On 19 September, the National Hurricane Center (NHC) began monitoring the potential for a non-tropical low pressure system to form several hundred kilometres southwest of the Azores, with the possibility of the low gradually acquiring tropical or subtropical characteristics.[5] Three days later, on September 22, a non-tropical low formed along a frontal boundary associated with the remnants of Hurricane Florence about 1,400 km (850 mi) south-southwest of the Azores.[6] The low quickly acquired subtropical characteristics as it meandered over the northeastern Atlantic, and at 15:00 UTC on 23 September, the NHC initiated advisories on Subtropical Storm Leslie.[7]