Two successive African wave disturbances from August 2006 are analyzed utilizing observations from NAMMA 2006, the ECMWF reanalysis, and output from a regional model to understand why the first wave does not develop over the eastern Atlantic, while the second wave does. The first wave leaves the African coast on 18 August 2006 and eventually forms Hurricane Ernesto over the Caribbean Sea, but genesis does not occur over the eastern Atlantic. The second wave sheds from the West African coast three days later on 21 August 2006 and quickly intensifies into Tropical Storm Debby just west of the Cape Verde Islands. The occurrence of the waves as well as their development is accurately captured in regional model simulations, allowing detailed thermodynamic and vorticity budget analyses. One reason for the difference in the development of the two waves is the structure of the wave disturbances themselves. The first (non-developing) wave tilts equatorward from the surface to the middle troposphere while the second (developing) wave leaves the African coast with a vertically-aligned center. Another reason for the difference is related to larger-scale environmental conditions, namely the presence of a dry Saharan air layer (SAL) outbreak coinciding with the passage of the first wave. A strong mid-level easterly jet maximum at 15¢ªN leaves the African coast at the same time as the first wave, about 10°Æ of latitude to the north of the wave center at 700 hPa. The associated mid-level anticyclone produces strong vertical and meridional shear near the first wave and is associated with low relative humidity values over the eastern Atlantic. In contrast, when the second wave leaves the African coast, the mid-tropospheric jet streak and dry SAL outbreak associated with the mid-level high pressure has passed to the west, weakening the vertical and meridional shear in low to mid-levels.