The U.S. Army Test and Evaluation Command (ATEC) Four-Dimensional Weather (4DWX) System is a next-generation meteorological and modeling support system that was developed to support Army research, development, test and evaluation in general and ATEC developmental and operational tests in particular. A key component of the 4DWX systems at Aberdeen Proving Ground's Aberdeen Test Center (ATC), Dugway Proving Ground's West Desert Test Center (WDTC), White Sands Missile Range (WSMR), and Yuma Proving Ground (YPG) is the Penn State/National Center for Atmospheric Research (NCAR) Mesoscale Model Version 5 (MM5). Depending on the range, MM5 is used with three or four nested computational domains to achieve the high resolution needed to resolve the local effects of complex terrain and other variations in land-surface characteristics. The 4DWX systems use MM5 in both analysis and forecast modes. Depending on the typical operational needs of the test center, MM5 is executed once or twice per day to generate 24- to 36-hour forecasts to assist in test planning and conduct. Following each 24-hour period, MM5 is run in an analysis mode in which the model solution is nudged toward the observed conditions. The result is a physics-based analysis of the three-dimensional structure of the atmosphere as it evolved throughout the period, which is archived for climatological and forensic purposes. The most recent 4DWX analysis capability is an MM5 analysis of existing conditions every 3 hours coupled with a short-term forecast, which supports real-time test go/no-go decisions. Whether used in the forecast or analysis mode, MM5 output is used to drive applications models such as a noise propagation model for ATC artillery tests, a dispersion model for WDTC smoke/obscurants and chemical/biological agent simulant tests, and an air drop model which predicts the impact point for an object released at an arbitrary time and location from an airplane. MM5 output also provides realistic synthetic atmospheres to support ATEC Virtual Proving Ground (VPG) modeling and simulation.