All hurricanes evolve, and most weaken, as they track northward, their size and strength affected by water, wind and terrain. And all hurricanes eventually die — a relatively quick downgrade to a tropical storm in the case of those, like Irene, that travel inland, a more lingering demise for those that trail out to the colder waters of the higher latitudes.
But Irene’s fall — from potential storm of the century to an also-ran in hurricane lore — was greater than most.
Meteorologists were quick to point out that the hurricane was, as forecast, a huge and severe storm, responsible for at least 16 deaths and damaging property from Florida to New England. Given its potential, they said, evacuations and transit shutdowns were well warranted. And they noted that although it was weakened when it hit New York, it was still a Category 1 storm, as predicted several days before, and was still causing extensive flooding even as a tropical storm.
But hurricane forecasters acknowledged that they did not quite call the storm right.
“We were expecting a stronger storm to come into North Carolina,” said James Franklin, chief of the hurricane specialist unit at the National Hurricane Center in Miami. “We had every reason to believe it would strengthen after the Bahamas.”
He added, “What we got wrong was the structure of the storm.”
Forecasters had expected that a spinning band of clouds near its center, called the inner eyewall, would collapse and be replaced by an outer band that would then slowly contract. Such “eyewall replacement cycles” have been known to cause hurricanes to strengthen.
While its eyewall did collapse, Irene never completed the cycle, Mr. Franklin said. “There were a lot of rain bands competing for the same energy,” he said. “So when the eyewall collapsed, there were winds over a large area.”
That led the storm to be much larger, but with the winds spread over a larger area, they were less intense. What hurricane specialists had forecast to be a Category 2 or possibly Category 3 storm when it hit eastern North Carolina early Saturday, with maximum sustained winds of 110 m.p.h. or higher, roared across the Outer Banks as a Category 1, with winds that were more than 10 percent slower.
After North Carolina, the storm weakened some more. But forecasters had always expected that, said Dave Radell, a meteorologist with the National Weather Service in Upton, N.Y. By traveling for a time across part of North Carolina, the hurricane was deprived of the heat and moisture of the ocean that it needed to thrive. Once it headed out over water again, east of Delaware and Maryland, it encountered slightly colder sea surface temperatures, which tend to weaken a storm as well. Finally, its energy was sapped when it encountered winds from an unrelated weather system that originated over the Great Lakes.
“Any combination of those factors will prevent a storm from intensifying,” Mr. Radell said.
“We also had a little drier air get wrapped into the system,” he said, which helps explain why most of the rain that fell in the New York area was contained in the front portion of the storm. There was little precipitation once Irene’s center passed.
The effect of unrelated winds on a hurricane, called wind shear, can be enormous, said Adam H. Sobel, an atmospheric scientist at the Lamont-Doherty Earth Observatory, part of Columbia University. “When the wind is different in either speed or direction at different heights, hurricanes don’t like that,” he said.
The differential winds can remove moisture from a storm, or distort its shape, which affects its ability to gain energy. Mr. Sobel said that Irene “seemed to come naturally into an area of shear.”
Mr. Franklin said that the hurricane center had done better at forecasting the movement of the storm, the predicted track barely budging in the past few days. But it was not surprising that the strength forecasts were off — the accuracy of such forecasts has hardly improved over the past several decades.