Forecast Cone Of Uncertainty: Facts And Myths

It's a staple of every hurricane season: the forecast "cone of uncertainty," which warns the public of the projected path of a tropical depression, storm or hurricane.

But there are some important things you might not know about it.

W​hat is the cone? Every year, the National Hurricane Center (NHC) issues forecast products for active tropical depressions, storms and hurricanes in the Atlantic and eastern Pacific basins. The forecast track of the storm is represented by a "cone" shape on their graphic and is almost universally accepted as the format for projecting a storm's path.

The color of the cone may vary among the NHC and media outlets. The Weather Channel and weather.com present the forecast cone in a dark red color.

F​orecast times are depicted along the cone's edge to provide an idea on timing. The Weather Channel and weather.com also indicate the NHC's wind intensity forecast in text boxes along the cone's edge.

W​hat does the forecast cone mean? The cone represents the most probable track of the center of a tropical depression, storm or hurricane over the next five days, assuming the storm lasts that long.

T​he most likely path of the center is usually down the center of a forecast cone.

H​ere at weather.com and The Weather Channel, we don't show a center line drawn through the cone because the whole point of a cone is to show uncertainty in the path and it can change with future forecasts.

W​hy is the forecast in the shape of a cone? The forecast cone is formed by enclosing the area swept out by a set of circles (not typically seen on the graphic, but shown below) along with the forecast track at 12-hour intervals.

The size of each circle is set at two-thirds of the historical forecast error over the previous five-year period. Based on those previous forecast errors, the entire track of the tropical cyclone can be expected to remain within the cone about 60 to 70 percent of the time.

The cone becomes wider as the forecast uncertainty increases with time, carving out the cone shape.

The size of this cone does not change from one storm to the next, from one forecaster to the next, or during the storm's history.

That's because some tropical storms and hurricanes are more difficult to forecast than others, due to uncertainties regarding steering winds and intensity.

The chart below indicates the average forecast track error for Atlantic tropical systems, up to 120 hours, from 2018 through 2022.

T​his is why the cone is a much better guide for a track forecast than any line drawn through the center of it.

There are important things the cone does not highlight. You cannot cover all of a storm's impacts by one forecast cone graphic.

The cone only indicates the forecast track of the center of a storm.

A storm isn't just a point along a line. It can range from a tiny storm to a giant, expansive hurricane. The cone does not have any information about the size of the storm.

T​hat matters because a storm's impacts often happen well outside the cone, sometimes hundreds of miles from the center of the storm. And the severity of impacts such as storm surge and the extent of winds, can depend on its size as much as its wind intensity.

Other impacts including rainfall flooding and tornadoes can also occur well outside the cone, not to mention well inland.

The National Hurricane Center, T​he Weather Channel and weather.com will include forecasts for rainfall, tornado threat, storm surge and winds to cover these impacts, in addition to the forecast cone.

T​he National Hurricane Center produced a helpful video explaining what the cone means and what it doesn't mean in the video below.

As we move into the future, forecasters hope tropical storm and hurricane forecasts will continue to improve and the "cone of uncertainty" will shrink even more.

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