An investigation of the spatiotemporal tendencies of the parameters of the probability density function describing precipitation in the last century in the contiguous United States reveals the existence of statistically significant local trends in both space and time. In such a case the distribution describing precipitation at a site is not constant in time, meaning that the population characteristics have not remained the same throughout the century. Here we show that when this variability is taken into account, it alters the effect of El Ni¿o-Southern Oscillation (ENSO) on precipitation significantly and may often lead to erroneous predictions of ENSO-related precipitation patterns. ¿ 2001 American Geophysical Union