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Irma is a rare hurricane. Here is why it is hard to figure out where it is heading

  • Hurricane Irma is an unusually strong storm.
  • Such storms are rare for a few simple reasons.
  • Still much uncertainty over the track it will take.

Hurricane Irma is already setting records.

An advisory sent out from the National Hurricane Center on Tuesday morning set the storm's initial intensity at 155 knots, making Irma "the strongest hurricane in the Atlantic basin outside of the Caribbean Sea and the Gulf of Mexico in the NHC records."

As it continues to grow in size and intensity, much uncertainty still remains over if and how it will impact the coast of the Southern United States.

Right now there are several potential paths Irma could take as it nears the tip of Florida and the Southern United States. As with all forecasts in meteorology the degree of uncertainty grows the further out the forecast extends, and if Irma ends up affecting the southern coast of the United States, it will not do so until the weekend.

NASA Sees Irma Strengthen to a Category 5 Hurricane.
Source: NASA
NASA Sees Irma Strengthen to a Category 5 Hurricane.

Right now, forecasters are pretty certain the storm will track either over the Greater Antilles — the group of larger Caribbean islands that include Jamaica, Puerto Rico, Cuba, the Caymans and the island that is home to Haiti and the Dominican Republic — or over the islands just to the north, such as Turks and Caicos and the Bahamas.

"And this is critical," said Neal Dorst a research meteorologist in the hurricane research division at the NOAA's Atlantic Oceanographic and Meteorological Laboratory. "If it goes over the larger islands it is going to be disrupted, because you have mountain chains and things like that, that tend to affect these storms."

There also is a low-pressure trough moving from West to East across the central United States, which could force Irma to make a northeastern turn.

"But that happens to be more than five days out," Dorst said.

Why Category 5 hurricanes are so rare

With speeds around 185 mph, Irma is a Category 5 hurricane on the Saffir-Simpson scale, a commonly used scale that attempts to measure potential property damage from storm winds.

A Category 5 storm has to have winds of at least 155 mph, and they are relatively rare. With winds that strong, there is a high likelihood that framed homes could be destroyed and damage to trees and power poles will result in power outages that last for weeks.

Out of about six hurricanes that form in the Atlantic every season, only about two to three are major hurricanes, with a Saffir-Simpson rating of 3 or above.

"Easily less than one of them is a Category 5," Dorst said.

There are three main conditions that have to be present in order for a hurricane to grow and sustain itself: warm water, moisture in the mid-atmosphere, and the absence of vertical wind shear, which is when winds at different altitudes vary in speed.

Sufficiently warm water is typically abundant across the tropical Atlantic during hurricane season, but wind shear and atmospheric moisture are far less consistent.

Vertical wind shear can essentially chop a storm up into pieces — hurricanes are cyclones, and they need to maintain a certain kind of vertical structure. If winds at the top and bottom of the storm are hitting the storm at different speeds, it can become too difficult to maintain their shape.

If a fast-moving wind were to hit the top of the storm, for example, it could tilt the cyclone to a less efficient angle, or break up the storm entirely.

There is also some evidence that dry air can help suppress the development of a storm if it gets into the core, called the eyewall. The dry air is thought to suppress the process of convection — the rising of warm moist air within the eyewall that fuels the storm's growth.

But Irma is "expected to remain within low vertical wind shear, a moist mid-level atmosphere, and high upper-ocean heat content as it moves west-northwestward during the next several days," according to a forecast issued Tuesday by the National Hurricane Center.

This will allow the hurricane to stay intense over the next several days, the NHC said.

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