Application of the National Lightning Detection Network in the analysis of the variability of warm season thunderstorm occurrence

A 20-year summertime thunderstorm climatology (1994–2013) from METAR station observations is compared to a National Lightning Detection Network (NLDN) lightning climatology at twenty first-order reporting stations across the continental United States covering the full spectrum of synoptic environments for thunderstorm occurrence. Results show that the NLDN lightning climatology within a radial distance of 10 km of a station best matches with the METAR thunderstorm climatology, in agreement with Bosart and Landin's (1994) statement that thunder can generally be heard within 10 km of a station, and the Federal Aviation Administration's thunderstorm reporting guidelines. There are, however, notable exceptions: 1) stations in Florida and Arizona exhibit months where the number of days with NLDN flashes within 5 km of the stations exceeds the number of METAR thunderstorm days, and 2) METAR thunderstorm reports for stations in the western United States (e.g., Denver, Bismarck and Salt Lake City) tend to occur with NLDN flashes at a farther radial distance from the station, e.g., at the 15–20 km radii. Additionally, stations in the central United States (e.g., Omaha, Oklahoma City and Minneapolis) are found to be the best verifiers of thunderstorm occurrence, with METAR thunderstorm reports >90% of the time lightning strikes within 10 km of the station.

In this presentation, we will explore the reasons for the verification differences between different regions of the United States in relation to standard thunderstorm forecasting variables, the number of flashes recorded by the NLDN, and whether a concurrent observation of precipitation is required for a thunderstorm to be reported. We will also examine whether there are detectable trends and interannual variability in thunderstorm occurrence across the United States, and notable correlations of mean annual thunderstorm days between stations. Finally, preliminary research has shown strong correlations exist between thunderstorm days and large-scale modes of climate variability, including the Madden-Julian Oscillation and El Nino Southern Oscillation. We will investigate the synoptic scale weather patterns and physical processes responsible for these correlations.