Statistical Characterization of Thunderstorm Severity and Associated Rainfall Intensities Over the South South Geopolitical Zone

Severe thunderstorm activity in Nigeria's South South geopolitical zone generates among the highest rainfall intensities observed anywhere in West Africa, creating significant flood, infrastructure damage, and aviation hazard risks. This study statistically characterizes thunderstorm severity and associated rainfall intensities across Rivers, Delta, Bayelsa, Akwa Ibom, Cross River, and Edo states using surface weather observations, lightning data, and sub-daily rainfall records from the period 2018 to 2023. Thunderstorm events were classified into ordinary cell, multi-cell, and organized convective system categories using an objective classification algorithm applied to 30-minute IMERG satellite rainfall composites. Maximum 15-minute and 1-hour rainfall intensities associated with each event category were compiled from automatic weather stations and radar-adjusted gauge estimates. Generalized extreme value distributions were fitted to the resulting intensity datasets to derive updated intensity-duration-frequency (IDF) curves. Results indicate that organized convective systems account for only 18 percent of thunderstorm events but contribute 54 percent of all recorded 15-minute rainfall intensities exceeding 60 millimetres per hour. Lightning flash rate analysis from the NASA LIS/OTD product confirms that the coastal zone between Port Harcourt and Warri experiences the highest thunderstorm electrical activity in the study region. Updated IDF curves for the five largest cities in the zone show return period rainfall amounts 15 to 28 percent higher than values in current engineering design standards. Keywords: thunderstorm, rainfall intensity, IDF curves, South South, convective systems.