Minor changes to seasonal air temperature and precipitation can have a substantial impact on the availability of water resources within large watersheds. Two such watersheds, the north-flowing Mackenzie and east-flowing Saskatchewan Basins, have been identified as highly vulnerable to such changes and, therefore, selected for study as part of the Climatic Redistribution of western Canadian Water Resources project. This project aims to evaluate spatial and temporal changes to water resource distribution through the analysis of a suite of hydroclimatic and streamflow variables. As part of this analysis, dominant summer (May–October) circulation patterns at 500 hPa for 1950–2011 are identified using the method of self-organizing maps. Surface climate variables associated with these patterns are then identified, including both daily air temperature and precipitation and seasonal Standardized Precipitation Evapotranspiration Index values. Statistical methods are applied to assess the relationships between dominant circulation patterns and the Southern Oscillation Index (SOI) and Pacific Decadal Oscillation (PDO). Results indicate that mid-summer (July–August) is dominated by a split-flow blocking pattern, resulting in cool (warm), wet (dry) conditions in the southern (northern) portion of the study area. By contrast, the shoulder season (May and October) is dominated by a trough of low pressure over the North Pacific Ocean. The frequency of weak split-flow blocking is higher during positive SOI and negative PDO, whereas ridging over the western continent is more frequent during negative SOI and positive PDO. Results from this analysis increase our knowledge of processes, controlling the distribution of summer water resources in western Canada. Copyright © 2014 John Wiley & Sons, Ltd.