Irene Forecasters Missed Storm’s Intensity While Correctly Predicting Path

Forecasters overestimated the strength of Hurricane Irene even as they accurately predicted the direction and timing of the storm’s destructive path along the U.S. East Coast.

“This was one of their better forecasts,” Hans Graber, a professor of marine physics at the University of Miami, said in a telephone interview. “Definitely, it helped to save lives.”

Irene was responsible for at least 18 deaths in the U.S. Still, it was a shadow of what roared across the Caribbean last week with winds of up to 120 miles (194 kilometers) an hour and hit North Carolina’s Outer Banks as a Category 1 hurricane. It was dropped to its current status before making landfall in New York City at about 9 a.m. yesterday.

“We’re still not where we want to be on intensity,” Ken Clark, a senior meteorologist with AccuWeather Inc., said in a telephone interview.

Irene may cost insurers as much as $3 billion to cover U.S. damage, with overall economic losses of $7 billion, according to Kinetic Analysis Corp., which predicts disaster impact. The U.S. suffered $35 billion in losses in nine separate events so far in 2011, according to the National Oceanic and Atmospheric Administration, tying a record for disasters causing more than $1 billion damage in a single year.

Steering Currents

While forecasters have gotten better at estimating the trajectories of storms because the winds propelling them, known as steering currents, have gotten easier to predict with computer models, projecting intensity is driven by dozens of variables that make modeling more difficult, Clark said.

Government officials relied on forecasts from the National Hurricane Center in making preparations, including ordering mandatory evacuations in New York and New Jersey. Dennis Feltgen, a spokesman for the Miami-based agency, said it accurately predicted Irene’s path when it said the storm would come aground in North Carolina before travelling north through New England.

“That’s exactly what it did,” Feltgen said.

The hurricane center also forecast Irene would reach Category 4 level, with maximum sustained winds of 131 mph. It only reached Category 3, with winds of 120 mph. Hours before the storm made landfall in New York, the center was forecasting it would arrive as a Category 1 hurricane. By the time it reached the city, it had weakened to a tropical storm with winds of 65 mph.

Feltgen said an overall analysis of the forecasting will have to wait until after hurricane season ends on Nov. 30.

Post Mortem

“We’re focused on operations right now,” he said. “Then we get to the post mortem.”

The National Hurricane Center “did an excellent job in nailing down the forecast,” Virginia Governor Robert McDonnell, a Republican, said yesterday in a news conference.

There were at least three storm-related deaths in the state, where more than 900,000 customers lost power.

Graber said the accuracy of the trajectory forecast hinged on the correct outlook for the ridge between two weather systems that served as a kind of valley through which the hurricane moved. Forecasters deploy multiple models and have gotten more accurate in recent years as they use past mistakes, such as the twists and turns of Hurricane Katrina in 2005, to hone them.

“Their skill is improving from season to season,” Graber said.

To contact the reporters on this story: William Selway in Washington at wselway@bloomberg.net; Timothy R. Homan in Washington at thoman1@bloomberg.net

To contact the editor responsible for this story: Mark Tannenbaum at mtannen@bloomberg.net

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