Why a hurricane's 'cone of uncertainty' matters — even for those who live outside of it

CBC meteorologist Ryan Snoddon explains what the cone of uncertainty is and how to interpret it.

CBC meteorologist Ryan Snoddon explains what the cone of uncertainty is and how to interpret it

Ryan Snoddon · CBC News · Posted: Sep 04, 2019 12:20 PM EDT | Last Updated: September 4, 2019

The cone of uncertainty.

The cone actually hasn't been around for all that long. It was first developed by the U.S. National Hurricane Center in 2001 and the agency began using it publicly in 2003, the year Hurricane Juan slammed into Atlantic Canada.

Did you know? For a 2-day forecast, the "cone of uncertainty" is 8% smaller in 2019 than it was last year. It's also 26% smaller than 5 years ago, 39% smaller than a decade ago, and less than half the size of 15 years ago... that's progress! <a href="https://t.co/aS21BwtyQz">https://t.co/aS21BwtyQz</a> <a href="https://twitter.com/hashtag/hurricanes?src=hash&ref_src=twsrc%5Etfw">#hurricanes</a> <a href="https://t.co/fSQ86CKvZg">pic.twitter.com/fSQ86CKvZg</a>
—@BMcNoldy

The good news is that because forecasts of hurricane tracks continue to improve and become more accurate, the cone of uncertainty has been shrinking with each new hurricane season.

However, a smaller forecast cone also means we need to be sure we understand what the cone is and what it represents when a hurricane is tracking into our region.

What is the cone of uncertainty?

The National Hurricane Centre (NHC) says the cone represents the probable track of the centre of a hurricane and is formed by enclosing the area swept out by a set of circles along the forecast track (at 12, 24, 36 hours, etc).

"The size of each circle is set so that two-thirds of historical official forecast errors over a five-year sample fall within the circle," the centre says.

There are some important takeaways there. Most notably, the cone represents where the centre of the storm might track, and it is based on how good or how bad the NHC forecast has been over the past five years.

So if we base this year's cone on NHC forecasts over the previous five years, the centre of the storm can be expected to remain within the cone of uncertainty roughly 60 to 70 per cent of the time.

The cone of uncertainty is a helpful tool, if you know how to interpret it correctly. (NOAA/NHC)

One of the most important things to remember when looking at a projection track and cone is that a hurricane is not a point.

Even if the forecast track is accurate and the centre of storm remains within the cone of uncertainty, impacts can be experienced well outside of the cone. Hurricanes, tropical storms and even post-tropical storms can be large systems and the cone does not take into account the size of the storm.

On this side of the border, the Canadian Hurricane Centre (CHC) has also added the cone of uncertainty to its official track maps in recent years.

The CHC begins issuing forecast bulletins when hurricanes, tropical storms, or post tropical storms are expected to impact Canada. (Environment Canada)

You may also notice on the CHC hurricane track map that some storm tracks are in red and some are in blue. The difference here is that when the CHC issues bulletins on a storm, it will appear in blue, while storms being tracked only by the NHC are in red.

Hurricane Dorian is expected to be a Category 1 storm by the time it arrives in the Maritimes this weekend. (Ryan Snoddon/CBC)

Long story short, the cone of uncertainty is a great way to quickly look at the latest NHC or CHC hurricane track map and identify whether or not you should be paying attention to a storm.