- A tropical storm or hurricane is much more than its category suggests.
- Its forward speed is important for the timing and magnitude of impacts.
- Its size also affects the extent of impacts and the severity of the storm surge it will generate.
A hurricane’s “category” provides information about 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 the maximum sustained winds of a storm.
While it is important to monitor the wind damage potential of a hurricane, no single metric can sum up all of a storm’s impacts.
Let’s look at two other aspects of hurricanes and tropical storms that are equally important, and what you can learn from each.
1. How fast is he moving?
In addition to the speed at which the winds of a storm are blowing, there is the speed at which the entire storm is moving.
You can find out how fast the storm is moving in our charts, like in the example below of Hurricane Isaias in 2020. This storm movement is updated with each advisory from the National Hurricane Center (NHC).
The speed of a storm’s progress depends on the large-scale winds surrounding it, acting as its flywheel.
A storm can slow down if its guiding winds collapse or are weak. It can accelerate if those guiding winds are strong, for example, when the storm is moving far enough north to feel the effects of the jet stream.
The average speed of a hurricane in low latitudes is 10 to 13 mph, according to NOAA’s Hurricane Research Division.
These lead-footed storms are difficult for meteorologists to stay ahead of and force those in the way to speed up their preparations, as the impacts will arrive sooner.
All other factors being equal, the faster a storm moves, the stronger the winds will be to the right of the center track, since the storm’s forward speed adds to its winds in that right half of the circulation.
Faster storms are also able to spread stronger winds farther inland before weakening.
A recent example of a fast moving was Isaias, which was accelerated by an unusually strong jet stream for early August.
Isaias’ destructive winds and tornadoes caused $3.5 billion in damage in the northeast alone. Up to 3 million customers lost power as a result of Isaias.
If a hurricane is at sea and slower moving, there is more preparation time for areas potentially threatened by the storm.
But when a storm slows 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 northwest Bahamas. Then the winds pushing the hurricane forward died down.
Dorian’s eyewall whipped the Northwest Bahamas for an unfathomable 52 straight hours at intensity category 4 or 5.
The intense winds brought a storm surge of up to 28 feet to Grand Bahama Island, according to the Bahamas Meteorology Department (BDOM). At least 74 residents have been killed in the Bahamas, with damage estimated at $3.4 billion, according to the BDOM.
Then there is the rain from a thunderstorm.
The amount of rain produced by a storm has little or nothing to do with the intensity of the storm’s wind, but rather with its speed of movement.
In 2017, Hurricane Harvey slammed into Texas at Category 4 intensity, then meandered excruciatingly slow near the Texas coast for four days.
Up to 60 inches of rain fell along the upper Texas coast causing catastrophic flooding. Harvey remains the nation’s second costliest hurricane or tropical storm ($143.8 billion in 2022 dollars) behind only Katrina.
In addition to checking the storm’s current advancing speed in a graph, the forecast track graph could also hint at a stagnant storm ahead.
When it no longer resembles a cone, but instead takes on the appearance of a circle, or there is little separation of the forecast points, it suggests that the storm should slow down or stop. You should expect prolonged impacts from heavy rains, storm surges and winds, as the last two examples illustrate.
2. Size matters
The category also doesn’t tell you the size of a hurricane.
Its size – especially the extent of its wind circulation – is important in determining the magnitude and extent of its impacts.
A larger storm blowing over a larger area of ocean for the longest time will generate the largest storm surge, if all other factors are equal.
Hurricane Ike’s 2008 winds were Category 2 upon landfall, but its giant size in the Gulf of Mexico generated a 15-20 foot storm surge that knocked out most structures on the Bolivar Peninsula in Texas.
Hurricane Katrina in 2005 weakened to a Category 3 on landfall, but its giant size in the Gulf of Mexico generated a US record storm surge of 28 feet along the Mississippi coast.
Hurricanes and tropical storms also tend to get larger as they move further north, under the influence of stronger winds aloft.
Sandy in 2012 had tropical storm-force winds 1,000 miles across – the largest Atlantic tropical cyclone on record since at least 1988 – before producing record storm surge along parts of the New Jersey, New York and Connecticut.
But just because a hurricane’s winds are strong doesn’t mean it’s necessarily important.
Hurricane Andrew in 1992 had only tropical storm force winds extending out to 90 miles from its center when it made Category 5 landfall in South Florida.
Hurricane Charley in 2004 was of a similar small size when it made landfall on the southwest Florida shoreline at Category 4 intensity and then rumbled northeast across the peninsula.
Smaller storms like these can produce intense swaths of wind damage near their center.
They can also grow rapidly more often because their small 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 get a little bigger in replace their eyewall with a more expansive outer eyewall, making its wind field more expansive.
So how do you know if the size of a current tropical storm or hurricane matters?
Meteorologists – including those at the NHC – will often note in a forecast whether a storm is large or small. In such cases, you will often see wind field graphs, such as Hurricane Irma in 2017 below.
Simply put, the larger the circles in graphs like the one below, the bigger the storm.
If a large hurricane or tropical storm is heading landward, 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 predicted path of its center.
Other key factors including its movement speed and size will determine the ultimate impact the next time a storm hits.
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