A broad circulation of disorganized thunderstorms is affecting much of Central America from Pacific to Atlantic. This is known as a ‘Central American Gyre’ which has been known to spawn tropical cyclones as well as flash flooding and landslides.

Note the low is centred over the SW Caribbean Sea but the envelop of moisture covers a broad area with enhancement on southern flank.

Heaviest rains target the SW exposed mountainous areas of Honduras and Nicaragua.

These CAG are large areas of enhanced convection, are typically fueled by phase 8 and 1 of the MJO and can drop large amounts of rain over an area extending from Pacific to Atlantic, especially with the presence of a southwesterly monsoon flow which connects the tropical Pacific with the CAG.

The topography of Central America and it’s close proximity to some of the deepest bath-tub warm water on earth can lead to catastrophic flash floods and landslides.

Even if a #TC does not form in association with the #CAG, the primary impact will not change.

Monsoonal flow on the southern side of the CAG is associated with enhanced tropical moisture & flooding rains will continue across Central America, the Yucatan & Greater Antilles. pic.twitter.com/2x6sG5iq9e

— Philippe Papin (@pppapin) October 4, 2018

As the entire circulation and it’s embedded mesovortices drift from western Caribbean to Gulf of Mexico this weekend, modeling shows one such mesovortex becoming dominant and possibly developing into a TC.

A Central American Gyre (#CAG) has continued to sit and spin over Central America. Embedded within are multiple mesovorticies forming in response to active convection stretching incipient vorticity.

The system remains primarily a heavy rainfall producer in the short term. pic.twitter.com/L022X1R9oc

— Philippe Papin (@pppapin) October 4, 2018

In contrast, the #ECWMF suggests that one mesovortex on the northeast side of the larger #CAG circulation will become dominant, possibly resulting in #TC genesis as broader circulation reaches the Gulf of Mexico. pic.twitter.com/qNOCAm9CLi

— Philippe Papin (@pppapin) October 4, 2018

According to the current GFS, the low becomes closed and tightens just off Cancun on Monday and intensifies over the GOM with a FL panhandle landfall Wednesday evening.

There are several examples in which tropical cyclones formed within CAG’s.

Nate formed from CAG just last October.

More on Central American Gyres

These gyres most often form in the late spring and early fall, when cold fronts become uncommon in this region of the world. They’re most common in September, but can be a source of tropical storms and hurricanes into November, and as early as May.

We typically see up to two gyres like this one set up each year, and they can spawn tropical storms in both the Atlantic and East Pacific basins, sometimes in each basin at the same time. Not all gyres produce tropical cyclones, but they all produce heavy rainfall.

Roughly 50 percent of Central American gyres have a tropical cyclone associated with them, according to Philippe Papin, Ph.D. candidate at the University of Albany. “When a tropical cyclone does occur, it tends to form on the eastern side of the [gyre] and rotates counterclockwise around the larger circulation.”

Gyre-like tropical systems are much more common in the western Pacific closer to southeast Asia, where the monsoon plays a larger role in the weather.

A notable example of gyre-induced tropical cyclone formation occurred in 2010 when Tropical Storm Nicole formed just south of Cuba from the gyre in late September.

Nicole was a short-lived and ill-formed tropical storm that tried to cross Cuba. It brought heavy rain to the Cayman Islands, Jamaica, Cuba and portions of South Florida.

Hurricane Stan in 2005 is another good example of a hurricane’s interaction with a Central American Gyre, according to Papin.

Following Stan’s dissipation over the mountains of central Mexico, it’s remnant spin became part of a larger gyre that caused heavy rainfall over Central America. While Stan’s direct circulation resulted in around 80 deaths,according to the National Hurricane Center, heavy rainfall resulting from the gyre took more than 1,000 lives across Central America.

Other examples include Tropical Storm Andrea (2013), Hurricane Ida (2009 – assist from the gyre), and Hurricane Patricia (2015 – assist from the gyre, not a direct result).

Tags: Caribbean, Central American Gyre, madden julian oscillation, ocean heat content