The dominant instabilities are examined for Rossby numbers between 10 and 100, and transitional Froude numbers centered about unity. This parameter regime is applicable to tropical cyclone (TC) perturbations with vertical lenghtscales comparable to the depth of the vortex. As the Froude number increases from its lower bound, the main cause of instability typically transitions from VR-VR wave interaction to IG wave radiation. The transition begins abruptly at a critical point where the two most unstable modes switch their order of dominance. In some cases, growth of the newly prevailing mode is almost completely driven by radiation. In other cases, the Froude number must continue to increase before radiation thoroughly controls the growth mechanism. Radiation driven instabilities at Froude numbers of order unity are shown to be quite fast and potentially relevant to real-world TCs. Critical layer stirring generally modifies the growth rate to an extent that cannot be ignored. The direction and magnitude of the modification are sensitive to subtle aspects of the potential vorticity distribution.This work is supported by NSERC and NSF.