The authors examined whether an initial disturbance and vertical wind shear were necessary factors for the formation of Hurricane Gabrielle (2001). Gabrielle formed a few hundred miles from the Florida coast only three days before making landfall. The formation of Gabrielle was simulated using the fifth-generation Pennsylvania State University-National Center for Atmospheric Research mesoscale model (MM5). The control simulation was compared to observations to establish the closeness of the model output to Gabrielle's actual formation. Additional experiments were subsequently performed to test the response of the developing storm to alterations in the initial conditions. The first additional experiment removed the initial disturbance, while the second reduced the vertical wind shear over and around the area of formation. It was found that when the initial disturbance was removed, Gabrielle did not form. When the vertical wind shear was reduced, a much weaker version of Gabrielle slowly formed. This provided direct evidence of a favorable influence by vertical wind shear in the formation of a tropical cyclone.