- There are several ways in which a tropical storm or hurricane can develop in the Atlantic.
- Tropical waves are one of the most common seeds of development.
- Fronts, thunderstorm clusters and non-tropical low pressure areas can also generate storms.
The development of hurricanes and tropical storms in the Atlantic can occur in a variety of ways, with some methods being more common than others.
Here’s a look at five of the seeds of tropical development that meteorologists watch for each year.
1. Tropical Waves
Tropical waves are one of the most commonly observed seeds of tropical development as they move westward from Africa across the Atlantic Ocean into the Caribbean Sea or the Gulf of Mexico.
These are not ocean waves in the sense of the kind you might play with on the beach, but rather areas of rotation, moisture and energy that, with the right mix of atmospheric conditions, can lead to hurricanes.
According to the National Hurricane Center, 60 tropical waves pass over the Atlantic each year. About one in five of these tropical waves develops into a tropical cyclone in the Atlantic basin, and some of them develop into tropical storms or hurricanes in the eastern Pacific. A full 85% of major hurricanes that reach Category 3 or higher begin as tropical waves in Africa.
(IN DETAIL: Tropical Waves 101)
Basic setup for tropical wave tracking through the Caribbean.
2. Dying cold fronts
Tropical storms can sometimes develop when cold fronts dry up in the Gulf of Mexico or western Atlantic, especially at the beginning and end of hurricane season.
The fronts are an immediate source of lift and rotation in the atmosphere, which can create a consolidated low-pressure system. If the low-pressure system develops sustained thunderstorm activity with favorable upper-level winds and warm water temperatures, a tropical depression or tropical storm can form as the front subsides.
Example of a stall front from the Gulf to the southeast coast and possible low pressure areas forming along this front.
3. Thunderstorm complexes
Another source of potential tropical growth is thunderstorm complexes from the southeast and the plains.
These complexes, called mesoscale convective systems, often cross the southern part of the United States early in hurricane season and usually lose strength as they move eastward. Occasionally, they can remain intact and move into the Gulf of Mexico or off the southeast coast with enough low-pressure vortices and stormy weather to produce a tropical depression or tropical storm.
A recent example of this type of tropical development was Hurricane Barry, which hit Louisiana in July 2019.
Barry’s formation was linked to a storm complex that formed in Kansas on July 4 and then moved across the Southeast into the Gulf of Mexico over the course of several days. The National Hurricane Center (NHC) even began monitoring this area of unsettled weather for possible development in the Gulf of Mexico while it was still over Tennessee on July 6.
After reaching the Gulf, Tropical Storm Barry formed on July 11 and briefly intensified into a hurricane upon landfall in Louisiana on July 13.
Track of the thunderstorm complex that became Barry (white line), and the track of Barry after it became a named storm (red line).
4. Central American vortex
Another trigger for tropical development is the so-called Central American vortex.
It is an extensive low-pressure area that, as the name suggests, sometimes forms over Central America and often causes torrential rains there.
Sometimes a smaller low pressure system will solidify and break away from the larger low pressure system or vortex that surrounds it. When this happens, a tropical depression or tropical storm can develop. The development of a tropical depression or storm is most likely at the beginning and end of the hurricane season.
(IN DETAIL: Central American vortex)
Structure of the Central American vortex.
5. Non-tropical low becomes tropical
Another source of development can be cold, non-tropical low pressure areas in the upper atmosphere.
Under the right conditions, these colder lows can sometimes form an area of low pressure near the Earth’s surface and turn into a hybrid subtropical storm. This type of storm gains some energy from just warm enough oceans, as is the case with tropical storms or hurricanes, but also has colder air aloft, usually from an area of low pressure aloft.
Occasionally, they can develop into full-blown tropical storms or hurricanes if thunderstorm activity is persistent enough to heat the entire vertical core of the low pressure system.
An example of this was Hurricane Fay in October 2014, which developed from a non-tropical depression to a subtropical storm and finally a fully tropical storm while circling the Atlantic Ocean. Fay then intensified to a Category 1 hurricane and made landfall in Bermuda just days after being classified as a non-tropical depression.
Satellite image of Fay after its landing in Bermuda on October 12, 2014.
(NASA)
