A method and system for creating permanent lesions in an area of target tissue, such as tissue at or proximate a junction between a pulmonary vein and the left atrium. The method may generally include positioning a medical device in contact with a pulmonary vein ostium, ablating the tissue, and recording a plurality of temperature measurements from one or more of three temperature sensors. The device may include an occlusion element in communication with a coolant source, a first sensor located distal of the occlusion element, a second sensor located proximal of the occlusion element, and a third sensor located in the occlusion element. One or more temperature measurements may be compared with each other to assess occlusion of the pulmonary vein, and/or may be compared with a set of reference temperatures to predict a real-time temperature within the target tissue.