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A combined observational and numerical modeling methodology was employed to examine the interaction of large-scale quasi-stationary waves with regional forcing in the drought of 1988 and flood of 1993 in the north-central United States. The study implements and tests a new lateral boundary condition closure approach enabling regional climate modeling sensitivity studies to account for anomalies external to the simulated domain. Thirty-day regional simulations of the continental United States using the MM5 regional model successfully reproduced the anomalous drought and flood conditions. In an additional set of simulations the observed large-scale quasi-stationary wave anomalies in the dynamic and thermodynamic fields were filtered out through adjustment of the observationally based lateral boundary conditions. In these filtered simulations the simulated meteorological fields over the drought/flood areas for both years tended toward those for normal years, suggesting that the proposed filtering technique can be used as a tool to test the regional response to external forcing. Comparisons of the control and filtered simulations illustrated the significant influence of large-scale anomalies on the strength and geographical distribution of the low-level jet and related impacts on rainfall in the central United States. [ABSTRACT FROM AUTHOR]