Verification of thunderstorm occurrence using the National Lightning Detection Network

Since the summer of 1983, the Department of Atmospheric and Environmental Sciences at the University at Albany, State University of New York, has organized a summertime, daily thunderstorm probability forecast contest. The contest involves predicting the probability to the nearest 10% that thunder will be heard during a 24-h period at ten locations across the continental United States, and the forecasts are verified by standard METAR reports. In recent years, however, there have been a significant number of instances in which a thunderstorm failed to be reported in the METAR observations despite reflectivity values exceeding 50 dBZ and the occurrence of a large number of National Lightning Detection Network (NLDN) detected flashes in close proximity (< 10 km) to the station.

Given that the NLDN has continuous space and time coverage, an average detection efficiency in excess of 95% and mean location errors of less than 500 m for cloud-to-ground (CG) lightning flashes, we examine the utility of using the NLDN to verify thunderstorm occurrence. In particular, a 12-year thunderstorm climatology (2001-2012) from METAR station observations will be compared to an NLDN lightning climatology at the ten stations currently used in the University at Albany's National Thunderstorm Contest and ten additional first-order reporting stations to cover the full spectrum of summertime, synoptic environments and rates of thunderstorm occurrence across the continental United States.

Preliminary results show 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 National Weather Service's thunderstorm reporting guidelines. There are, however, notable exceptions: 1) Stations in Florida, e.g., Miami and Tampa, have months where the number of days with flashes within 5 km of the stations exceed 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 agree best with the number of NLDN lightning days further from the stations, i.e., at the 15-20 km radii. Additional questions to be addressed in the presentation include: (i) are there detectable trends and interannual variability in thunderstorm occurrence for each station, (ii) are there notable correlations or anticorrelations of mean annual thunderstorm days between stations, and (iii) how do mean thunderstorm day soundings and standard thunderstorm forecasting variables vary from station to station.