Pat Shingleton: "Line Storms..."
Seafarers believed a major storm would occur around the end of summer, closer to the autumnal equinox in the middle Ages. When the sun crossed the equator or "line," on its journey to the southern hemisphere, its rays would also move in a line across the equator. Sailors would refer to these systems in September and October as a "line storm." The belief that the sun's rays created the storm was in part correct. Cool air from the north collided with super-warm southerly air, activating more tropical storm activity. The traditional period for the peak storm activity is the 60-day interval targeted around September 10. By tradition, the Caribbean becomes active and New England also becomes vulnerable. With Irma and Jose active, possibly there's some validity to this theory.