A weather disturbance known as a 'tropical wave' was detected off the Atlantic coast of Africa on September 25th of this year. Although it sounds like a tropical wave is a surfing term, in fact, it is used to describe a meteorological event where an elongated area of low pressure - an 'atmospheric trough' - forms along a north-south axis. Tropical waves tend to move westerly across the tropics causing cloudiness or storms, but can also lead to tropical cyclones.
This particular event was dubbed 'Invest 97L' at the time - an 'invest' being meteorology-speak for a weather disturbance which is being monitored for cyclone development. As it moved westward over the next three days, Invest 97 reached the West Indies, becoming an organized weather system with sufficient strength to be dubbed Tropical Storm Matthew.