In many systems, the occurrence of nitrification in chloraminated service reservoirs is the first sign of chloramine loss. Once the system is nitrified, recovery is difficult. To prevent the occurrence of nitrification, the risk has to be assessed well in advance. If chloramine residual, temperature and ammonia concentrations are predicted, the biostability concept could be used to assess the risk of nitrification. In this paper, the reservoir acceleration factor (FRa), which calculates chloramine decay rates within the reservoir from parameters measured online, has been used to forecast the most important parameter, chloramine residual. Results showed that FRa better shows the reservoir status than nitrite levels and the errors in the forecast residuals are less than 0.10 mg/l when predicted 10 d in advance. The distinct advantage of this approach is that it utilizes the online measurements as input variables such as temperature, inlet and outlet chlorine levels, hydraulic retention time obtained from reservoir level and flow meter readings. The approach opens an avenue to develop an online risk assessment tool, which will inform the utility of an imminent nitrification episode well in advance. The higher error than the measurement error (Ã‚±0.03 mg/l) could be overcome by improving the model.