“It’s fairly uncommon, but it does happen every once in a while,” he said.

If the storm does loop back around, however, expect it to have weakened substantially. The current forecast calls for the hurricane to be reduced to nothing more than a tropical depression, with wind speeds of no more than 40 miles per hour, Dr. Landsea said. But he warned, “Our wind-speed forecasting skill is less than our storm-track forecasting skill,” so “we don’t want to make any assumption that there’s not going to be any impact when it comes back around.”

Marshall Shepherd, director of the atmospheric sciences program at the University of Georgia, said in an email response to questions that while there have been loops before, this one is “still neat and rare to see.” He also predicted that “if the high is strong enough, the clockwise steering flow will loop the storm (likely weakened) back to Florida.”

That other storm in the Atlantic, Tropical Storm Nicole, is still too far to the east to interact with Matthew much, Dr. Landsea said, and it is unlikely to affect the United States.