An active trough of low pressure exited the East Coast of the United States on May 12, and by the following day it passed near Bermuda. Initially the trough was located only near the surface, though as cold air from behind it advanced eastward, the trough extended into the mid- to upper-levels of the atmosphere. By May 14 a closed low pressure area developed in seclusion from the westerlies. Continued cold air from behind the trough, about 12° F (7° C) colder than usual for the time of year, extended across the low in conjunction with warm upper levels and warm sea surface temperatures, resulting in large amounts of instability. The polar trough gradually weakened as the low organized, and on May 15 it developed into a subtropical depression while located about 300 miles (480 km) south of Bermuda.