The Hurricane and Severe Storm Sentinel (HS3) is a five-year mission specifically targeted to investigate the processes that underlie hurricane formation and intensity change in the Atlantic Ocean basin. HS3 is motivated by hypotheses related to the relative roles of the large-scale environment and storm-scale internal processes. HS3 addresses the controversial role of the Saharan Air Layer (SAL) in tropical storm formation and intensification as well as the role of deep convection in the inner-core region of storms. Addressing these science questions requires sustained measurements over several years due to the limited sampling opportunities in any given hurricane season. Past NASA hurricane field campaigns have all faced the same limitation: a relatively small sample (3-4) of storms forming during the campaigns under a variety of scenarios and undergoing widely varying evolutions. The small sample is not just a function of tropical storm activity in any given year, but also the distance of storms from the base of operations.