Recently assembled long-term gridded precipitation records for the western U.S. based on station records have revealed three distinct cool season precipitation regimes from 1916-2003 which are associated with observed shifts in the Pacific Decadal Oscillation (PDO) in 1946-47 and 1976-77. The early part of the record from 1916-1946 can be broadly characterized as dry and moderately variable. Wide spread drought was common during this period, but variations from year-to-year were modest. The period from 1947 to 1976 was wetter overall, especially in the Pacific Northwest (PNW) and California (CA), but year-to-year variations were relatively small and extreme high or low precipitation was rare during this period. Starting in the mid 1970's a marked change in cool season precipitation variability is evident. The coefficient of variation increased substantially, the time series displayed more persistence from year to year, and the precipitation variations in different regions in the West became more synchronous. These changes in cool season precipitation are corroborated by observed changes in streamflow variability noted in past studies. This change in cool season precipitation variability occurred at the same time as rapid increases in both global and regional temperatures, but it is not clear that these changes in climate are directly related, nor is there any compelling evidence of consistent trends in cool season precipitation volumes in the West associated with 20th century warming. In contrast to cool season precipitation, warm season precipitation in most of the West has been steadily increasing throughout the 20th century, and systematic changes in the variability of warm season precipitation are not evident.

These observed changes in precipitation variability have a number of important implications for water resources management in the West. Model simulations suggest that cool season flood risks have increased markedly across most of the West since the mid 1970's, for example, and the frequent occurrence of multi-year drought in recent decades has had important implications for water resources management in the West, particularly in CA and the southwest. Long retrospective simulations of hydropower production suggest that increasing synchronicity of precipitation variations since the mid 1970's has increased the West's vulnerability to hydropower shortfalls associated with drought. Increasing warm season precipitation may help mitigate warming-related impacts associated with earlier snowmelt or increases in ET which would otherwise tend to reduce late summer soil moisture and baseflow.