lived supercell.

A detailed radar analysis of the evolution of the long-lived tornadic supercell will be presented.  It will be shown that southeasterly backed flow in the cool pool increased the low-level shear, thus increasing the horizontal vorticity tilted into the vertical.  The local radar analysis will further show that supercell dynamics were present, with a tornadic signature of varying intensity evident during the supercell lifecycle.