A Lightning Climatology of Pennsylvania, with Urban Heat Island Applications

The spatial and temporal patterns of USPLN cloud-to-ground lightning data for the five-year period 2009-2013 have been analyzed for Pennsylvania and nearby surrounding areas. Two standard metrics were employed: stroke density and flash days. Five-year mean and monthly climatologies were created for the region of study. In general agreement with previous work, the highest stroke densities of 8-12 strokes km-2 are found in western Pennsylvania, with minima of 1-2 strokes km-2 found in the northeastern part of the state. In general, stroke density remains moderate over the Appalachians, decreases on the downslope side, then increases again at lower elevations well southeast of the mountains. Of the lightning recorded in the area of study, over 70% of the strokes occurred during the months of June, July, and August. The cities of Philadelphia and Pittsburgh were studied in an attempt to discern any urban heat island influence on convection. Only maritime tropical air mass days in June, July, and August were considered to focus on days that lacked synoptic-scale forcing. A hybrid metric that combines stroke density and flash days suggests that there was increased lightning production downwind of Philadelphia.