Easterly Waves: The Negative Impact of Low Relative Humidity in the Mid-Levels
When an embryonic tropical wave entrains mid-level air with low relative humidity, chances are that it will not become a tropical cyclone. That's because mid-level air with low relative humidity entrained into the central-core thunderstorms causes stronger downdrafts to develop (entrainment promotes evaporational cooling that promotes sinking parcels of air to become more negatively buoyant...check out this idealized schematic describing the impact of entrainment of air with low relative humidity.
In turn, stronger downdrafts are able to penetrate into the boundary layer, injecting lower theta-e air (air that's more stable), which, acts to snuff out new convection in the vicinity of the central core. As a result, new convection is forced to develop farther away from the central core. Air pressure then decreases away from the core in response to the convective warming associated with these new thunderstorms. In turn, the pressure gradient decreases across the cloud cluster, thus weakening the already fragile system. The die is now cast and the system fizzles (or at least doesn't develop any further).