In this study, to identify the source of moisture for the widespread and heavy rainfall that occurred on 21 and 22 January 2007 in the western and southwestern regions of the country. To achieve this, we conducted a synoptic and thermodynamic analysis of the rainfall. Daily precipitation data from 45 synoptic stations across 7 provinces were obtained from the Iranian Meteorological Organization and analyzed. First, we identified the rainfall through data from ground stations. Then, using upper atmosphere data and relevant maps, we analyzed the widespread and heavy rainfall in the west and southwest of the country. The results show that on the day of the onset of precipitation, the Sudanese and Mediterranean low-pressure troughs merged on the eastern edge of the Mediterranean, and on the day of the peak of precipitation, the Sudanese low-pressure system independently caused the rainfall of this period. The study of moisture flow maps showed that the Indian ocean and Red, Oman and Persian Gulf Seas played a major role in strengthening and providing moisture to the Sudanese low-pressure system during the occurrence of precipitation. At high levels, on the day of the onset of precipitation, the establishment of blocking in the west of the study area and the displacement of the trough associated with it over the study area and the expansion of the trough axis resulting from it to southern Arabia and the location of the western half of Iran in the east and in front of the trough and its association with low surface pressure have advanced simultaneously. The establishment of the subtropical jet stream on the front of the trough and high divergence played a major role in strengthening the Sudanese low-pressure system, sucking moisture from southern water resources, and as a result, widespread rainfall in the west and southwest of the country.