Sunday’s storms have been described in many ways: a rain bomb, training storms, flash floods.

The devastating storm that flooded parts of Connecticut and swept away roads was far more than infrastructure could handle. Twelve-plus inches of rain fell in a short period, or three months worth of rain for the state.

But what exactly is a 1,000-year flood? No, it’s not a flood that happens once only every 1,000 years, but instead a flood that has a one in 1,000 chance of happening in any given year, as the U.S. Geological Survey explained. That calculation — how often flooding events are likely to happen, is part of how flood risk maps are derived.