JACKSONVILLE, Fla. -- – You’ve seen it for years and it has become synonymous with hurricane season.
It’s the forecast cone.
The cone is created by the National Hurricane Center to show not just the forecast track, but also the potential uncertainty about the track.
But did you know every year the forecast cone changes? And we now know what the cone will look like in 2023.
How the forecast cone is created
The forecast cone isn’t necessarily a cone.
It is actually a series of circles that become connected to create a cone.
When the National Hurricane Center forecasts potential movement, it begins by creating exact future positions of the center.
These positions are created at three hours after the forecast is issued, at 12 hours, and then every 12 hours after that for five full days.
After the positions are set, a line is created. This is the line you often see passing through the center of the forecast cone.
But each position carries a calculated uncertainty. This uncertainty is used to create a circle around the forecast position.
These circles get larger and larger and the forecast goes from hours to days. This is because the uncertainty of any forecast increases as you move forward in time.
A series of nine circles are created. Then the circles are enclosed by their outer edges to create the forecast cone.
How is the uncertainty decided?
The National Hurricane Center, or NHC, uses historical forecast errors of the past five years to determine the uncertainty.
The goal is for the center of the system to remain in the forecast cone more than 66% of the time. This means the rest of the time, the center of the system will drift outside the forecast cone.
This is an important point as the National Hurricane Center is acknowledging that over 30% of the time the forecast cone will not be completely accurate.
As uncertainty increases, the forecast cone increases in size to convey this uncertainty.
Because the NHC uses the past five years of forecasts, the forecast cone changes from year-to-year.
The shrinking forecast cone
The first forecast cone was issued by the NHC back in 2002, and the cone has seen some dramatic improvements.
In 2002, the forecast cone was quite simply enormous, showing the high uncertainty that existed over 20 years ago.
The forecast cone with what would become Arthur in 2002 was literally the size of all of New England.
As forecasting the path became better and better, the cone shrank in size quickly.
By 2007, the forecast cone looked a bit more typical but was still quite large in the later time periods.
The forecast cone has continued to shrink, but it is no longer shrinking dramatically.
The NHC forecasts have become quite accurate and significant improvement is becoming harder and harder to achieve.
Going forward, there will likely be minimal changes in the forecast cone.
The 2023 forecast cone
The NHC has announced the size of the forecast cone for 2023.
The cone is roughly the same as in 2023, with just some minor changes in the medium and long range.
The 2023 cone will be exactly the same as 2022 for the first 24 hours. The ranges are 16 nautical miles from the center at 3 hours, 26 nautical miles at 12 hours and 39 nautical miles at 24 hours.
At 36 hours, the forecast cone will be 2% larger, or 1 nautical mile. This is a result of higher forecast errors in this time period.
The cone will actually be a touch smaller at 60 and 72 hours. This is a result of higher forecast errors in the medium-range over the past five years.
At 60 hours, the cone will be 4% smaller and at 72 hours, the cone will decrease by 1%.
The forecast cone will then expand in the longer range, again due to increased forecast errors.
At 96 hours, the cone will be 2% larger. At 120 hours, or five days, the cone will be 3% larger. This is also the biggest jump in the forecast cone, with a 5 nautical mile extension.
Again, the forecast cone is designed so that center will exit the forecast cone 1/3 of the time.
So the new forecast cone will look similar to the previous ones, but it does not mean the storm is guaranteed to track inside the cone.