Lightning Jumps as a Predictor of Severe Weather in the Northeastern United States

Severe weather events in the northeastern United States can be challenging to forecast, given the importance of interaction of convection with complex terrain. Complex terrain also poses challenges for the warn-on forecast due to the lack of surface observations in critical areas and radar beam blockage. To supplement existing observations, this study explores using lightning to forecast severe convection in areas of complex terrain in the northeastern United States. A sudden increase in lightning flash rate, or a lightning jump, is indicative of a strengthening convective updraft. Lightning jumps may key forecasters in on particular convective cells that have an increased probability of becoming severe.

This study assesses the value of using lightning jumps to forecast severe weather during the 2014 and 2015 convective seasons, defined as June–August, in the northeastern United States. Total lightning data from the National Lightning Detection Network (NLDN) is used to calculate lightning jumps using a 2-sigma lightning jump algorithm with a minimum threshold of 5 flashes min⁻¹. Following individual cells, lightning jumps are verified against severe weather reports from the Storm Prediction Center (SPC). If a lightning jump occurs within 45-minutes prior to a severe report, it verifies as a hit. If a lightning jump occurs with no corresponding severe report, it is classified as a false alarm.

There is a high probability of detection (POD; 75%) and a high false alarm rate (FAR; 55%) for severe-weather days during the 2015 convective season. Previous studies that applied a similar methodology to limited areas outside of the Northeast using lightning mapping arrays have a similarly high POD (79%), but a much lower FAR (36%). The anomalously high FAR of 55% for our sample is likely due, in part, to the location and timing of some convection, occurring in regions of low population density and/or overnight. These results suggest lightning jumps have limited value when used alone, but also suggest that storm reports have significant gaps.