The main aim of the current paper is to discuss the important effective variables, particularly synoptic patterns, in creating flood in Masoule basin using flood discharge data in Komadoul station during 20 year period (1984-2003). To achieve the aim, daily average and instantaneous maximum of discharge in mentioned station were used. Data accuracy was done using the data for Chomesghal station and technical report from the unexpected events section in the area. In the study period, among the 181 identified flash flood events, 61 events were chosen between years 1996-2003. Then, based on the most effective creator parameters, they were classified into two main groups; events which were due to atmospheric instability and synoptic patterns and those occurred as a result of combination rainfall and short length of main channel. About 51% and 49% of destructive floods in the study area belong to the first and second groups, respectively. Using the synoptic maps of surface level, 700 and 500 HP, and based on the position of cyclone and anticyclone on basin and their movement over the moisture resources, three specified patterns was distinguished. The cyclonic systems come from Mediterranean Sea and Black Sea, while the anti-cyclonic models mainly come from Caspian Sea and rarely from Black Sea. The result of the research also showed that mentioned systems, based on their characteristics or establishment period on the basin, have been made flood with different magnitude. Among the sampled events, 34, 13, and 14 events were adjusted with one, two and three consecutive day rainfall, respectively. From the temporal point of view, distribution of flood shown that most of them were occurred between Sept. and Oct., late winter and early spring.
)