A new 3D Tracer tool is coupled to the WRF model to analyze the origin of the moisture in two extreme Atmospheric River (AR) events: the so-called <q>Great Coast Gale of 2007</q> in the Pacific Basin, and the <q>Great Storm of 1987</q> in the North Atlantic. Results show that between 80 % and 90 % of the moisture advected by the ARs, as well as between 70 % and 80 % of the associated precipitation have a tropical or subtropical origin. Local convergence transport is responsible for the remaining moisture and precipitation. The ratio of tropical moisture to total moisture is maximized as the cold front arrives to land. Vertical cross sections of the moisture suggest that the maximum in humidity does not necessarily coincide with the Low-Level Jet (LLJ) of the extratropical cyclone. Instead, the amount of tropical humidity is maximized in the lowest atmospheric level in southern latitudes, and can be located above, below or ahead the LLJ in northern latitudes in both analyzed cases.