The summertime sources and transport pathways of water vapor into the interior of southwestern North America are examined in a high-resolution mesoscale model simulation. A complex multi-stage transport pathway is associated with a pronounced diurnal cycle of deep convection and low-level wind systems. Daytime thermal pumping of moisture from the Gulf of California towards the Sierra Madre Occidental (SMO) leads to widespread, deep convection. At night, land surface and atmospheric cooling generate drainage flows down both sides of the SMO. A nocturnal, low-level jet forms above the western foothills and transports moisture north into the Sonoran Desert. Moist air draining to the east mixes with onshore flow from Gulf of Mexico and flows into the northern Chihuahuan Desert. The diurnal flow of moisture up and down the SMO and the formation of nocturnal, low-level jets transport moisture into the core regions of the NA monsoon in NW Mexico.