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.”…

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.”