Maps: New Orleans 3 hour rainfall 1-in-50 and 1-in-100 annual exceedance probabilities
The above maps are "rainfall frequency and magnitude maps" produced by the Southern Regional Climate Center in 1997. The maps show the expected return periods for different quantities of rainfall over different periods of time.
In New Orleans, 3-hour rainfall totals of 7 inches are expected once every 50 years, based on historical data. 3-hour rainfall totals of 9 inches or more should occur once every 100 years.
New Orleans experienced 1-in-50 and 1-in-100 year rainfall events in the same areas, only two weeks apart.