Because the depression was so big, it was quite slow to develop, combined with some easterly wind shear in the eastern Atlantic and because of the Saharan Air Layer to the north as it moved south of the Cape Verde islands. The actuual strengthening was slowed down because convection was very slow. However, on September 13, the depression strengthened into Tropical Storm Helene. The storm slowly strengthened over the warm waters of the eastern Atlantic thereafter as it tracked west-northwest.