Upon moving to the northwest of the Leeward Islands, Betsy entered conditions favorable for marked development. An upper-level trough centered a short distance north of the tropical storm enhanced outflow conditions and speed divergence. Under these conditions, Betsy proceeded to quickly intensify, and reconnaissance missions tasked by the United States Air Force and Weather Bureau indicated that the tropical storm had reached hurricane intensity by 0000 UTC on August 30, centered roughly north-northeast of Puerto Rico. By coincidence, forecast responsibilities were transferred to the Weather Bureau Office in Miami, Florida at the same time; however, the hurricane was still under the purview of the Weather Bureau per se. Due to increasing atmospheric pressure heights to the north, Betsy drastically slowed in forward speed and intensification, and remained stationary for a period of time on August 31 before it began to drift westward. On September 2, Betsy began to quickly intensify, and after strengthening to a Category 3 hurricane-equivalent – a major hurricane on the modern-day Saffir–Simpson hurricane wind scale – the small hurricane attained Category 4 intensity and reached an initial peak intensity with winds of 140 mph (220 km/h) at 0000 UTC on September 4, while situated well north of the Turks and Caicos.