A tropical wave moved off the coast of West Africa into the tropical Atlantic on September 12, producing disorganized showers and thunderstorms, but doing so in an environment just marginally favorable for development.[135] By September 15, the wave had moved generally westward to about midway between Africa and the Lesser Antilles.[136] There the showers and thunderstorms continued within the northern portion of the wave.[137] The disturbance then transitioned into a trough of low pressure on September 17, as it moved slowly northward.[138] After showing little signs of development over the next two day, a low-pressure formed within the trough and convection began to organize around it.