Analysis of Extreme Rainfall Event Resulting in floods on June 28th 1394 in Alborz Province

Flash floods caused by short heavy precipitation are one of the most important natural hazards. The main objective of the study is to provide a synoptic and thermodynamic mechanism analysis of the severe rainfall leading to floods on July 28th, 1394, in Alborz province. According to Sijan and Kondor rain-gauge stations as well as Karaj synoptic station, precipitation features and the occurrence of rainfalls in the warm season were determined. Furthermore, anomaly data, geopotential heights, vertical velocity (omega), and vorticity in the days prior to the precipitation at 500 and 850 hpa and sea level were determined and drawn based on GRADS. The instability indexes and the Skew-T thermodynamic graph were analyzed based on Tehran synoptic station. The main synoptic features of the rainfall include the cold weather in the upper atmosphere, creation of quasi-stationary trough, and surface earth low-pressure, injection and continuous supply of moisture from Caspian Sea and northwestern streams. The dominant pattern at sea level before and during the precipitation was thermal low pressure in Pakistan. The vertical velocity (omega) at the time of the rainfall strengthened  and more than 500 hPa and 850 hPa was observed. On the day of the precipitation, the amount of positive vorticity reached +9 due to the movement of the upper atmosphere meridional flows. The rainfall with Showalter Index in severe unstable conditions as well as Precipitation Index enjoyed the necessary water vapor for heavy rainfall in the region. High temperature difference between the upper level in atmosphere and ground surface during warm seasons causes heavy precipitation. Hence, such knowledge could be reliable source for flood warnings before the occurrence of the crisis.