An analysis is presented of the viscous - inviscid interaction region around the tip of a flame spreading over the surface of a solid fuel in a forced laminar high Reynolds number air stream that opposes the flame propagation. Through the interaction, the vaporization of the solid and the thermal expansion of the gas originate an adverse pressure gradient upstream of the flame tip, which leads to a decrease of the shear acting on the small region controlling the flame spread rate. Under certain realistic conditions this adverse pressure gradient may separate the boundary layer upstream of the vaporizing region of the solid, leading to a new mode of flame spread with a higher spread rate determined by the flow in the whole interaction region.