More than 1 million people in the Carolinas were ordered to evacuate days before Hurricane Florence hit landfall.

Government officials order coastal evacuation even when it’s sunny at the beach with not a cloud in the sky and no hint of the ominous threat thousands of miles away other than from satellite images. People who know I study hurricane evacuations have often asked me to explain this curious decision.

In the end, evacuation planning is part science, part skill based on experience, and part luck.