Hurricane Katrina is one of the most destructive natural disaster in U.S. history. The infrequency of severe coastal hurricanes implies that empirical probability estimates of the next big one will be unreliable. Here we use an extreme-value model together with interpolated best-track (HURDAT) records to show that a hurricane of Katrina's intensity or stronger can be expected to occur, on average, once every 21 years somewhere along the Gulf coast from Texas through Alabama and once every 14 years somewhere along the entire coast from Texas through Maine. The model predicts a 100-year return level of 83 ms-1 (186 mph) during globally warm years and 75 ms-1 (168 mph) during globally cool years. This difference is consistent with models predicting an increase in hurricane intensity with increasing greenhouse warming.