Monthly distributions of NLDN and GLD360 cloud-to-ground lightning

National maps of cloud-to-ground lightning flash density (flashes/km2/year) for one or more years have been produced since Vaisala's National Lightning Detection Network (NLDN) was first deployed across the contiguous 48 states in 1989. However, monthly national lightning maps have not been shown. For this study, monthly maps were compiled in 20 by 20 km squares from 2004 to 2008 for the lower 48 states.

Monthly flash density maps were prepared, along with monthly maps of the percentage of the average year's lightning. From January through March, results typically show banded structure associated with small numbers of thunderstorms over the southeastern states. Lightning then spreads north and westward until by June, all of the U.S. except the far southwest is experiencing lightning. New England, the northern Rockies, and the Florida peninsula have a small percentage of their lightning activity outside the months of June, July, and August. Arizona and portions of adjacent states have their highest incidence in July and August. Flash densities throughout the continental U.S. drop off quickly in September.

During summer months, NLDN monthly maps are compared with those from the new Vaisala Global Lighting Dataset GLD360. Results show that lightning over all of Mexico, Central America, and the western Caribbean complement and extend the NLDN coverage in very reasonable patterns.