X-YEAR, Y-DURATION RAIN

The magnitude of rainfall which has a 1-in-X chance of being exceeded in any future one-year time period with a duration of Y (hours or days). X-year rains must have durations associated with them; e.g., 25-year, 6-hour rain, 50-year, 24-hour rain, 100-year, 10-day rain, etc. These values are statistically derived using past rainfall records. Also referred to as Rainfall Duration-Frequency. Also see X-Year Flood.