Nowadays, one of the most significant problems is that to recognize how the severity of heavy precipitation and floods may alter in future time in comparison with the current period. The purpose of this research is to understand the impact of future climate change on storm water and probability of maximum flood for future time period. Zayandeh rud river basin in Iran is selected as a case study. Forecast of future climatic parameters based on temperature and precipitation of the upcoming period (2006-2040) is completed with using the HadCM3 model and based on RCP 2.6, 4.5, and 8.5 emission patterns. Also, climate change model is downscaled statistically with applying LARS-WG. In the next step, the probable of maximum precipitation is measured through synoptic method and then, in order to model maximum storm water under the climate change effects, the HEC-HMS for simulating rainfall-runoff model is used. Also, the Snowmelt Runoff Model (SRM) is applied to model snow melting. The results of this research indicate the maximum of probable precipitation in the basin for the period of 2006-2040 under the scenario RCP 2.6, can rise by 5% and by the scenarios of RCP 4.5 and RCP 8.5 can decrease by 5% and 10%, respectively in comparison with the current period 1970-2005.