Not Just The Category: 2 Other Things You Should Know About Hurricanes And Tropical Storms

A hurricane's "category" provides information on its winds, but there are other important factors to keep in mind that can determine a storm's impacts from surge, wind and rain.

The ubiquitous "Category 1, 2, 3, 4 or 5" descriptor associated with a hurricane is based on the Saffir-Simpson Hurricane Wind Scale. This scale developed in the 1970s is strictly a scale of a storm's maximum sustained winds.

While that's important to monitor for the wind damage potential of a hurricane, no single parameter can encapsulate all the impacts of a storm.

Let's delve into two other aspects of hurricanes and tropical storms that are also important, and what you can glean from each of them.

1. How Fast Is It Moving?

In addition to how fast a storm's winds are blowing is how fast the entire storm is moving.

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, acting as its steering wheel.

A storm can slow down if its steering winds collapse or are weak. It can speed up if these steering winds are strong, for instance, when the storm moves far enough north to feel the effects of the jet stream.

An average hurricane's speed in the lower latitudes is 10 to 13 mph, according to NOAA's Hurricane Research Division.

Fast Movers

These lead-foot storms are challenging for meteorologists to stay ahead of and require those in the path to rush their preparations to completion as impacts will arrive sooner.

All other factors equal, the faster a storm moves, the stronger the winds will be to the right of the center's path, since the storm's forward speed adds to its winds in this right half of the circulation.

Faster-moving storms are also able to spread stronger winds farther inland before weakening.

One recent example of a fast-mover was Isaias, which was accelerated by an unusually strong jet stream for early August.

Isaias' damaging winds and tornadoes inflicted $3.5 billion in damage in the Northeast alone. Up to 3 million customers were without power in the wake of Isaias.

Slow Crawlers

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.

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.

2. Size Does Matter

The category also doesn't tell you how big a hurricane is.

Its size – specifically how large its wind circulation extends – is important to determine the magnitude and extent of its impacts.

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.

Winds in 2008's Hurricane Ike were Category 2 at landfall, but 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 while in the Gulf of Mexico generated a record U.S. storm surge of 28 feet along the Mississippi coast.

Hurricanes and tropical storms also tend to become larger as they move farther north, under the influence of stronger upper-level winds.

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 record storm surge along parts of the New Jersey, New York and Connecticut coasts.

But just because a hurricane's winds are strong doesn't necessarily mean it's large.

Hurricane Andrew in 1992 only had tropical-storm-force winds extending up to 90 miles from its center when it made its Category 5 landfall in South Florida.

Hurricane Charley in 2004 was of similar small size when it plowed ashore in southwest Florida at Category 4 intensity, then roared northeast through the peninsula.

Smaller storms like these can produce intense swaths of wind damage close to their center.

They can also rapidly develop more often as their tiny core of thunderstorms concentrates heating near the center. But they can also be more fragile and susceptible to hostile conditions such as wind shear and dry air.

Smaller, more intense hurricanes eventually become somewhat larger by simply replacing their eyewall with a more expansive outer eyewall, making its wind field more expansive.

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.

Put simply, the larger the circles in graphics like that below, the larger the storm.

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.

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

Other key factors, including how fast it moves and how large it is, will determine the ultimate impact the next time a storm strikes.

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