Why a Hurricane's Size and Speed Matter as Much as Its Category
All eyes are on the Gulf Coast and the Southeast as Isaias makes its way to the region. While most forecasts focus on the category of a tropical weather maker, experts note that that is not the only metric that matters. A storm's Saffir-Simpson Hurricane Wind Scale category designation simply takes into account its wind speed, falling on a scale of 1 to 5. This is undoubtedly an important metric, but it is not the only factor that influences the overall impact of a storm.
Here are the two other important factors to consider when evaluating your personal risk.
Speed of the Storm
The large-scale winds surrounding a storm determine how fast it is moving. Fast steering winds naturally support a faster storm speed. Conversely, a storm is more likely to slow its speed if the steering winds are weak. According to NOAA's Hurricane Research Division, the average speed of a hurricane churning in the lower altitudes is 10 to 13 mph.
Why does speed matter? Storms moving at a fast clip are more difficult for forecasters to monitor. It stands to reason that this unpredictability makes it more challenging for residents in the path to prepare for impact. Faster-moving storms also come with stronger winds to the right of the eye, resulting in a greater degree of impacts for those in this path. Lastly, storms chugging along are able to send their strong winds farther inland before they weaken.
On the other hand, slow creepers give people more time to prepare for the impending arrival. However, these crawlers also come with the threat of prolonged impacts, as it takes longer for the storm to exit the region. This means that those in the line of fire could be dealing with hurricane-force winds and destructive storm surge for hours.
Most consequentially, the speed of the storm also determines how much rain falls over any given area. Areas that get caught up in a storm that is slowing its speed should be prepared for more significant impacts.
Physical Size of the Storm
A storm's category designation also does not tell how big it is in size. This size is measured by how far the winds extend from the center of the feature.
A larger storm spinning over a greater portion of the ocean for a longer time will produce the greatest amount of storm surge. For instance, while the infamous Hurricane Katrina had weakened to a Category 3 storm prior to landfall, its massive size as it roamed the Gulf supported the development of the record-breaking 28-foot storm surge along the coastline of Mississippi.
Tropical weather events typically grow in size as they move to the north and come under the influence of more powerful upper-level winds. 2012's Sandy is a good example of a large storm that packed tropical-storm-force winds that extended for 1,000 miles, making it the largest Atlantic storm on record since 1988.
But do not discount a storm just because it is small. It is important to note that storms that are smaller in size can still generate immense winds near the center of circulation. Smaller storms are also more vulnerable to the process of rapid intensification, as they tend to heat up more quickly.
On the flip side, the smaller size makes these storms more susceptible to the weakening impacts of dry air and wind shear. Smaller storms can grow quickly simply because they are able to replace their eyewall more frequently, leading to a bigger wind field.
The takeaway from all of this is that although a storm's category designation is a useful piece of information, it does not tell the whole story. It is always a good idea to pay attention to these other two metrics as well.
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