Climatological Characteristics and Objective Prediction of Thunderstorms over Alaska

Abstract

Archived lightning data from the Bureau of Land Management automated network of direction-finding stations in Alaska were examined to determine the seasonal, diurnal, and spatial distributions of cloud-to-ground lightning, including the effects of the underlying topography on the timing, location, and magnitude of the lightning activity. The interior of Alaska was found to exhibit a continental type climate with a pronounced afternoon maximum in lightning activity and no significant nocturnal activity. Over 90% of the strikes were found to occur during June and July, the time of maximum solar insulation. A relatively high incidence of positive flashes was found during all phases of thunderstorm occurrence. High flash accumulations over mountainous regions clearly revealed the affinity of lightning activity for elevated terrain. Maximum activity was found to occur earliest in the higher elevations, moving to lower elevations later in the day. Linear screening regression analysis and the Model Output Statistics approach were used to statistically relate lightning data to Nested Grid Model forecast fields. Operational equations were derived to provide objective thunderstorm probabilities for use as guidance by forecasters in estimating the potential hazard of wildfire initiation in Alaska. The primary prerequisite for the formation of thunderstorms over Alaska was found to be large-scale static instability followed by secondary contributions from local convergence in the wind and moisture fields. Low-level moisture convergence was also found to be very important to the production of high flash accumulations on any given day.