NASA/Dryden Flight Research at Edwards Air Force Base lies in a region of southern California that is affected by weather from the surrounding coastal, desert, and mountainous regions. The terrain, climate, and weather of southern California are characterized by tremendous extremes. For example, encompassed in this region are the highest (Mt. Whitney) and lowest (Death Valley) points in the continental United States. Understanding how the atmosphere and complex terrain drive the local weather, in particular, the wind is very important to flight test operations. One such project that has benefited by the use of a high resolution meteorological model is the NASA X-43A hypersonic research vehicle program. This project is a joint program between Langley Research Center in Hampton Virginia, Dryden Flight Research Center in Edwards California, and some industry. The X-43A employs scramjet technology, is an unpiloted, air-breathing craft which will operate at speeds up to Mach 10. The model employed for this project is the Penn State/NCAR (National Center for Atmospheric Research) MM5 model. This is a "community model" in the public domain, available at no cost, and is used all over the world. The model has an excellent reputation in the atmospheric community and is easily modified for a specific project. The model has multiple nest capabilities, nonhydrostatic dynamics, four dimensional data assimilation capabilities, many physics options, uses terrain-following coordinates, and is portable to a wide range of computing platforms. MM5 is used for a broad spectrum of theoretical and real-time studies from global scales down to cloud-sized scales, including such phenomena as monsoons, cyclones, hurricanes, mesoscale convective systems, land-sea breezes, fronts, mountain valley circulations, and urban heat islands. On June 2, 2001, launch day, the plan was to drop the PegasusŪ rocket from the B-52 bomber off of the coast of southern California, ignite the rocket and boost the X-43A up to approximately 95,000 feet and a speed of about Mach 7. At this altitude, the X-43A would start its engine and travel under its own power for less than 10 seconds at a speed of Mach 7 to Mach 10 covering about 17 miles. The craft would then coast and splash down into the Pacific Ocean. Unfortunately, just after release from the B-52, the rocket fired its engine, began its ascent, and went out of control. Several hundred miles off of the coast of California, the rocket with the X-43A attached had to be destroyed just 51 seconds after being released from the B-52. NASA has a second experimental craft that it is readying for flight in the near future. Along with the meteorological data, the MM5 model was used to analyze the atmospheric conditions surrounding the launch day of the X-43A. These results will be presented in detail. Special consideration is given to the differences between the model results and the preflight atmospheric condition determinations using only rawinsonde and synoptic meteorological data. It is anticipated that in support of the next X-43A launch that the meteorologists at Dryden Flight Research will implement the MM5 in near-real time to forecast the atmospheric conditions.