This work examines the origin of atmospheric water vapor arriving to the North American Monsoon (NAM) region over a 34-yr period (1981–2014) by using a Lagrangian diagnosis method. This methodology computes budgets of evaporation minus precipitation by calculating changes in the specific humidity of thousands of air particles advected into the study area by the observed winds. During the NAM wet season, on average the recycling process is the main water vapor source, followed by the supply of moisture from the Gulf of California. However, the water vapor transport that generates synoptic-scale rainfall comes primarily from the Caribbean Sea, the Gulf of Mexico and terrestrial eastern Mexico. An additional moisture source over the southwestern US is also identified in association with synoptic rainfall events over the NAM region. A high (low) moisture supply from the Caribbean Sea and the Gulf of Mexico from 4 to 6 days before precipitation events is responsible for high (low) rainfall intensity on synoptic scales during the monsoon peak. Westward propagating mid to upper level inverted troughs (IVs) seem to favor these water vapor fluxes. A 200 % increase in the moisture flux from the Caribbean Sea is related to the occurrence of heavy precipitation in the NAM area, accompanied by a decrease in water vapor advection from the Gulf of California.