The impact of HDOB data on hurricane forecasts is determined through numerical experiments using the 3D and 4D variational versions of the NRL Atmospheric Variational Data Assimilation System (NAVDAS and NAVDAS-AR), the Coupled Ocean-Atmosphere Mesoscale Prediction System-Tropical Cyclone (COAMPS-TC), and the Navy Operational Global Atmospheric Prediction System (NOGAPS). The numerical experiment is set up to use both the mesoscale and global models to forecast the tracks and intensities of tropical cyclones from the 2010 hurricane season, with particular focus on Hurricane Earl from that season. Experimental runs that include the additional HDOB data are compared against control runs without the additional HDOB data. The adjoint of the numerical models is used to determine the observation impact of HDOB and its benefit to the forecast. Preliminary results for Hurricane Earl show that HDOB data are beneficial to forecasts and reduce intensity and track errors, particularly as the storm begins its extratropical phase.