 The system formed on the northern edge of the monsoon trough south of Guam . Prior to its formation , low @-@ level westerlies were firmly in place south of the 10th parallel eastward to the International Date Line . On July 20 , the monsoon trough segmented , with the middle segment becoming Andy ( Bess formed from the eastern segment ) . On July 22 , the area south of Guam showed increasing thunderstorm activity and organization . That morning , it became a tropical storm . Andy 's center briefly became exposed on July 23 before drifting back under the thunderstorm activity . After looping south of Guam , the cyclone moved northwest and strengthened . Andy turned westward near the 18th parallel on July 25 as the ridge to its northeast strengthened . The system became a strong typhoon for a prolonged period on July 28 and July 29 before striking southern Taiwan . 
