Instability of Synoptic severe along with abundant moisture has a major role in flood event. At the present study, the synoptic situation of the Sarbaz flood basin has been investigated. For this purpose, two databases were needed. First one contains daily discharge data of Pirdan hydrometric station and daily rainfall census in neighbor stations of basin and second database include maps of ground surface and 500hp level in confine of latitude 10° till 60° N and longitude 10° till 90° E that are prepared from the National Oceanic and Atmospheric Administration (NOAA). Because of the flood type of this watershed in the summer and winter season, strongest floods for each season are selected. Thus, provided maps are belonging to two days before flood, start day of flood and maximum of flood. Results showed that in the ground surface of summer type, establishment of two low pressure centers on Persian Gulf and Pakistan, spread of Persian Gulf low pressure flow to south-east of Iran and entrance of its warm and wet air into the low pressure of Pakistan, deepening of west troughs of Caspian sea and presence of low pressure pitch of Pakistan in front of this troughs were causing instability. establishment of high system of Iran and spread of its prongs, as rich, to north Caspian Sea and forming of deep troughs between high systems of Iran and low pressure in the west of India at 500hp level has caused instability in south-east of Iran that is located in front of troughs. In the ground surface of winter type, continuum of low pressure from east Africa to north-east of Pakistan with southwest-northeast direction was formed. Troughs that are emanated from the Aral Lake high pressure and prongs of high pressure in west of Caspian Sea have main roles in instability and control of this continuum path. In 500hp level, establishment of deep trough in west and center of Iran and presence of low pressure systems of ground surface of southeast of Iran in front of this trough have intensified instability.