A U.S. Thunderstorm Climatology Based on 25 Years of NLDN Observations (1993−2017)

Since the inception of the NLDN in the late 1980s, the focus of examining longer-term thunderstorm activity has shifted from maps illustrating the average annual number of thunderstorm days to flash density maps. The research presented here examines both lightning density and estimated thunderstorm day distributions over the contiguous U.S. for the period 1993 to 2017. A geographic information systems (GIS) approach was employed to determine cloud-to-ground (CG) flash count and thunderstorm day occurrence for each of the 9,095 days of NLDN lightning flash data available from NCEI. More than 559.8 million CG flashes were detected by the NLDN over the contiguous U.S. during the 25 years of this study. A thunderstorm day was assumed for all locations within either 5 nautical miles (9.6 km) or 10 nautical miles (18.5 km) of a lightning flash. These distances are used to determine TS and VCTS surface weather observations at ASOS locations that rely on NLDN data for detecting thunderstorms. Summaries of the CG flash results include their annual, monthly and diurnal variability, along with a map of the annual flash density. These results are similar to those from prior research that used data samples of fewer years than the 25 years examined here. The geographic distributions of thunderstorm days derived here from NLDN flashes allow direct comparison to distributions developed from surface observations of thunder since the early 1900s. The NLDN-based thunderstorm day results better characterize the spatial variability of thunderstorm days in the western U.S. than the studies based on surface weather observations due to increased observation availability. Also, year-to-year changes in thunderstorm day activity appear less sensitive to the increase in the detection efficiency due to upgrades in the NLDN than CG flash frequencies. The years studied (1993-2017) were divided into five consecutive 5-year groups. Maps illustrating the long-term variability of thunderstorm activity across the subgroups will also be examined.