In the days before Helene made landfall, it drew a lot of energy from the very warm waters of the Gulf of Mexico, strengthening quickly from a Category 1 to a powerful Category 4 hurricane.
High sea surface temperatures meant there was a lot of moisture in the atmosphere capable of producing a vast amount of rain and this moisture was carried deep inland by the storm.
Most very powerful hurricanes do strengthen rapidly but what was unusual about Helene was the sheer scale of the storm’s rainclouds and wind field coverage.
Despite the storm making landfall on Florida’s Big Bend, even cities such as Miami, many hundreds of miles away, experienced wind gusts of more than 70mph (110km/h), due to Helene’s vast size.
It also meant the extreme rainfall took a long time to pass over any location in Helene’s path.