At a Glance

Tropical storms in the Atlantic Basin can form in several different ways.Tropical waves are most commonly monitored for development in hurricane season.Fronts, thunderstorm clusters and non-tropical low-pressure systems can also generate tropical storms.

Tropical storms can form in more than one way, and the is a reminder of the multiple ways for tropical development that forecasters have to monitor.

Here's a look at some of the potential triggers for tropical development that we watch for every hurricane season.

Dying Cold Fronts

The early part of hurricane season typically has tropical storms that are born closer to land.

Some of this early-season activity comes from cold fronts stalling out before they eventually fizzle in the Gulf of Mexico or the western Atlantic Ocean.

The fronts are an immediate source of lift and spin in the atmosphere, and that can sometimes give birth to a consolidated area of low pressure. If the low is located in an area with favorable upper-level winds and warm sea-surface temperatures, then a tropical depression or storm can occasionally form as the front dies away.

The system we are watching for development near the northern Gulf Coast this week can partially trace its origin to this type of dying front trigger.

The convergence of surface winds along a stalled front can sometimes generate the development of a low-pressure system that eventually grows into a tropical storm.

Tropical Waves

Tropical waves are one of the most common weather features tracked for tropical development. This is particularly true in the heart of the hurricane season from August into October.

They are discrete areas of horizontal spin in the low- to mid-levels of the atmosphere, which can sometimes form into a tropical storm when conditions in the atmosphere and ocean are conducive. You can see tropical waves on infrared satellite imagery as clusters of thunderstorms originating over northern Africa, which can then push westward into the tropical Atlantic Ocean.

About track across the Atlantic Ocean each year, according to the National Hurricane Center.

Major hurricanes (Category 3 or higher on the Saffir-Simpson Hurricane Wind Scale) are typically responsible for the majority of the damage in any hurricane season. It turns out roughly 85 percent of all Atlantic major hurricanes have origins traceable to tropical waves. About 60 percent of all tropical storms and Category 1 and 2 hurricanes are spawned by these waves.

On the other hand, many tropical waves cross the entire Atlantic Ocean and Caribbean Sea without triggering tropical cyclone development.

Thunderstorm Clusters

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

These clusters, called mesoscale convective systems, often cross through the southern tier of the United States during the early part of hurricane season and lose forward steam as they bubble eastward.

Sometimes those thunderstorm clusters spawn what's called a , or MCV. If the MCV can remain intact and move into the Gulf of Mexico or off the Southeast coast, they can sometimes spin up a tropical depression or storm.

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

Barry's formation can be tied to an MCV that developed in Kansas and then moved across the Southeast into the Gulf of Mexico over the course of several days. The MCV helped lead to the organization of Tropical Storm Barry in the northern Gulf, which then briefly intensified into a hurricane at landfall.

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 atmospheric and oceanic conditions, sometimes these 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 has colder air aloft, usually from an upper-level low-pressure system.

These systems can sometimes transition to fully 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 is 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. and made landfall in Bermuda just a few days after it was classified as a non-tropical low.

Monsoon Trough

Another trigger for tropical development is called a monsoon trough, and they exist in several ocean basins around the world.

One of those monsoon troughs stretches from the western Caribbean Sea into the eastern Pacific Ocean. Surface winds converge in this east-west band located near and on either side of Central America, giving rise to spin in the atmosphere and thunderstorm activity.

Sometimes an area of low pressure will consolidate and break away from the parent monsoon trough. When that happens, a tropical depression or storm can develop.

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