El Niño-Southern Oscillation-related hydroclimatic variability in the western United States is characterized by a north–south dipole pattern of precipitation anomalies with opposing signs. Here I use a high-resolution dataset to analyze spatiotemporal patterns in the transition zone between the centers of opposite association. Results indicate that the transition zone is spatially limited west of the continental divide, consisting of a narrow zone within the 40–42°N latitude band and portions of eastern Washington, eastern Oregon, and western Colorado. The transition zone was narrower when the Southern Oscillation Index and the Pacific Decadal Oscillation (PDO) were in a constructive phase and during the negative Atlantic Multidecadal Oscillation phase. Although the transition zone has remained remarkably stationary across the Great Basin, shifts (likely PDO-related) have occurred over time in the West Coast states. Implications for water resource planning in river basins underlying the transition zone vary by region.