Convective lines with parallel precipitation develop in environments with significant line-normal low-level wind shear, but also significant along-line environmental shear throughout the troposphere. Such systems appear to be capable of very rapid upscale growth because, as hydrometeors are advected along the convective line, the surface cold pool also extends rapidly along the line owing to the local production of outflow (as opposed to advection of the cold air). Notably, the along-line flow within the convective line is considerably weaker than that of the environment. The velocity of this air that has been processed by the convection, and not the environmental storm-relative flow, is actually responsible for the line-parallel transport of hydrometeors (and development of the PS region). Indeed, as time passes, the along-line shear within the convective line decreases. Because PS systems produce most of their outflow in very close proximity to the surface outflow boundary, their cold pools may also strengthen quite rapidly. Intensification of the outflow, along with the diminished line-parallel shear within the line, implies that such systems will commonly evolve toward a trailing precipitation structure in time.