Evolution, destination and characteristics of three westward propagating storms with associated impacts over Nigeria during August 2017 Atlantic hurricane season

This study investigates three westward propagating convective systems, associated with African easterly waves (AEW), which impacted Nigeria and subsequently migrated into the Atlantic Ocean. Convective variables and rainfall products were examined from National Oceanic and Atmospheric Administration National Center for Environmental Protection/Climate Prediction Center (NOAA NCEP/CPC) and ERA5 reanalysis. Two of the three storms were initiated during the early afternoon hours (between 12 and 13Z) and merged into a larger system during the late afternoon (around 15Z-19Z), consistent with previous studies that deep convective systems are most common in the late afternoon. It was observed that moderate–strong convective available potential energy (CAPE), between 1500 and 2000 J/kg, dominated the environment before and after initiation. One important finding is the presence of strong vertical wind shear northwest of the vicinity where all of the storms initiated. Strong moisture convergence was also observed before and during initiation of the three storms. During its widespread coverage over Nigeria, 3-h accumulated rainfall reached 60 mm for all the three storms, and the ice water content (IWC) of the storms was found to be significantly higher than the liquid water content for all of the three cases. The skew-T diagram indicated that the height of the three systems reached the tropopause and was consistent with the altitudes of the IWCs. Annually, during the Atlantic hurricane season, in July–September, westward propagating storms often resulted in heavy rainfall and associated floods, which frequently caused severe damages to properties and loss of lives in most West/Central African countries.