Pre-Matthew TW 2016

The tropical wave which eventually became Hurricane Matthew

A tropical wave, also known as an open wave to distinguish it from a tropical cyclone, is an area of low pressure that forms in tropical regions, oriented north to south, typically containing heavy rain and thunderstorms. Tropical waves are the source of most strong tropical cyclones in the Atlantic and East Pacific basins. Tropical waves generally move east to west. Most tropical waves have origins in Western Africa's Sahel region, and typically develop between May and November. Tropical waves can carry winds of up to tropical storm force, but lack the closed circulation needed for classification as a tropical cyclone.

Atlantic basin tropical waves

Tropical waves

Tropical wave formation in the Atlantic basin.

Tropical waves are the source of the majority of hurricanes, especially major hurricanes, in the Atlantic basin. Tropical waves in the Atlantic typically develop over central or western Africa from May to November, with a peak in late August and early September. From May to late July, the waves usually struggle to develop in the eastern tropical Atlantic due to cooler sea surface temperatures, increased wind shear, and dry air from the Saharan Air Layer. Also, during this time of year, tropical waves typically exit the African coast at too low of a latitude to quickly develop. However, in August and September (especially from mid-to-late August to mid-September), these waves often develop quickly after exiting the coast of Africa, often developing into Cape Verde-type hurricanes. In rare cases, these waves can develop almost immediately after leaving Africa's coast, such as Hurricane Fred in 2015. By early October, the waves typically are weaker in strength and thus rarely develop far east in the tropical Atlantic.

It is often difficult for models to distinguish which tropical waves develop and which do not.