Invest 90L in the Western Caribbean Sea Likely to Be a Tropical Storm or Hurricane Threat for Gulf Coast This Weekend

Jonathan Belles
Published: October 4, 2017

A tropical depression is expected to develop later today in the western Caribbean Sea, and is increasingly likely to pose a threat to parts of the U.S. Gulf Coast as a tropical storm or hurricane by this weekend.

(MORE: Hurricane Central)

The disturbance, called Invest 90L by the National Hurricane Center (NHC), is currently located in the southwestern Caribbean Sea off the coast of Nicaragua and is expected to slingshot northward through this week and into the upcoming weekend.

(MORE: What is an Invest?)

The U.S. Air Force Reserve will fly a reconnaissance mission into Invest 90L this afternoon to gather more information about the disturbance's structure and intensity. If a closed circulation of low pressure is found with sufficiently persistent thunderstorms, the system could be classified as a tropical depression at that time.

Nate will be the name given to the next Atlantic tropical storm.


Potential Development Area and Satellite Imagery

Environmental conditions over the western Caribbean Sea and southern Gulf of Mexico are already favorable for development.

Wind shear is currently low over Invest 90L, but shear is expected to climb somewhat in the coming days as the system lifts northward.

Western Caribbean Sea water temperatures are currently in the mid- to upper 80s, about 2 to 5 degrees above average. Temperatures of 80 degrees are generally supportive of tropical storm or hurricane development.

(MORE: Why Floridians Should Worry About the Tropics in October)


Current Sea-Surface Temperatures

In addition to the warm surface temperatures, the warmth runs hundreds of feet deep in the northern and western Caribbean Sea and parts of the Gulf of Mexico, by far the greatest ocean heat content anywhere in the Atlantic basin right now.

This bathtub-like water has been relatively untapped this hurricane season. The last tropical system to pass through this area was the dormant tropical remains of Harvey in late August as it passed through the entire Caribbean, but that system did not use much, if any, heat from this area. Before that, Tropical Storm Franklin formed in the western Caribbean Sea in early August.

(MORE: September 2017 Was the Most Active Month on Record for Atlantic Hurricanes)

U.S. Threat This Weekend

This weekend, upper-level high pressure over the northern Gulf Coast is expected to weaken as a southward plunge in the jet stream carves into the central U.S.

Therefore, the tropical system is expected to be pulled north into the Gulf of Mexico, steered by the combination of upper-level high pressure centered near or east of the Bahamas and what is known as a Central American gyre. More on this gyre is located at the bottom of this article.


Upper-Level Steering Winds For the Weekend Tropical System

It is now likely the system will landfall along the northern Gulf Coast, somewhere between Louisiana and Florida, Sunday. It remains too soon to tell where exactly this landfall will occur. 

Most guidance also suggests this landfall will most likely be as a tropical storm or low-end hurricane, however, intensity forecasts are notoriously tricky this far out in time.

For now, all interests along the U.S. Gulf Coast, particularly from Louisiana to Florida, should monitor the progress of this system closely. We'll have the latest forecast updates here at weather.com and will add details as they become clearer in the coming days.

(MORE: 2017 Already a Top 10 Most Active Hurricane Season)


Model Forecast Tracks

Caribbean, Mexico, Central America Impact

The tropical system will likely track close to, if not over parts of Mexico's Yucatan Peninsula – including Cancún and Cozumel – Friday, perhaps lingering into early Saturday. 

The main impacts, there, will include bands of locally heavy rain, elevated surf, and some stronger wind gusts. 

These Central American gyres are notorious for flooding rainfall over Central America and Mexico, and this one will be no exception.

Areas of locally heavy rain are likely to persist at least into part of the weekend from eastern Mexico into Guatemala, Honduras, El Salvador, Nicaragua, Costa Rica and, perhaps parts of Panama.

This heavy rain may trigger dangerous flash flooding and mudslides, particularly over the mountainous terrain of Central America. 

(MORE: How a Borderline La Niña Could Impact the Rest of the 2017 Hurricane Season)


Forecast Rainfall

What Spawned This? More on Central American Gyres

This possible tropical depression or storm will originate on the eastern end of a larger feature, currently forming in the far western Caribbean, called a Central American gyre.

This "gyre" is a large, broad area of low pressure over the Central American isthmus and western Caribbean Sea. This feature can lead to the development of a tropical cyclone in the Caribbean Sea and/or in the eastern Pacific Ocean.

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 eastern 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.

Satellite images showing the evolution of Nicole from a gyre, Sept. 26-29, 2010.

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, its 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).


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