Analysis of synoptic patterns creative showery precipitation (Case Study: rainfall whit amount of 132 mm on 2012/10/13 in Behshahr town)

Showery precipitations are one of the natural hazards that knowing about their occurrence and extent of their effects can help to reducing the environment harms. Iran has the arid and semiarid climate. Low short duration and be showery precipitation are some of the precipitation regime characters in this country. Because of its geographical location in the north of Iran and having the humidity conditions, Behshahr town has the heavy rainfall in every year, especially during the summer and early autumn. The purpose of this study is synoptic survey of Showery rainfall of Behshahr town with 132 mm which has been occurred in 2012/10/13. in this study, the daily synoptic maps of weather at the earth and 500 HP levels on eleventh and thirteenth days of October in 2012 have been used which these maps included the geo-potential height, sea level, omega, relative humidity, and specific humidity maps. The results showed that two types of air masses were effective in occurrence of showery precipitation in study area which included the income European low-pressure and high-pressure systems and troughs arrived from west and southwest to Iran. So, synoptic conditions which are necessary to creation the showery precipitation are advection of cool air over Caspian Sea by high pressure system that is an income European system, ascending the warm and humid air masses by low pressure system income European and addition to, the warm and humid convection of west and Southwest by the troughs.