On September 4, the wave encountered a mid- to upper-level trough of low pressure several hundred miles east of the Lesser Antilles. Resistance from the trough split the wave in two; the southern portion proceeded through the Windward Islands, while the northern portion of the wave moved toward the northwest in tandem with the trough. As the trough became negatively tilted, or oriented northwest-to-southeast, strengthening anticyclonic flow on its east side promoted periods of enhanced convection close to the tropical wave. These convective bursts, largely driven by diurnal cycles, led to the development of a broad surface low on the morning of September 12 as the disturbance turned north-northwest through the Turks and Caicos Islands. At 21:00 UTC, the National Hurricane Center (NHC) upgraded the disturbance to a potential tropical cyclone, given that it posed a threat to land but its circulation did not yet meet the organization necessary to designate a tropical cyclone. This facilitated the issuance of tropical storm warnings in the Bahamas. Observations from ships and nearby islands early on September 13 indicated that the low-level center had become better defined, and satellite imagery depicted the formation of a narrow curved spiral band in the disturbance's northeastern quadrant. Based on this evidence, the NHC estimated that a tropical depression formed around 18:00 UTC on September 13, approximately 85 miles (140 km) east of Eleuthera in the Bahamas. It intensified into Tropical Storm Humberto six hours later. The newly formed cyclone slowly turned northwestward as it tracked toward a weakness in the Azores high. Dry air and wind shear imparted by the trough that contributed to Humberto's genesis now inhibited organization, and the tropical storm more closely resembled a subtropical cyclone rather than a fully tropical one during September 14. As the day progressed, reconnaissance missions found a more aligned storm center, an indication that upper-level winds were beginning to relent. Banding features flourished, upper-level outflow expanded, and a formative inner core became apparent. By 00:00 UTC on September 16, Humberto intensified into the season's third hurricane as it temporarily stalled about 175 miles (280 km) east-northeast of Cape Canaveral, Florida. From that point, a broad upper-level trough over the East Coast of the United States steered the cyclone sharply toward the northeast. While moving over sea surface temperatures of around 84°F (29°C), Humberto intensified during the next day.