Although it is centered in northwestern Mexico, the Mexican monsoon also has been shown to impact portions of the southwestern United States. To provide more information about the spatial distribution of monsoonal impacts in the Southwest, this study employed multiple linear regression modeling to reduce local topographic impacts on monsoonal precipitation to reveal intense monsoonal activity within the Gila River basin. The precipitation data were daily precipitation totals from 115 stations from June 16--September 15 of 1996--2002. An intense monsoonal zone was found in the south-central portion of the basin. Therefore, intense monsoonal activity associated with the Sierra Madre Occidental in northwestern Mexico extended into south-central and southeastern Arizona but not into New Mexico.