Analysis: Hurricane Irma - A Historic Storm for Florida

Another way of looking at the amount of rainfall produced by Hurricane Irma over the U.S. is by plotting the Average Recurrence Interval (ARI), as shown below in Figure 6. What we see is that almost all of Florida received 24 hour rainfall values that, on average, have a 1 in 25 to 1 in 200 chance of occurring in any given year. The areas of highest 24 hour precipitation mentioned above have a less than 1 in 1000, or a 0.1%, chance of happening in Florida in a given year. Even coastal Georgia, affected by a severely weakened Irma, had very rare rainfall totals with some coastal towns seeing a day’s worth of precipitation that statistically have a 0.2% chance of occurring in a given year. By comparison, the highest rainfall amounts produced by Andrew can be expected once in every 10 to 50 years. A 24 hour maximum rainfall ARI has been chosen for Irma, because the majority of rainfall in most areas occurred over this time span, as evidenced by the rainfall mass curve plot at the point of highest precipitation in the analysis domain (Figure 7).