The overall structure of Laura remained subtropical but there were indications that it was acquiring tropical characteristics. During the afternoon, satellites revealed that Laura had developed a deep warm core, a characteristic of a tropical cyclone, but the wind field remained subtropical in nature. By the nighttime hours, the radius of the strongest winds contracted to 80 mi (130 km), much smaller than what is typical of a subtropical cyclone. However, Laura was still under the influence of an upper-level low and cloud tops remained shallow. However, the cloud tops became colder overnight and Laura was on the verge of becoming a tropical cyclone by the morning of September 30, but strong interaction with the upper-level low meant it was still subtropical. Despite moving over colder waters, a water vapor satellite image determined that Laura had separated itself from the upper-level low, signifying it had developed into a tropical cyclone. Laura was upgraded to a tropical storm at a relatively high latitude, 41.2°N.