We utilize the adjoint to understand how small perturbations of moisture, winds and temperature evolve into downstream disturbances that impact the wave guide and cyclones. The adjoint diagnostics indicate that the intensity of severe winds and precipitation in these storms just prior to landfall was highly sensitive to the moisture and temperature fields, and to a lesser degree the wind field. Despite large differences in structure and evolution, cyclone development was always critically sensitive to mesoscale filaments of water vapor within the larger-scale plumes of enhanced moisture, often similar to an atmospheric river in the vicinity of the waveguide. We also place the results from several of the storms in the context of a very active waveguide that occurred during December 2013-February 2014, which serves to further highlight the importance of low- and mid-level moisture sensitivity along water vapor plumes. The results of this study underscore the need for accurate moisture observations and data assimilation systems that can adequately assimilate these observations in order to reduce the forecast uncertainties for these high-impact extratropical cyclones. However, given the nature of the sensitivities and the potential for rapid perturbation and error growth, the intrinsic predictability of these severe cyclones may be limited.