Dynamic stall dominates the aerodynamic performance, the robustness, and the wake dynamics of vertical axis wind turbines. To better assess the dynamic stall onset and its associated unsteady effects, this paper analyzes experimentally the evolution of the leading edge suction vector on a sinusoidally pitching airfoil based on time-resolved surface pressure measurements and particle image velocimetry. During the dynamic stall stage, we linked the shear layer evolution with the evolution of the leading edge suction. The dynamic stall development prior to dynamic stall onset consists of two stages: a primary instability stage and a vortex formation stage. The transition between the stages is marked by a maximum in the leading edge suction. During the primary instability stage, the leading edge suction increases linearly while the shear layer height with respect to the airfoil's surface also increases linearly. During the vortex formations stage, the leading edge suction decreases linearly while the shear layer rolls up and creates a dynamic stall vortex. The maximum leading edge suction increases nearly linearly with the normalized effective unsteadiness. The leading edge suction at dynamic stall onset appears to be independent of unsteadiness of the pitching motion.