Lightning observations in northern Colorado snowstorms

Lightning flashes during snowstorms occur infrequently compared to warm-season convective storms. The rarity of such events may pose an additional hazard because of their unexpected nature. In addition, because cloud electrification in thundersnow events leads to only few lightning discharges, studying thundersnow events may offer additional insights into mechanisms for charging and possible thresholds required for lightning discharges. In this study, we present observations of four Northern Colorado thundersnow events that occurred in the 2012-2013 winter season. Detailed total lightning information was collected by the Colorado Lightning Mapping Array allowing for the analysis of lightning characteristics and storm charge structure of these cases. Lightning data from CONUS lightning detection networks are also examined. Dual-polarization radar data from the WSR-88D radars near Denver (KFTG) and Cheyenne (KCYS) are analyzed to uncover the microphysical structure of the lightning-producing storms. The occurrence of four thundersnow events in one season strongly disagrees with previous climatologies, implying that thundersnow may be more common than previously thought. Most of the lightning flashes occurred within convective cells, in which polarimetric radar data reveal the collocation of graupel and pristine ice crystals. However, some flashes occurred in snow bands lacking any apparent convective structure. In some of these cases, depolarization streaks were observed in the radar data, indicating sufficiently strong electric fields as to orient pristine ice crystals. In several cases, flashes occurred in cells that were located over the Palmer Divide, a topographic feature that may have enhanced updrafts. Similarities among the different cases are described.