How Hurricanes Develop

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A​tlantic hurricane and tropical storm development can occur in several ways, with some methods being more common than others.

H​ere's a look at five of the seeds for tropical development that meteorologists watch for each year.

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1. Tropical Waves

Tropical waves are one of the most common seeds watched for tropical development as they move westward from Africa across the Atlantic Ocean to the Caribbean Sea or Gulf of Mexico.

These are not waves in the ocean in the sense that you can play in them at the beach, but rather areas of spin, moisture and energy that hurricanes can form from under the right mix of atmospheric conditions.

According to the National Hurricane Center, 60 tropical waves track across the Atlantic Ocean each year. Roughly one in five of these tropical waves becomes an Atlantic basin tropical cyclone, and a few of them become tropical storms or hurricanes in the eastern Pacific. A whopping 85% of major hurricanes that reach Category 3 or higher get their start as a tropical wave from Africa.

(​IN DEPTH: Tropical Waves 101)

2. Dying Cold Fronts

Tropical storm development can sometimes occur when cold fronts stall out and eventually fizzle in the Gulf of Mexico or the western Atlantic Ocean. That's especially the case both early and late in the hurricane season.

The fronts are an immediate source of lift and spin in the atmosphere, which can give birth to a consolidated area of low pressure. If the low gains persistent thunderstorm activity amid favorable upper-level winds and warm water temperatures, then a tropical depression or storm can form as the front dies away.

3. Thunderstorm Complexes

Thunderstorm complexes that come from the Southeast and Plains are another source of potential tropical growth.

These complexes, called mesoscale convective systems, often cross through the southern tier of the United States during the early part of hurricane season and usually lose steam as they bubble eastward. Occasionally, they can remain intact and move into the Gulf of Mexico or off the Southeast coast with enough low-pressure spin and stormy weather in order to generate a tropical depression or storm.

A recent example of this type of tropical development was Hurricane Barry, which hit Louisiana in July 2019.

Barry's formation was tied to a complex of storms that developed in Kansas on July 4, then moved across the Southeast into the Gulf of Mexico over the course of several days. The National Hurricane Center (NHC) even began to track this area of disturbed weather for possible Gulf development when it was still over Tennessee on July 6.

Once it reached the Gulf, Tropical Storm Barry formed July 11 and then briefly intensified into a hurricane at landfall in Louisiana on July 13.

4. Central American Gyre

Another trigger for tropical development is called the Central American Gyre.

It's a broad area of low pressure that sometimes sets up, as its name implies, over Central America, where it often produces flooding rain.

Sometimes a smaller area of low pressure will consolidate and break away from the parent larger area of low pressure, or gyre. When that happens, a tropical depression or storm can develop. Tropical development from them is most likely to happen early and late in the hurricane season.

(​IN DEPTH: Central American Gyre)

5. Non-Tropical Low Becomes Tropical

Another source of development can come from cold, non-tropical areas of low pressure in the upper atmosphere.

Under the right conditions, sometimes these colder lows can develop an area of low pressure near the Earth's surface and transition into a hybrid subtropical storm. This type of storm gains some energy from just-warm-enough oceans, like tropical storms or hurricanes do, but also have colder air aloft, usually from an upper-level low-pressure system.

Occasionally, they can become full-fledged tropical storms or hurricanes if thunderstorm activity is persistent enough to warm the entire vertical core of the area of low pressure.

An example of this was Hurricane Fay in October 2014, which transitioned from a non-tropical area of low pressure into a subtropical storm, and eventually a fully tropical storm while it was spinning in the Atlantic. Fay then intensified into a Category 1 hurricane and made landfall in Bermuda just a few days after it was classified as a non-tropical low.