Rapid intensification (RI) is a form of tropical cyclone (TC) intensification that is particularly challenging to predict. Understanding RI is essential for improving hurricane intensity forecast skill and hurricane preparedness. However, the physical processes that lead to RI are not well understood. Hurricane Earl, the second-strongest TC of the 2010 North Atlantic basin season, underwent RI on August 29, in its first day as a hurricane. The Advanced Hurricane Weather Research and Forecasting (AHW) model produced a simulation of Earl that was unsuccessful in predicting intensification, followed closely by a simulation successfully forecasting RI. The purpose of this study is to investigate the environmental and storm structure characteristics that led to Earl's simulated RI. The unsuccessful and successful simulations were first compared in terms of the environmental vertical wind shear, because this parameter is often negatively correlated with TC intensification. Area averages of the deep-layer and mid-layer vertical wind shear over the storm suggest that this parameter did not influence the intensification of the successful simulation. Initially, the successful simulation featured greater relative humidity throughout the troposphere within the storm's circulation. This led to a greater upward mass flux throughout the troposphere and a rapid intensification of circulation in the middle troposphere before any significant change occurred at the surface. After this deep vortex had been established, the mass flux (and lower-tropospheric convergence) continued to increase, leading to RI. These results provide a basis for further research to better understand and predict the development of RI.