LIGHTNING IN TORNADIC THUNDERSTORMS OVER
THE NORTHEASTERN UNITED STATES

Thomas J. Galarneau, Jr.*, Sheryl F. Honikman,
Alicia C. Cacciola, and Lance F. Bosart
Department of Earth and Atmospheric Sciences
The University at Albany/SUNY
1400 Washington Avenue
Albany, New York USA

Kenneth D. LaPenta, John S. Quinlan,
and Glenn Wiley
National Weather Service Forecast Office,
Albany, New York USA

Implementation of the WSR-88d radar has resulted in significant improvement in the ability to warn the public of tornadoes. However, there remains room for improvement. For example, even though the tornado outbreak of 31 May 1998 across New York and Massachusetts was well forecast, only 13 of 51 tornado warnings had an actual tornado verification. This paper reports on an attempt to use cloud-to-ground (CG) lightning flash characteristics as a discriminator for tornado warnings over the northeastern US. Previous CG lightning studies (e.g., Kane 1991; Seimon 1993; McGorman and Burgess 1994) have established that tornadic supercell thunderstorms may have distinct lightning flash rate and polarity signatures in some cases. Over the northeastern U.S. significant terrain variations can also influence where and when deep convection occurs and is an important aspect of the false alarm problem. For example, Bracken et al. (1998) demonstrated that terrain-channeled southerly flow provided favorable shear profiles for tornadic supercells in the case of the Great Barrington, MA, tornado event of 29 May 1995. There were also indications from this and previous studies that CG flash rate patterns are influenced by underlying terrain variations. Accordingly, we will use CG lightning data collected by the National Lightning Detection Network (NLDN) to help address the false alarm problem.

2. Data/Methodology:

A total of 73 tornado days occurred over New York, New Jersey, Pennsylvania, and New England during the 1993-1998 period. A computer program was written to process the data from the NLDN on the 73 tornado days. For each tornado day the CG flashes were plotted on a base map centered on the position of the reported tornado touchdown. Tornado touchdown reports were obtained from Storm Data. Given that reported tornado positions as archived in Storm Data are subject to some uncertainty and error, we checked the reported tornado positions against archived 88D data for KENX (Albany, NY). In cases of discrepancy (none found for F2 and higher cases) the reported tornado position was subjectively "corrected" based on the WSR-88D observations. This initial study is restricted to F2 and higher tornadoes (15 tornadoes over six events).

3. Preliminary Results

We show in fig. 1 the percent of total CG flashes binned into two minute periods, relative to 12 min on either side of the time of tornado touchdown. Normalization was accomplished by weighing every case relative to the case with the highest number of flashes. The results suggest that the percent of total flashes peaks at t-10 (min) before the time of reported tornado touchdown (t-0) and then declines slowly to t+6 (min) with a slight secondary peak at t-0. These results are similar to those found by Kane (1991) in a limited number of cases. We also stratified the CG flashes by polarity. Shown in Fig. 2 is the percent positive CG flashes relative to t-0. There is a slow increase in the percent of positive CG flashes throughout with roughly 6% of the CG flashes having a positive polarity at t-0. Note, however, that there is the suggestion of a short-lived decrease in the percent of positive CG flashes between t-0 and t+2.

4. CONCLUSIONS

Based upon a limited sample, no robust CG flash variation signatures are apparent in the sample of F2 and greater tornadoes. Future work will expand the sample size to include the remainder of the tornadoes in the five year period.

5. Acknowledgement

This research was supported by COMET Project Number 09915806.

6. References

Kane, R. J., 1991: Correlating lightning to severe local storms in the Northeastern United States. Wea. and Forecasting, 6, 3-12.

MaCGorman, D. R., and D. W. Burgess, 1994: Positive cloud-to-ground lightning in tornadic storms and hailstorms. Mon. Wea. Rev., 122, 1671-1697.

Seimon, A., 1993: Anomalous cloud-to-ground lightning in an F5-Tornado-producing supercell thunderstorm on 28 August 1990. Bull. Amer. Meteor. Soc., 74, 189-203.

U.S. Department of Commerce, 1993-1998: Storm Data, 35-40.

*Corresponding author address: Thomas J. Galarneau, Jr., University at Albany/SUNY, Department of Earth and Atmospheric Sciences, 1400 Washington Avenue, Albany, New York 12222 USA; email: tomjr@atmos.albany.edu