Patterns of variability of the evaporative sources of Mississippi River basin precipitation are investigated. Time series of selected regions' evaporative contributions to warm-season precipitation in the Mississippi and its subbasins are produced on a 15-day basis for the years 1963 to 1998. Source region contributions are estimated by backward-trajectory analysis, which tracks water vapor backward in time from its site of condensation to its point of evaporation. The National Centers for Environmental Prediction (NCEP) reanalyses are used for the required atmospheric dynamics and observed data from NCEP Climate Prediction Center for precipitation. Correlation and principal component analyses of the source contribution and precipitation anomaly time series reveal a spatial pattern consisting of internal (Missouri, Red/Arkansas, and Lower Mississippi) and external (Texas/Mexico and Caribbean) sources, which explains statistically much of the interannual variability in warm-season precipitation. Further analysis of the pattern reveals that the external sources play a more important role than internal sources in the anomalies of precipitation. Examining a subset of the period, from 1979 to 1998, moisture flux analysis confirms the role of the external sources by showing the strong association between the precipitation anomalies and flux across the southern boundaries of the basin (which includes the contributions from Texas/Mexico and Caribbean). An index is introduced, based on measurable surface pressure differences between centers over the Atlantic Ocean and central United States (Mississippi watershed). The index is potentially useful in forecasting slow variations of precipitation supply in the basin.