Keith Blackwell, a hurricane forecaster and research scientist who teaches at the University of South Alabama, used microwave satellite imagery to document that Katrina sported a double eyewall. "Generally," Blackwell said, "double eyewall storms have a broader wind field than single eyewall storms, and they produce a one-two punch as each eyewall makes landfall. The winds push water toward the shore, and this water surge is greatest when the eye crosses the coast."

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