It’s Atlantic hurricane season: Do you understand what ‘the cone’ really is?

Ocean
Ocean (Photo by Gilberto Olimpio from Pexels)

When weather teams issue forecasts, they come with certain specifics to help people understand what’s happening.

In the case of a hurricane, tropical storm or tropical depression, it usually includes a track forecast cone, commonly known as the cone of uncertainty.

Developed by the National Hurricane Center, this weather staple can sometimes cause confusion. Here’s what you need to know about “the cone.”

[And here’s a sample graphic, if you scroll down on this page, from the NHC).


Time

The cone of uncertainty’s main function is to show a tropical system’s projected track five days into the future, and it is used in both the Pacific and Atlantic basins. Within the cone is usually a dot indicating the forecast position of the storm’s center, normally at 12-hour intervals.

Size and shape

You will notice that the cone is narrow close to the present time, and gets wider as it projects farther ahead in time. That’s because the forecast position error gets larger the longer into the future it gets.

The cone doesn’t get as wide in instances where the computer models show a lot of agreement in the storm’s track. Conversely, in some situations in which there is a lot of uncertainty in a storm’s eventual path, the cone gets very wide to reflect the many path scenarios that may result.

Impacts

It is critical to understand that the cone of uncertainty tells us nothing about the storm’s size or specific impacts. Hurricane and tropical storm impacts extend far out from the storm’s center, and the cone only shows the potential track of the center of the storm.

Impacts can and frequently do spread outside of the cone.

Because the public generally only focuses on the center of the storm, the National Hurricane Center (and most broadcast meteorologists) no longer use the “skinny line” that connects the dots in the cone, as this makes you focus on the cone itself conveying a storm’s position possibilities, meteorologist Paul Gross said.

Path

Even though hurricane forecasting has improved to the point that 60% to 70% of tropical systems remain within the cone during their lifespan, it is important to remember that timing and strength errors still occur.

It may seem odd that timing is that big of a deal. After all, what’s the difference between a hurricane hitting at 9 a.m. versus hitting at 4 p.m.?

The difference is high and low tide, Gross said.

A storm hitting at high tide has a higher and more destructive storm surge than a storm hitting at low tide.

Furthermore, some notable storms recently experienced unexpected rapid intensification before landfall. If emergency managers are expecting, for example, a Category 1 storm, then they evacuate people from areas expected to inundate in a Category 1 storm. But if that storm suddenly strengthens to a strong Category 2 storm, then people who did not evacuate because they were deemed not to be in a high-risk area are suddenly caught in a storm surge they did not expect.

This is why you need to focus on more than just a storm’s path as shown in the cone of uncertainty.

Bottom line

The cone of uncertainty is a great piece of guidance that conveys a storm’s eventual path. However, there are important details that it does not provide.

That’s why it is so important to rely on our weather team for the information you need to keep you safe. Our experienced meteorologists know the impacts tropical systems bring, and will always provide you with the information you need.