The Probability of Severe Weather in Storms in Which Most Cloud-to-Ground Flashes Lower Positive Charge

Early studies of storms which produce cloud-to-ground lightning dominated by flashes lowering positive charge to ground (+CG flashes) reported that they often produced large hail or tornadoes. However, subsequent studies using a threshold of 20-40% for the ratio of +CG flashes to total CG flashes found no significant relationship to severe weather. We gridded 11 years of data from the National Lightning Detection Network to count +CG and –CG flashes having estimated peak currents ≥15 kA in grid cells with dimensions of 15 km x 15 km x 15 min. These grid dimensions were chosen because 15 km corresponds roughly to the horizontal dimension of typical thunderstorm cells and 15 min is roughly half the typical duration of a cell. Grid spacing was stepped in space every 5 km along both x and y over the entire contiguous United States and was stepped in time every 5 min. To focus on storms dominated by +CG flashes, we identified all grid cells satisfying one of four sets of thresholds: cells in which +CG flashes constituted ≥80%, 90%, or 100% of ≥10 CG flashes per 15 min or constituted100% of ≥20 CG flashes per 15 min. These thresholds are much larger than those used in most previous studies of storms that produce +CG flashes.

We previously reported that most storms satisfying our thresholds were in a swath from Kansas and eastern Colorado roughly northward through Nebraska, the Dakotas, and Minnesota. The environmental properties we found to be associated with +CG domination in this region are properties often associated with severe storms. For example, the mean 0 – 6 km environmental shear and mean storm-relative winds at the equilibrium level were greater in these storms than in storms with –CG domination. However, different combinations of conditions appear to be conducive to +CG-dominated storms in different regions. For example, in the southern part of this region, the mean cloud-base height of +CG-dominated storms was higher and mean warm-cloud depth was shallower, as others have reported, although their mean instability was less. In the northern part of this region, the reverse was true: mean cloud-base height was lower, mean warm-cloud depth was greater, and mean instability was greater in these storms than in storms with –CG domination. In this study, we examine whether storms satisfying the above thresholds for +CG flashes tend to produce any particular type of severe weather and whether the relationship to either the probability of severe weather or to the type of severe weather that is favored varies from region to region.