Hurricane Helene is now forecast to reach catastrophic Category 4 strength by the time it makes landfall in Florida on Thursday, the National Hurricane Center says, with storm surge potentially climbing to 20 feet along some parts of the coast.

Of all the hazards that hurricanes bring, storm surge is the greatest threat to life and property along the coast. It can sweep homes off their foundations, flood riverside communities miles inland, and break up dunes and levees that normally protect coastal areas against storms.