Hurricane Ian A Gulf Coast Danger, Regardless Of Its 'Category' Strength

I​an should become a strong hurricane in the western Caribbean Sea or Gulf of Mexico. B​ut you may have noticed from the forecast path graphic that Ian's winds are then expected to diminish as it nears the Gulf Coast.

Given that, will Ian be that big of a deal once it reaches the Gulf Coast?

Yes, and t​here are important reasons why Ian will be dangerous regardless of what category it is at landfall.

W​hat The Saffir-Simpson Scale Is And Is Not

Y​ou're probably at least somewhat familiar with the Saffir-Simpson Hurricane Wind Scale and its ubiquitous "Category 1, 2, 3, 4 or 5" descriptors.

This scale was developed in the 1970s and is strictly based on a storm's maximum sustained winds.

That's important for the wind damage potential of a hurricane, but no single parameter can encapsulate all the threats of a storm.

Here are two other important factors that will also play a big role in Ian's final impact on the U.S..

1. Size Does Matter

A hurricane's category doesn't tell you how big it is, or in other words, how large its wind circulation extends. That's important to determine the magnitude and extent of its impacts.

Ian's size will also increase by the time it's in the Gulf of Mexico. This typically happens in hurricanes and tropical storms as they move farther north.

And here's the key: Ian will maintain a large circulation even if it loses some wind intensity before landfall.

A larger storm blowing over a greater area of the ocean over the longest time will generate the highest storm surge if all other factors are equal.

That means Ian's storm surge will likely be dangerous, regardless of how strong its winds are at landfall, or what category of a hurricane it is.

Recent history has provided several examples.

Winds in 2008's Hurricane Ike were Category 2 at landfall along the upper Texas coast after dropping from Category 4 levels north of the Caribbean Sea.

However, its giant size in the Gulf of Mexico generated a 15 to 20 foot storm surge that wiped out most structures on Texas' Bolivar Peninsula.

Hurricane Katrina in 2005 weakened to a Category 3 at landfall, but its giant size in the Gulf of Mexico generated a record U.S. storm surge of 28 feet along the Mississippi coast.

Sandy in 2012 had tropical-storm-force winds 1,000 miles wide – the largest Atlantic tropical cyclone on record since at least 1988 – before it produced a record storm surge along parts of the New Jersey, New York and Connecticut coasts.

So how can you tell if the size of a current tropical storm or hurricane is large?

Meteorologists – including those at the NHC – will often note in a forecast whether a storm is large or small. In cases like that, you'll often see graphics of the wind field, such as for Hurricane Irma in 2017 below.

If a large hurricane or tropical storm is headed for landfall, expect its storm surge to be higher and, in general, its impacts to be more widespread.

2. Need For Speed

A storm's forward speed also matters.

You can find out how fast the storm is moving in our graphics, such as in the example below from Hurricane Isaias in 2020. This storm motion is updated with each advisory from the National Hurricane Center (NHC).

A storm's forward speed depends on the large-scale winds surrounding it that act as its steering wheel.

I​n Ian's case, we expect it to slow down somewhat around the time of its landfall.

If a hurricane is out to sea and moving slower, there's more preparation time for areas potentially threatened by the storm.

But when a storm slows down near or over land, its impacts are not only prolonged, but also greatly amplified.

In 2019, Hurricane Dorian exploded into a Category 5 hurricane as it approached the northwestern Bahamas. Then the winds pushing the hurricane forward collapsed.

Dorian's eyewall lashed the northwestern Bahamas for an unfathomable 52 straight hours while at Category 4 or 5 intensity.

The intense winds drove a storm surge of up to 28 feet on Grand Bahama Island, according to the Bahamas Department of Meteorology (BDOM). At least 74 residents were killed in the Bahamas, with damage estimated at $3.4 billion, according to the BDOM.

Then there's a storm's rainfall.

How much rain a storm produces has little or nothing to do with the storm's wind intensity, but rather how fast it moves.

In 2017, Hurricane Harvey slammed into Texas at Category 4 intensity, then took an agonizingly slow meander near the Texas coast for four days.

Up to 60 inches of rain fell along the upper Texas coast, triggering catastrophic flooding. Harvey remains the nation's second-costliest hurricane or tropical storm ($143.8 billion in 2022 dollars) behind only Katrina.

Besides checking for the storm's current forward speed in a graphic, the forecast path graphic could also hint at a stalling storm ahead.

When it no longer resembles a cone, but rather takes on the appearance of a circle, or there's little separation of forecast points, it suggests the storm is expected to slow down or stall. You should expect prolonged impacts from heavy rain, storm surge and winds, as the last two examples illustrated.

A hurricane is much more than its "category" or even the forecast path of its center.

Other key factors, including how large it is and how fast it moves will determine the ultimate impact from Ian, and storms in the future.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.