During the 20th century, three major decadal-scale moisture anomalies occurred in the western and central USA: an early 20th century period of abnormally heavy precipitation, the Dust Bowl drought of the 1930’s, and the drought of the 1950’s. A coterminous U.S. grid of the Palmer Drought Severity Index (PDSI) is used to map these large-scale moisture regimes from the instrumental record and from a reconstructed PDSI record derived from tree-ring chronologies. The tree-ring record extends to A.D. 1500 and is examined for analogs of the 20th century moisture regimes in terms of similar duration, intensity, and location. The early 20th century pluvial persisted from 1905 to 1923 and is the wettest period observed in the western USA over the 500-year reconstructed record. However, close analogs to the pluvial occurred in the early 19th and early 17th centuries. The Dust Bowl drought was focused in the northern Plains and northern Rockies from 1931 to 1940. No close analogs were found that match the severity of this extreme event, although at least four droughts of similar geographical position and duration were identified in all but the 16th century. The 1950’s drought extended from 1950 to 1957 and was concentrated in the Southwest. Ten droughts of similar spatial coverage and duration replicate the 1950’s drought in the past 500-years. A megadrought occurring in the 16th century far exceeded the severity and duration of the 1950’s drought and may have been the most widespread, severe, and sustained drought of the last 500 years. The megadrought (See Figure showing the peak 10 years of this 21-year drought) rivals and likely even surpasses the severity and duration of the Dust Bowl drought (See Figure).