The National Hurricane Center started using a forecast cone for tropical systems in 2001.

Like anything else, it’s a tool, and it serves a purpose, but it can be misunderstood.

What You Need To Know

  • Many people base their hurricane preparation and evacuation plans around the forecast cone

  • The cone helps communicate the track most likely taken by the center of tropical systems over time

  • The width of the cone has nothing to do with the storm’s expected diameter

  • People can be led astray by the cone, as it doesn't account for storm size, intensity or impacts

People rely on the cone. And in a general sense, they see themselves as in or out of the cone, which, to them, means safe or not safe.

But many don’t know that the cone represents a static, one-dimensional dot, or the center of the storm at its initial location. As it moves, it’s plotted at an accuracy of 2/3 skill circles or 200 miles of error or 400 miles wide. Then the dots are connected to the circles representing future movement.

At four and five days out, the cone isn’t a bad messaging tool because it alerts people, but within 1-3 days, it can hurt people because they see themselves as in or out of the cone, and as you get closer to the event, the cone gets smaller.

As history has proved, we know hazards can extend outside of the cone.

The error that the cone gives is that those outside of the cone feel a false sense of security, and those inside the cone have an exaggerated chance of being directly in the tropical storm or hurricane’s path.

It is a simplified visualization that sometimes doesn’t accurately communicate hazards to those affected.

Sometimes, people see the cone size as the size of the tropical system, which is not the case.

And when a tropical system has left or dissipated, it’s also confusing because the cone ends up looking like the actual wind swath of a storm.

Here's an example from Ian’s first advisory projected out to day five. According to the cone, the storm could have gone to Daytona Beach, Havana, Cuba or as far as the Bahamas. 

People look at the cone and think, "well it will probably be in the center" which was Fort Myers at this point, but they don’t know that based on the cone’s definition, it could equally be at any of those locations. 

By advisory nine, the cone shifted about 200 miles to the northwest. So Panama City and the Florida panhandle were included in the cone and Fort Myers was on the edge of the cone, when not 36 hours before they were in the center.

This message can be confusing and not keep people as safe as they could have been had they seen other tools.

Another tool that meteorologists use with the cone is showing wind speed probabilities. It considers the storm’s size and says there is at least a 33% chance (or more than 1/3 of the time) that you’ll see tropical storm force winds (in yellow contours) or hurricane winds (in red contours). 

Showing this with the cone still shows the NHC forecast, which it based on the Monte Carlo run data. 

It shows you where the tropical storm force winds are projected (greater than 34 knots) and where hurricane-force winds are projected (greater than 64 knots), which sends a clearer message to everyone, which is every meteorologist’s goal. 

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