Mesoscale analysis of the cavendish, VT bow-echo tornado on 21 July 2003

On 21 July 2003, a well-defined mesoscale convective vortex (MCV), produced by a decaying mesoscale convective system (MCS) that moved across the Central Plains the previous evening, triggered a severe MCS that tracked from northeastern Pennsylvania and eastern New York into southern Vermont by 0300 UTC 22 July 2003. Radar reflectivity data revealed a line-echo wave pattern within the MCS that appeared to be responsible for producing widespread severe weather across New York, Pennsylvania and southern Vermont. The 0000 UTC Albany, NY sounding on 22 July 2003 indicated a very unstable layer aloft with elevated Convective Available Potential Energy (CAPE) values of 2700 Jkg-1 and minimal amounts of Convective Inhibition (CIN). In addition, a strong low-level jet of 50 knots at 850 hPa helped to enhance the shear across the forecast area. As the system traveled across the southern tier of New York, the convective line intersected a pre-existing surface thermal boundary that extended from northeastern Pennsylvania into southern Vermont. The thermal boundary was created from convection that moved across the region earlier in the day. At the intersection point, a particularly strong cell exhibiting supercellular characteristics evolved into a bow echo and subsequently produced a series of tornadoes from the eastern Catskills into southern Vermont. The remnants of this strong cell crossed into Burlington’s county warning area near Cavendish around 0230 UTC 22 July 2003, producing significant damage. A detailed ground and aerial damage survey revealed extensive tree damage with a path length and width of about 3.5 and 0.5 miles, respectively. An analysis of the radar and damage survey data suggests that this damage occurred within the northern book-end vortex and not at the leading edge of the bow echo at or north of the apex. There are limited studies about the convective mode(s) that produce tornadoes in Vermont. Previous observational studies over the Central and High Plains have shown that the intersection point of a primary convective system and a preexisting boundary is a preferred location for tornadogenesis and that the cells at the intersection point can evolve into a bow echo structure. This study examines WSR-88D radar data, meso-surface analysis, and upper air data to understand the mesoscale environment that result in this type of tornadic evolution in northern New England. The identification of the pre-storm environment associated with this evolution will help forecasters in the warning decision making process across the BTV CWA, where issuing tornado warnings is a relatively infrequent occurrence.