On July 19, 2015, very heavy convective rainfall in a very short period of time led to a terrible flashflood in Kan and Sijan. High intensity rainfall in a small area of about 300 km2 was one of the main causes of this devastating flood. In order to study these convective precipitations, observation data, radar products, meteorological maps were used and the WRF regional forecast model was used to simulate precipitation. The model was implemented with three nested domains with horizontal resolution of 18, 6 and 2 km and 1 hour time steps. The results indicate that the thermodynamic and convective conditions are suitable for the occurrence of severe instabilities in the afternoon of the day (19th July 2015). Also, the trend of changes in instability indices and precipitation resulting from the model output at 12, 13, 14 and 15 Greenwich mean time indicates its maximum value at the time of the flood. The development of low-pressure system in the central regions of Iran and Tehran and the establishment of high-pressure plums on the Caspian Sea, has caused a severe pressure gradient on the Alborz mountain range, which is accompanied by the passage of the through the middle and upper levels of atmosphere and cold weather at these levels. At the time of storm occurrence in the Tehran region, the active cold front adjacent to the ground due to its arrival in Tehran area at the maximum temperature, has been intensified and led to the instability severity enhancement and caused sudden heavy rain (flash flood) in the west of Tehran. With the movement of the system to the east, due to the reduction of temperature gradient and the effects of surface friction, the activity of the storm is reduced.
Key words: Convective rainfall, Atmospheric patterns, Flashflood, Kan and Sijan Regions