The National Oceanographic and Atmospheric Administration's Coastal Services Center website shows historical hurricane tracks for the Canadian Maritimes for 2003 to 2009. (NOAA)

Sophisticated computer models that replaced instinct with cold, hard mathematics have helped forecasters predict where a storm like Hurricane Earl is going about twice as accurately as 20 years ago.

Last year, they proved it: The three-day forecast was as accurate as the here-it-comes, one-day warning used to be in the 1980s. In the 2009 hurricane season, the one-day forecast predicting where a storm would hit was off by only 85 kilometres on average.

But Earl is the type of storm, big and in a tricky location, that can defy expectations. Its predicted track shows the eye passing just off the U.S. East Coast, dancing so close to shore that a slight wobble could turn that miss into a mess.

Even if the eye should remain offshore, high winds that extend about 320 kilometres from the centre could reach inland.

A small shift could "bring the centre of Earl directly in contact with the Outer Banks, hence the need for the (hurricane) warning," National Hurricane Center director Bill Read said Wednesday. North Carolina has begun removal of people from the Outer Banks.

East Coast storms can be more predictable than those in the Gulf of Mexico because they do not usually make the sharp twists and turns taken by some Gulf storms.

Hurricane Earl a 'forecasting nightmare'

Still, meteorology Prof. Kerry Emanuel of the Massachusetts Institute of Technology called Earl "a forecasting nightmare in a way."

A small shift could 'bring the centre of Earl directly in contact with the Outer Banks, hence the need for the (hurricane) warning,' National Hurricane Center director Bill Read says. (Associated Press)That is why Read and others emphasize that the forecast is not a precise projection of Earl's movements. It is a line surrounded by a "cone of uncertainty."

About one out of three times, the eye of the storm will move out of the cone, said Timothy Schott, tropical cyclone program leader for the National Weather Service in Silver Spring, Md.

"We're very confident about the track. We're confident about the intensity," Read said.

Because of uncertainties, however, the track cannot be narrowed to "a skinny line on a map," he said. "That's why we have errors."

Those errors are nothing compared with what they used to be.

When Max Mayfield joined the hurricane centre in 1972, forecasters had some computer models, but their calculations were based more on history, not the physics of the current atmosphere.

Mostly forecasters used their knowledge and plain old "feel," said Mayfield, who later became the centre's director and is now retired.

1972 forecast off by 724 km

In 1972, the average two-day forecast was off by about 724 kilometres; last year it was 130 kilometres. The margin of error used to be so big that when a storm hit the Leeward Islands, far to the southeast of the United States, forecasters started alerts for Florida and up the East Coast, Mayfield said.

He credits the improved forecasts to better observations of storms and improved computer models.

"We have a lot more confidence in the models than we used to," Mayfield said.

Many if not most of the models now look at the shifting dynamics of the atmosphere to see what forces are guiding a hurricane. That type of calculation takes faster computers, which are now more readily available.

Hurricanes avoid high-pressure systems, which act almost like brick walls, and follow low pressure troughs, which act like bowling alley gutters guiding storms. The models essentially predict where the walls and gutters will be.

In some ways, those computer models have gotten so reliable that hurricane specialists half-jokingly grouse that they will soon become messengers instead of forecasters, said Hugh Willoughby, a professor at Florida International University and former head of the weather service's hurricane research division.

Intensity forecasts need work

There also are far more computer models churning data and making predictions, said MIT's Emanuel. That makes a consensus more likely, he said.

But the weather service's Schott said that is only half the story. Despite years of research, forecasters still have not significantly improved their forecasts on storm intensity. They are not certain why storms suddenly get weaker or stronger.

That is why planes and drones are continuously flown into Earl for more information, especially about the way energy is exchanged between the ocean and the storm itself, Schott said.

"While we pride ourselves that the track forecast is getting better and better, we remain humbled by the uncertainties of the science we don't yet understand," Schott said. "This is not an algebra question where there's only one right answer."