Previous studies on the prediction of flight delay focused mainly on delays due to poor flight schedule, operational disruptions and airport capacity. Weather change, however, is often the root cause of flight delay. For the need of smooth operations, it's important for airport operators, airline staffs and air traffic controllers (ATC) to predict flight delays in abnormal weather conditions. In this research, a mathematical real-time delay forecasting model is derived to estimate the delays of each flight, in the cases of runway direction change and airport closing, on the basis of the actual performance of airport operations. To validate the developed delay forecasting model, a SIMMOD-based simulation model was developed, calibrated and validated using ATC data and observed operational data. Compared to the outputs of the SIMMOD model, the average error of the delays derived from the mathematical formulas is only 0.78 minute. The result suggests that the developed formulas can be used to effectively predict flight delays and, with this information, better decisions can be made in advance.