The headwaters of the Rocky Mountains contribute considerable water resources to the growing populations of the western United States. Hydroclimatic variations of the past provide context for the potential ranges of moisture availability in the future. For example, paleoclimatic reconstructions from lake levels and pollen data indicate extremes in moisture availability (both wet and dry periods) in the Rocky Mountains over the past 6,000 years. The concept of a north–south dipole to describe variations of past precipitation within the Rocky Mountains may not capture the range of synoptic processes that can lead to extreme wet or dry conditions. Modern precipitation data are used to identify recent anomalously wet and dry periods in the Rocky Mountains for use as modern climate analogs for past extreme hydrologic conditions. Using the North American Regional Reanalysis data set (32-km resolution), the surface and atmospheric climatic controls of extreme wet and dry periods can be assessed to provide context on the first-order precipitation dynamics that may have contributed to changes in past moisture availability during the Holocene. A hierarchy of controls led to extremes in moisture availability in the region. Large-scale atmospheric anomalies associated with the shape and position of the jet stream influence secondary atmospheric mechanisms like rising (leading to wet conditions) and sinking motions (leading to dry conditions). Simultaneously, small-scale controls such as topography and soil moisture anomalies also influence such extremes. The various scales of climatic controls during modern extreme wet and dry periods can be used as analogs for past extremes to contextualize the range of variability for such events in the past as well as for potential extremes in the future.

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