Heavy and torrential rains are one of the hazardous phenomena of atmosphere which always cause several severe damages over Iran. For this reason in every developmental program their intensity and frequency should be understood and predicted. The main factor to their development and intensification are the atmospheric pressure patterns and motions. Therefore, in order to predict their occurrence, in this study we extracted the highest daily precipitation of days with 90^th percentile and above of the year for the 40 selected stations during the 1980- 2009 period. Of these days we selected the days in which at least 25 percent of the stations had the highest daily precipitation. For the selected days we extracted the 500 hPa and sea level pressure maps. Then we clustered the sea level

maps through the use of factor analysis and cluster analysis. We extracted five pressure patterns for the whole country explaining 77.63 percent of the total variation. In each pressure pattern we divided the daily rainfall intensity of its days by the daily intensity of all selected rainy days. In this way we developed an index which measures the share of each pressure pattern in the hazardous capacity of the country,s torrential rain days.

The results showed that during all of the torrential rain days there was a deep trough in the region between the Mediterranean Sea and Iran at the 500 hPa level which is accompanied with a low pressure center over Iran. The upper level trough brought the atmospheric instability and the low level pressure system brought the moisture from the western and southern water bodies and intensified the upper level instability. But the variations of their location and intensity caused different pressure patterns and rainfall intensities. Thus predicting the position and intensity of these pressure patterns are very important to monitor the torrential rainfall episodes of the country.

The CP3 pattern with 30 percent of the days showed the highest during this pattern the western trough was bifurcated to the west of Iran but merged to the east. Thus tow maxima of intensive rain days were developed over the south and northwest of the country. The surface low center was deepened up to the northeast of the country bringing moister from the southern water bodies to the country. This pattern was dominant through the fall and winter seasons. The CP4 pattern with 12 percent frequency was the least occurring pattern. This pattern caused the heavy rains of the western parts of the country. The very deep trough to the west of Iran cause very heavy rains in the region. The CP1 pattern was strong in the north and northeast of the country where as the CP2 and CP5 patterns were dominated the southern parts of the country.