We examine the initial conditions that lead to backscatter plume development in the equatorial F region ionosphere. By analyzing ALTAIR radar data we obtained the following principal results: (1) Prior to plume development, an altitude modulation of electron density contours exists in the bottomside F layer. These quasi-periodic variations were mapped spatially over a 1200-km east-west distance by using incoherent scatter measurements and found to consist of three cycles with an average spatial wavelength of 400 km. (2) The wave structure observed in the bottomside electron density contours grew in amplitude at a rate consistent with that predicted by the generalized gradient drift instability. (3) Bottomside backscatter developed on the west walls of all three altitude-modulated regions followed by plume development from near the crests of these regions.