Current windage yaw warning methods of transmission lines are generally implemented by on-line monitoring devices, and these methods rarely involve effective evaluation on flashover risk of the air gap between line and tower after windage yaw, which leads to weak applicability and unsatisfactory warning effect in engineering. This paper takes the traditional windage yaw research achievements into account, and proposes a method for windage yaw flashover warning of transmission lines based on numerical weather prediction. Firstly, wind speed predicted in grid is mapped into the tower by using inverse distance weighted interpolation algorithm. Wind speed vertical to line is calculated by using the angle between wind direction and line trend. Then, cup type tower is taken to solve the minimum air gap by rigid rod method as an example. Finally, the flashover voltage of the minimum air gap under standard atmospheric conditions is amended by air density, humidity and rainfall to obtain the flashover voltage under the forecast weather conditions, which is utilized to realize windage yaw flashover warning by compared with the system operating voltage. This method provides a viable and effective risk control measure for windage yaw flashover on transmission lines which are in strong wind area but the on-line monitoring devices are difficult to install on, and it has the vital practical significance on power grid disaster prevention and mitigation caused by windage yaw. This work is supported by Science and Technology Project of State Grid Corporation of China (Real-time Evaluation and Warning of Electrical Reliability for UHV Transmission Line Based on Meteorological Parameters).