“Insights into Lightning Climatology Derived from Twenty-Five Years of USA National Lightning Detection Network (NLDN) Maps: 1989-2013”

Cloud-to-ground lightning data from the National Lightning Detection Network (NLDN) are summarized in annual maps, 1989-2013, for the continental United States. We have not altered the recorded data in any way with the exception of deleting all positive lightning reports with peak currents less than 15 kA. All data were analyzed with a spatial resolution of 20-km. We analyze total (negative and positive) flashes for ground flash density, the percentage of positive lightning, the negative and positive first stroke peak currents and the flash multiplicity. The Gulf Stream is "outlined" by higher flash densities off the East Coast of the United States. The annual percentage of positive lightning to ground varies from less than 2% over Florida to values exceeding 25% off the West Coast. There is a relatively sharp transition from low to high median negative peak currents along the Gulf and Atlantic coasts of the USA. No sharp transitions are observed for median positive peak currents. Relatively lower positive peak currents occur throughout the southeastern USA. Over the twenty-five year period, we note that the latitude of maximum flash density has apparently increased from 28 to 33 degrees latitude.