depression slid slowly northwestward between the contiguous upper vortex and a large high-pressure region over the central Atlantic. A region of lighter wind shear ahead, as well as an anticipated weakening of the upper cyclone, prompted the NHC to introduce the possibility of a tropical transition. Later, satellite observations showed a rapid improvement in the depression's structure; convective banding wrapped completely around the broad inner wind field, which produced gusts to 65 mph (105 km/h). Accordingly, the NHC upgraded the depression to Subtropical Storm Otto at 21:00 UTC on October 6, about 215 mi (345 km) northeast of Grand Turk. Although satellite images continued to display a classical subtropical cyclone, with Otto's center and the vortex aloft nearly collocated, data from a Hurricane Hunters aircraft indicated a weak warm convective core was developing within the mid levels of the circulation—a feature normally present at the upper levels of tropical cyclones. Moreover, a contraction of the large wind field reaffirmed that Otto was entering the final stages of its tropical transition. Although convective activity briefly weakened overnight, weakening shear and considerably warmer sea surface temperatures permitted small patches of thunderstorms to refire on the morning of October 7. As Otto meandered northward, temperature contrasts from forecast models indicated that the warm core within the circulation had ascended to the upper levels of the cyclone. In consequence, a burst of deep, tropical convection with extreme cloud top temperatures of approximately −112 °F (−80 °C) occurred over the center. Having shed the last of its subtropical characteristics, Otto became a warm-core system and was operationally declared tropical at 1200 UTC that day. Over the course of October 8, Otto began to accelerate to the northeast under the increasing influence of a vertically deep trough off the US East Coast. Conditions aloft remained conducive, and the storm's convection deepened symmetrically over the center to form a large, well-defined central dense overcast. By 1200 UTC, Otto's winds had reached 75 mph (120 km/h) while a mid-level eyewall began to form, prompting the NHC to upgrade the storm to a Category 1 hurricane south of Bermuda. Further strengthening ensued as the newly formed hurricane proceeded over the warm waters of the central Atlantic; microwave imagery revealed the eye, though obscured, was vertically well established, marking Otto's peak strength with estimated winds of 85 mph (140 km/h). Upon peaking in intensity, Otto had become fully embedded within