The twentieth-century pluvial in the western United States

Persistent, widespread wet conditions in the western United States in the early twentieth century have been noted in a number of studies. Here, we investigate the character of this pluvial, which covered a roughly 9-state region and lasted about 13 years. Paleoclimatic data used to evaluate the period in a long-term context indicate that the twentieth-century pluvial is an extremely rare event, as previous studies have suggested, even when assessed in the context of a 1186-year reconstruction of regional drought. An analysis of twentieth-century climate data, characterizing precipitation seasonality, intensity, and frequency, shows that the pluvial was primarily a result of winter season, heavy to moderately heavy precipitation events, during a handful of extremely wet winters. Temperatures were also anomalously cool. The combination of duration, intensity, and spatial extent make this an unusual event, not only in twentieth century, but in the past 12 centuries.