Radar Observations of Northeastern United States Tornadoes

During recent years, there have been several major tornado outbreaks in the northeastern United States. The introduction of the WSR-88D radar produced significant improvement in the National Weather Service's ability to warn the public of tornadoes during these events. Radar analysis of a large number of northeastern tornadoes will provide a better understanding of tornadic thunderstorm morphology and the radar characteristics that best identify impending tornadogenesis, leading to additional improvement in warning accuracy and lead time.

While supercells produced greater than half of the more than 80 tornadoes examined, a variety of storm structures were associated with the tornadic thunderstorms. More than a third of the tornadoes developed with bowing lines or cells, with most of these tornadoes forming on the bulging portion of the bow. Occasionally, supercells were observed in lines of thunderstorms, or bowing line segments evolved into supercells. Appendages or hooks were observed in less than half the tornadoes, and boundary interactions may have played a role in the evolution of about one fifth of the tornadic storms. Gate to gate rotating velocity couplets of varying intensities were identified with a majority of the tornadoes. However, more than a third of the tornadoes (mostly F0 and F1) were associated with non-rotating wind maxima and may have been indicative of gustnadoes. A number of tornadoes produced no discernable radar signature due to time and spatial sampling limitations of the radar.

Preliminary analysis showed that the WSR-88D calculated shear of the gate to gate rotating velocity couplets associated with many tornadoes may be useful in identifying tornadic storms. The effects of beam spreading with range in both the actual shear calculation and the ability of the radar to resolve storm features required that the observed shear values be adjusted for range. The adjusted shear values, which provide evidence of the strength of small scale (1 km or less), low-level mesocyclones were compared to the larger scale (1 km to 10 km), midlevel mesocyclone strength in order to assess a storm's tornadic potential.