Where we seek tropical development
Tropical cyclones – tropical depressions, tropical storms and hurricanes – do not form spontaneously. They need a spark to start the convection that starts the process of tropical cyclogenesis – or the organization of thunderstorms into a distinct tropical system.
This spark comes from a larger disturbance in the atmosphere. The sources of these disturbances are diverse and often change depending on the time of year.
The most famous spark are the disturbances that roll over Africa every three to five days – disturbances called African Eastern Waves (or AEW), since the ripple in the 5,000 to 10,000 foot wind field resembles to a wave, but in the atmosphere.
East African surges are the most notorious as studies suggest they account for around 50-60% of named Atlantic storms and are responsible for 85% of our Category 3, 4 and 5 hurricanes.
Overall, however, the East African waves do not become the dominant source of our tropical systems until later in July and especially in August and September.
At the beginning of the season, such as during the first half of June, we often see tropical systems developing closer to the United States in the Gulf of Mexico, in the Caribbean, and even off the southeast coasts.
The most common sources of formation are areas of low pressure at the end of cold fronts that are always plunging south, or, as we have seen in recent weeks and with Tropical Storm Alex in early June, the area of wider rotation between the Atlantic and the Pacific. known as the Central American gyre.
As NHC hurricane specialist Dr. Philippe Papin discovered in his research, this larger Central American vortex is a more significant spark for the May and June systems, but tends to be less active in July and August.
Over the next few weeks, we will begin a period of transition, as we branch out across the Atlantic and the atmosphere provides us with other sources of tropical sparks.
By analyzing where tropical systems formed historically the last week of June and the first week of July, we can get an idea of where these disturbances are coming from.
Off the east coast and in the Gulf of Mexico, we continue to see occasional storms forming from locked fronts or pre-existing rotation in the Central American Gyre.
We are also beginning to see, however, the occasional East African wave forming in the eastern Atlantic.
Off the southeastern United States, we will occasionally find large non-tropical thunderstorm complexes (called mesoscale convective systems) that form over land and emerge over water to become tropical systems.
This scenario occurred in 2014 when Category 2 Hurricane Arthur formed from a large storm complex moving through Georgia and South Carolina.
In general, the greatest threat to the continental United States still comes from systems forming closer to home.
Although about one in five storms form in the Eastern Atlantic once we get to late June and early June (usually East African waves), only one in recent memory – Elsa in 2021 – has actually landfall in the continental United States.
So we’re starting to look to the eastern Atlantic, but the fast-forming house ales are always the most threatening at this time of year.
For now, thankfully, we don’t see anything brewing on the horizon. Some of our longer range models suggest more defined disturbances entering the Eastern Atlantic from Africa in the coming weeks, but nothing exciting so far.
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