We investigate the atmospheric origins for the regional hydrologic variations in the three key river basins draining along the North American west coast – Fraser River Basin, Columbia River Basin, and the Sacramento-San Joaquin River Basin, emphasizing the changing character of their co-variability on decadal and longer time-scales and the role of progressive tropical oceanic warming. Observed extratropical climate variability and the hydrologic impacts are examined in the context of: (a) the changing amplitude and spatial scales of atmospheric teleconnections related to slow oceanic changes during the late 20th century, and (b) changes in the ENSO-related sensitivity of the nature and spatial coherence of hydroclimatic extremes in the western North America. From the standpoint of assessing the key stressors, and possible adaptation to changing water supply, this work provides: (a) a diagnosis and assessment of the synchronous and asynchronous wet and dry periods across the three basins, that exemplify increased vulnerability for hydropower production and agricultural water supply, and (b) the possible role of Indo-Pacific ocean precursors in determining the spatial scales of winter precipitation response over western North America.