Lightning in tornadic thunderstorms over the Northeastern United States

Over the 1993-1998 period, there have been 73 tornado days over New York, New England, New Jersey, and Pennsylvania. Although tornado warning accuracy and lead time has improved greatly (lead times have improved by as much as 20 minutes) with the use of the WSR-88D, tornado warning false alarm rates are still too high (on 31 May 1998, only 13 of the 51 tornado warnings issued were actual tornadoes). The purpose of this poster is to report on an attempt to use cloud-to-ground (CG) lightning data collected by the National Lightning Detection Network to help address the false alarm problem.

A computer program was developed to isolate single tornadic thunderstorms and monitor storm CG lightning characteristics (i.e. percent positive, frequency, proximity in storm) in relation to tornado touchdown time. Tornado touchdown time is defined as the reported time the tornado touches the ground as noted in storm data. The reported touchdown times can be in error for many reasons. Archived WSR-88D data as available was used to verify tornado touchdown times. The single tornadic cells were isolated by using the lat/lon of the tornado touchdown as a reference.

Preliminary results show that in 50% of the cases looked at so far, the frequency of CG flashes decreased (to 1-5 CG flashes per minute) during the tornado life cycle. The frequency then returned to pre-tornadic levels (5-10 CG flashes per minute) after the tornado ceased. In all of the cases studied so far, positive strikes are rare (at most, 2 strikes for the whole duration of the storm), and 75% of all CG lightning flashes tend to occur ahead of the tornado (with respect to storm motion).