A linear two-layer model is used to elucidate the role of prognostic moisture on quasi-geostrophic (QG) motions in the presence of a mean thermal wind. Solutions to the basic equations reveal two instabilities. The well­documented baroclinic instability is characterized by growth at the synoptic scale (horizontal scale of~ 1000 km) and systems that grow from this instability tilt against the shear. Moisture-vortex instability -an instability that occurs when moisture and lower-tropospheric vorticity exhibit an in-phase component- exists only when moisture is prognostic. The instability is also strongest at the synoptic scale, but systems that grow from it exhibit a vertically-stacked structure. When moisture is prognostic, baroclinic instability exhibits a pronounced weakening when the thermal wind is easterly. On the other hand, moisture-vortex instability is strongest in this mean state. Based on these results, it is hypothesized that moisture­vortex instability is the dominant instability in humid regions of easterly thermal wind such as the South Asian and African monsoons.