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A​ large, bloated area of low pressure called the Central American Gyre (CAG) can be an early and late-season source of tropical development, and a relentless faucet of flooding rainfall.

T​hese CAGs can form from May through June, then again most often in October and November following a lull during the summer. More than 40% of years see a CAG form.

T​hey form as the typical east-to-west flow of wind and moisture from the Caribbean to the Eastern Pacific backs up over Central America. This atmospheric constipation slows down flow across the region and tends to collect moisture and spin from southern Mexico to Nicaragua and Honduras.

T​he gyres last on average for three days but can last up to a week.

Dangerous Flood Threat

The biggest impact from CAGs is often very heavy rainfall that can last a week or more. As the large low sits over or near Central America, it wraps deep moisture into Central America from the Pacific, the Caribbean and Gulf of Mexico.

The broader area can pick up several inches of rain, but the rainfall totals can rise as high as multiple feet in areas with terrain.

That could lead to life-threatening flash floods and mudslides.

Why CAGs Matter For Tropical Development

These CAGs can spawn or influence tropical storms in the Caribbean Sea, Gulf of Mexico or eastern Pacific Ocean.

In 2020, one such CAG formed in late May and guided two tropical systems in a counter-clockwise loop.

The larger gyre pushed eastern Pacific Tropical Storm Amanda inland into Guatemala, then steered its remnant into the Bay of Campeche, where Tropical Storm Cristobal soon formed. The CAG forced Cristobal to make a weird loop over southeast Mexico before Cristobal finally turned north toward a Gulf Coast landfall on June 7, 2020.

Roughly 50 percent of CAGs have a tropical cyclone associated with them, Philippe Papin, a hurricane specialist at the National Hurricane Center and expert on CAGs, previously told weather.com.

"When a tropical cyclone does occur, it tends to form on the eastern side of the [gyre] and rotates counterclockwise around the larger circulation," said Papin.

D​eadly Tropical History

The combination of Amanda, Cristobal and the CAG over nine days dumped up to 34 inches of rain in southeast Mexico, 42 inches in El Salvador and 26 inches in Guatemala. Forty-three deaths were attributed to the flooding or landslides in Central America.

In early October 2017, a CAG spawned Hurricane Nate, which made a Category 1 landfall along the Mississippi Gulf Coast.

But the early stages of Nate as a tropical depression and storm, combined with the CAG, soaked Central America. Some parts of Costa Rica picked up 10 to 19 inches of rain.

Flooding and mudslides killed 44 people and destroyed thousands of homes, particularly in Costa Rica and Nicaragua, according to the National Hurricane Center's final report.

In 2005, Hurricane Stan dissipated over the mountains of central Mexico, but its remnant spin became part of a larger gyre that triggered heavy rainfall over Central America.

While Stan's direct circulation resulted in about 80 deaths, severe flash flooding and mudslides from the gyre claimed an estimated 1,000 to 2,000 lives across Central America, according to the National Hurricane Center.