Decade-scale precipitation increase in Great Plains at end of 20th century

During the 1980-1999 period, many regions in the central and southern Great Plains experienced the longest and strongest increase in average annual precipitation of the century. The size of the increase ranged from 6 to 12% of mean annual precipitation and from 25 to 60% of interannual precipitation variability. Precipitation increased for dry, average, and wet years, though not equally for each category. Generally, precipitation in very wet years increased less than in average and dry years. The probability of occurrence of dry years was greatly reduced compared to earlier in the century, whereas the probability of average years remained about the same and the probability of wet years increased. The observed decade-scale precipitation increase is attributed to a reduction in the number and severity of dry years, as well as to an increase in the number of wet years, though very wet years did not increase as much. The seasonal distribution of the increase in precipitation showed no statistically significant prevalence during any one particular month, yet qualitative considerations suggest that early summer and autumn months capture more of the annual precipitation increase. In the northern and northwestern Great Plains, a similar precipitation increase, but of smaller proportions, was observed during the 1990-1999 period.