The specific mechanisms that produce lightning have been the subject of countless studies and research projects. Theories regarding the near-storm environmental conditions favoring lightning production have been proposed and tested with reasonably good results. The most prominent indicators of potentially high lightning activity include sufficient available moisture (to ensure the presence of graupel essential for electrification) and instability (for vigorous updrafts and rapid charge separation). However, even with this understanding, attempts to operationally forecast lightning activity and density in the 6 to 24 hour time frame have been limited. Typically, numerical weather prediction (NWP) models forecast these ideal precursor conditions with a reasonable degree of accuracy; however, they may not necessarily predict the specific characteristics within and around particular storms. Additionally, NWP models often have difficulty precisely predicting convection, including its initiation, movement, timing, and extent of coverage. These along with other factors present significant challenges to forecasters attempting to predict lightning activity.
During the summer of 2008, the NWS Forecast Office in Raleigh, North Carolina tested a method for forecasting the potential for extreme amounts of lightning over central North Carolina. An operational checklist, consisting of ten key parameters drawn from past studies of lightning production and extreme lightning events, was created. Checklist parameters include the most unstable Convective Available Potential Energy (CAPE), CAPE in the -10C to -30C layer [to assess buoyancy in the mixed phase layer (Deierling et al. 2006)], normalized CAPE [to estimate potential updraft strength (Blanchard 1998)], and two lightning predictors provided by the NWS Storm Prediction Center. Thresholds for each parameter (low, medium, and high) were assigned based on results presented in peer reviewed research papers and presentations, as well as on qualitative observations during extreme lightning events in central North Carolina in 2007. This checklist served as the basis for the daily lightning predictions. From June through August 2008, forecasters on the overnight shift filled out this checklist each day and evaluated the results. On days when the majority of the checklist parameters were in the “medium” or “high” categories and the forecaster determined that extreme lightning had a good chance of occurring, the potential for exceptional amounts of lightning could be mentioned in the early-morning Hazardous Weather Outlook product.
The advantages of a multi-parameter method with forecaster input versus a single model approach are numerous: a poor forecast by a single model does not necessarily doom the entire lightning forecast, as would be the case with an automated single-model forecast; the forecast can be based on the best model, rather than always relying on one particular model; and the forecaster can weigh each parameter differently based on the particular weather situation and his/her heuristic experiences.
A case example in which the checklist correctly indicated the potential for storms to produce high-density lightning will be presented. Future plans for this project include verification of the checklist against observed lightning strikes in central North Carolina, toward the goal of improving and refining the checklist methodology.