The overall goal of the WRF Model project is to develop a next-generation mesoscale forecast model and data-assimilation system that will advance both the understanding and prediction of mesoscale weather, and accelerate the transfer of research advances into operations. The model is intended to improve forecast accuracy across scales ranging from cloud to synoptic, with priority emphasis on horizontal grid resolutions of 1-10 kilometers. The WRF software has a modular, hierarchical design that provides good portability and efficiency across a range of foreseeable parallel computer architectures. The model incorporates advanced numerics and data-assimilation techniques, a multiple relocatable nesting capability, and numerous state-of-the-art physics options. It is well suited for a wide range of applications, from idealized research simulations to operational forecasting, and has the flexibility to accommodate future enhancements. A research version of WRF is maintained and supported for broad community use, and workshops and tutorials are offered to assist a rapidly growing user base. Numerous real-time forecasting experiments are being conducted to evaluate WRF performance in a variety of forecast applications. In addition, the model is being comprehensively tested for an ensemble approach to be implemented operationally at NCEP.