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Several aspects of the interaction between midlatitude and subtropical systems are investigated using a case study and a potential vorticity (PV) framework. Our case study occurred on 25 November 2009; Jeddah and other regions in Western Saudi Arabia were hit by heavy rainstorms. The analysis of absolute, relative, and potential vorticity implies the significance of the lower level dynamics in the initiation of this case of cyclogenesis. The impact of the severe convective weather process caused more than 90 millimeters of rain to fall in Jeddah in just four hours. The analysis indicates that the heavy rainfall was due to the existence of an upper level cold trough in the Eastern Mediterranean and a warm blocking high situated over southeasternSaudi Arabiaand theArabian Sea. In addition, an evident low level shear line set up in the northwest of Jeddah, and the southeast movement of the shear line caused dynamic lifting and unstable energy release over Jeddah. The water vapor transport occurred primarily below 700 hPa, and a low level jet transported the water vapor from the Red Sea to centralSaudi Arabia. Furthermore, the blocking high in southernSaudi Arabiawas favorable for maintaining water vapor passage for a long time. The topography of Jeddah also played a role in the enhancement of convection.