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Hurricane Dorian started as a disorganized system forecasters thought might fizzle out in the eastern Caribbean as a mere tropical storm. Instead, Dorian became an historic Category 5 hurricane that decimated the northern Bahamas, where it stalled for two days before coming within miles of the East Coast and churning up the coast.

As with many hurricanes before it, the collective mood oscillated from tense to relieved and back again with each new forecast that showed a new track. 

Among residents, there was some confusion and frustration. People wanted to know: Why were forecasters unable to figure out exactly where Dorian was going?

Hurricanes are predicted in probabilities, not guarantees, scientists told TCPalm this week, and one small adjustment can change the track of a big storm in a big way.

More: Efforts being made to get relief supplies to the Bahamas running into a variety of issues

And: How Fort Myers and Naples are pitching in to help the Bahamas after Hurricane Dorian

Chaos theory

The behavior of hurricanes is highly dependent on the conditions around them, said Dr. Allison Wing, professor of atmospheric science at Florida State University. Climate systems are understood through the mathematical theory of chaos, Wing said, meaning small changes in conditions can have a big impact over time.

It’s also known as the butterfly effect. 

When Dorian formed as a tropical storm over the eastern Caribbean, experts thought the storm might lose strength as it passed over Puerto Rico or Hispaniola. But a slight change in the track early in its life cycle meant Dorian bypassed the islands, giving it the opportunity to intensify to a monster Category 5 hurricane.

Small change, big impact.

To be able to predict how a hurricane like Dorian that formed in the eastern Caribbean will behave once it reaches Florida, scientists need to know how it started. And that, Wing said, puts scientists at a disadvantage from the beginning. 

“We never know the precise initial condition, the precise temperature and wind measurements at every single location on Earth at every single time,” Wing said. “So we're always starting at an imperfect state and those small differences grow.”

When scientists don’t have information about the variables they need to inform the computer models that predict a hurricane’s behavior, they make their best guess based on the information they have, Wing said. This educated guesswork leads to a great deal of uncertainty in hurricane forecasting.

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Cone of uncertainty

When scientists try to predict a hurricane’s behavior, they do so in terms of probabilities. These probabilities help scientists understand where a hurricane might go, while weighing the uncertainty that cannot be avoided in hurricane predictions.

Enter the cone of uncertainty. 

The cone of uncertainty is meant to convey the probability of where a hurricane might go, said Dr. Corene Matyas, professor of Earth system science at the University of Florida. 

“The cone is only representing a two-thirds chance of where the center of a hurricane might go,” Matyas said, “so (impacts) could easily extend outside the cone.” 

The center of the storm will land outside the storm one-third of the time, Matyas said.

Despite criticism about the last-minute forecast change showing a northward turn when previous models showed Dorian going across the state, Wing said the storm never actually left its projected path and always remained within the forecast cone. 

The cone is often misunderstood, said Kevin Rodriguez, a meteorologist for the National Weather Service in Melbourne. People mistake the center line of the cone for the predicted landfall.

The only thing the center line shows is the center point for the range of error in the prediction, Rodriguez said. All forecasters are saying is, odds are the center of the hurricane will hit somewhere within the cone, with no more or less probability placed on the center line.  

So why does the National Hurricane Center include it on maps?

"We need the points in order to draw the cone. Each point is a concentric circle," NHC spokesman Dennis Feltgen said, adding the interactive map defaults to the cone without the center line.

Users can customize the map, adding the past and forecast track; warnings and watches; wind radii, different knots and arrival times; and even subtracting the cone.

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Earth's atmosphere

It’s not enough to merely understand the conditions and probabilities surrounding a specific hurricane. The atmosphere is a system that spans the entire planet.

Movement in atmospheric currents on the other side of the world can impact hurricanes in this part of it. In order to model a system that size, a lot of computer power is required, Rodriguez said. 

Today’s computers are better than they were 20 years ago, Rodriguez said, but they simply cannot account for every single variable in a planet-sized system that changes from second to second. 

“The atmosphere is dynamic, it’s always changing," Rodriguez said. “It’s very hard for the computer models to grasp it. They don’t have the resolution at this time.”

Four high pressure systems, part of this dynamic atmospheric system that made Dorian so hard to predict, stalled the hurricane over the Bahamas for nearly two days, Rodriguez said. It wasn’t until these systems shifted that Dorian continue was able to continue north. 

Small change, big impact. 

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