WASHINGTON — As Hurricane Irma barrels dangerously toward Florida, scientists say that a perfect mix of meteorological conditions has conspired over the past week to make the storm unusually large and powerful.



“You need just the right ingredients for a hurricane of this magnitude to last for so long,” said Phil Klotzbach, an atmospheric scientist at Colorado State University. “And Irma has had them all.”

Weather forecasters had already expected this summer to be an active hurricane season in the Atlantic Ocean because of warmer-than-average ocean surface temperatures, which provide fuel for hurricanes, as well as weaker-than-average wind shear, which can help to dissipate storms.

But even in that context, Irma was special.

Irma initially developed near Cape Verde off Africa’s coast on August 30, and a ridge of high pressure kept the storm from wandering off harmlessly into the cooler northern ocean. Instead, it was pushed inexorably westward, with plenty of time to intensify over the warm Atlantic waters.