Introduction :
Synoptic systems control precipitation patterns. One of the synoptic systems aspect that could determines the rainfall amount and spatial distribution, is geographical distribution of synoptic patterns. Moreover , changes in synoptic patterns in each region could explain daily weather conditions. Identifying synoptic systems in each region could provide a clear picture of that region climate. Climate as one of the basic structure of the environment has direct interaction with human life and activity. Identification of atmospheric circulation in each region determines the dominant weather patterns that help to understand the biological conditions in that region .
In present study, the behavior of synoptic patterns corresponding ? with precipitation during September months among 30 years ( 1976 – 2005 ) in Guilan province is addressed . Guilan is located in north of Iran. It is one of the wettest regions in the country. Where the most precipitation occurs at the second half of September .

Methodology :
In order to achieve a comprehensive view of daily synoptic conditions with heavy precipitation in Guilan, lowest levels including Sea level pressure, wind advection, humidity advection in 850 and 700 (hPa) heights are analyzed. The framework of atmospheric investigation has been chosen between 0 to 70 degree in Eastern hemisphere and 10 to 60 of Northern hemisphere. This area includes all possible systems that might affect southern Caspian coast. This area has the dimensions of 29 × 21 pixels and has been covered by 609 cells. The period under investigation is the months of September during 1976-2005.
The desired atmospheric data have been acquired from the National Center for Environmental Prediction/National Center for Atmospheric Research ( NCEP / NCAR ) in 6 - hours ( 00 : 00 , 06 : 00 ,12 : 00, 18 : 00 GTM ) and 2.5 × 2.5 degree resolution. Daily precipitation data from 23 stations have been chosen from IRIMO (Islamic Republic of Iran Meteorological Organization) and MOP (Ministry of Power).
Percentile method has been used for extraction of days with heavy rainfall in September . The relationship between synoptic patterns and heavy rainfall in September has been calculated based on Pearson correlation coefficient. Based on Lund correlation method, circulation patterns of sea level pressure patterns have been classified. 72 days out of 77 days of heavy rains during September were classified. The remained 5 days were unclassified, because they were abnormal in compare with the period under investigation. The patterns of daily mean sea level pressure, 850 and 700 hPa height and advection convection, humidity advection and daily precipitation maps were drawn for each pattern.

Results and Discussion :
The result of this paper showed that the systems during heavy rainfall occurrence in Guilan are high pressure on the North-West of Black Sea, Northern Russia, and North West Europe as well as on the Norwegian Sea, North Sea and West Black Sea - Mediterranean Sea. Humidity advection shows that the increasing in humidity in all patterns occurred all over Guilan. In all patterns , presence of deep trough and its front axle is appeared. Thus, convection conditions at the surface, could justify instability as well as heavy rainfall in the region. In some patterns, the wind advection was cross Caspian Sea through Black Sea. Therefore, in addition to Caspian Sea, Black Sea was involved in supply of heavy precipitation of Guilan. In all under investigated patterns, most of the produced precipitation among the coast is notecible in compare to the southern regions.

Conclusion :
The results of the five patterns showed that high-pressure systems (North West Black Sea, Northern Russia – sub polar, North West Europe - the Norwegian Sea, North Sea and West Black Sea - Mediterranean Sea) are appeared when heavy precipitation took place. The trough formation in upper levels, fluxes high-latitude cold air toward Caspian Sea and Black Sea as well as front trough axis location over Guilan, provided a convection conditions on the surface made suitable conditions for instability and occurrence of heavy precipitation. In some patterns, in addition to the moisture of the Caspian Sea, Black Sea was involved in the heavy precipitation.

Keywords : Correlation , Sea Level Pressure , Heavy Precipitation , September , Guilan Province .