Fujiwhara effect
On September 24, ship reports near an area of disturbed weather east of the Lesser Antilles prompted a Hurricane Hunters flight. By the time the aircraft investigated the system, they discovered a tropical storm with winds of , which was named Ilsa by the Weather Bureau. Subsequent analysis estimated that the storm became a tropical depression earlier that day. Ilsa quickly intensified into a hurricane on September 25, by which time it was located about southeast of Hurricane Helene. Over the subsequent few days, the two hurricanes underwent the Fujiwhara effect, in which Ilsa turned to the north and Helene turned to the northeast. Ilsa quickly intensified on September 26, developing a well-defined eye and spiral rainbands. Early on September 27, it reached peak winds of 110 mph (175 km/h), and subsequently it weakened. On September 29 the hurricane turned to the northeast and accelerated, becoming extratropical and dissipating by the next day.