The North American Monsoon (NAM) system controls the warm season climate over much of southwestern North America. Characterized as a semi-arid environment, understanding of the regional behavior of the hydroclimatology and its associated modes of variability is critically important to effectively predicting and managing perpetually-stressed regional water resources. This work explores the hydroclimatology of northwestern Mexico, i.e. the core region of the NAM, by developing a detailed hydroclimatology from 15, unregulated, headwater basins along the Sierra Madre Occidental mountains in western Mexico. The present work is distinct from previous studies as it focuses on the intra-seasonal evolution of rainfall-runoff relationships and contrasts the sub-regional behavior of the rainfall-runoff response. It is found that there is substantial sub-regional coherence in the hydrological response to monsoon precipitation. Three physically-plausible regions emerge from a rotated Principal Components Analysis of streamflow and basin-averaged precipitation. Month-to-month streamflow persistence, rainfall-runoff correlation scores and runoff coefficient values demonstrate regional coherence and are generally consistent with what is currently known about sub-regional aspects NAM precipitation character.