It has been shown by many previous researches that an abrupt increase of precipitation over the North American Monsoon (NAM) region from June to July is accompanied by a decrease in precipitation over the Great Plains for the same period. The mechanisms underlying this apparent out-of-phase relation between NAM and the Great Plains precipitation are not clear. In particular, the moisture sources for both the NAM precipitation and the Great Plains rainfall are not entirely understood. In this study, the North American precipitation for two periods, from 1998 to 2000 and from 1982-1988, is simulated using a regional climate model (RCM) with prescribed lateral boundary conditions from the NCEP/NCAR reanalysis. The regional climate model has 30 km horizontal resolution and 23 vertical levels. The RCM underestimates the overall amount of NAM precipitation, however, the out-of-phase rainfall pattern between NAM and Great Plains in terms of July and June rainfall difference is well simulated. We are currently analyzing the difference in moisture transport between June and July in terms of the integrated moisture transport in the upper, middle and lower layers. The focus is on whether the seasonal variation in rainfall over NAM and the Great Plains is a result of the changes in moisture transport pattern, if so, which part of the moisture transport is responsible for the out-of-phase relation between NAM and the Great Plains. RCM experiments with ECMWF reanalysis as lateral boundary conditions will be carried out to test the sensitivities of our results.