Irma

Figure 1: Irma’s immense swirling eye, captured by NOAA’s GOES-16 satellite on September 5th, 2017.

The 2017 hurricane season has proven to be an extraordinary one, and one that has been masterfully forecasted. There was Harvey, which brought seemingly impossible rainfall amounts to Texas (recently featured in our blog). Jose, another storm that briefly reached intense hurricane status and made a slow dance in the Atlantic for over two weeks. Another, Maria, was a storm that strengthened from a tropical storm to a catastrophic category 5 hurricane in less than 30 hours and bore into Puerto Rico, completely cutting off power to the island and causing catastrophic damage. And then there’s Irma, a hurricane that maintained category 5 status longer than any other tropical cyclone on earth, and cut a swath of destruction from the Lesser Antilles up through northern Florida. This storm, Irma, will be one not soon forgotten and had it not been rivals with the aforementioned tropical systems, it would most likely be considered to be the major storm event of 2017 for the mainland United States.

After traversing the entirety of the Atlantic, from western Africa to the Caribbean, and garnering as much energy as possible from the warm ocean waters, Irma made direct impact on the Leeward Islands, Bahamas, Virgin Islands, and Cuba. Other islands, such as the U.S. territory of Puerto Rico, also suffered severe damage from Irma. Irma ultimately lead to a total of at least 38 deaths in the Caribbean. With Irma weakening to a category 3 after moving over Cuba, it strengthening once more into a category 4 hurricane in the roughly 100 miles between northern Cuba and Cudjoe Key, Florida, where the direct impact on the mainland U.S. occurred. Irma made landfall in the extreme western part of the forecast track, which turned out to be the best possible track for Florida, as the direct impacts of Irma were not felt in the heavily populated southeastern side of to peninsula.

On the minds of many is how Hurricane Irma compares to Hurricane Andrew, another historically intense hurricane to hit Florida in 1992 as a category 5 – the strongest category possible on the Saffir-Simpson scale of hurricane wind intensities – that brought death and destruction across about a 15 mile-wide path that cut through southern Florida from the Atlantic to the Gulf. Indeed, Andrew was a much smaller storm than Irma, as evidenced in the side-by-side comparison below in Figure 2. This meant that Irma’s hurricane-force winds were felt over a significantly larger area than Andrew’s. The majority of the state of Florida, in fact, experienced some form of hurricane force winds as Irma moved northward.

Irma Vs Andrew

Figure 2: A satellite composite comparison of the sizes of Hurricane Andrew (northwest) and Hurricane Irma (southeast). Source: Miami New Times

Andrew Irma Path

Figure 3: The path of Hurricane Andrew in 1992 (left) and Hurricane Irma (right), via NOAA.

Highest wind speeds for Hurricane Andrew (175 mph) and Hurricane Irma (185 mph) were both exceptionally high, as were their storm surges. There are now 50 reported deaths in Florida due to Hurricane Irma, while Hurricane Andrew directly or indirectly led to 45 deaths in the state of Florida. What is perhaps the biggest difference between Andrew and Irma is the sheer amount of rainfall that Irma dumped across Florida as compared to the amount dropped in southern Florida by Andrew. Below, the total storm precipitation for Andrew and Irma are plotted in figures 4 and 5, respectively. Hurricane Andrew, moving swiftly across Florida and into the Gulf, dropped around 10 inches of rain in its strongest rainbands, whereas Irma in her slow peddle up through the entirety of Florida dropped around 20 inches in the hardest hit locations near Vero Beach and Gainesville, Florida. The near constant battering of strong rain bands flooded much of Florida, causing close to 50 billion dollars in damage, making it the fourth costliest hurricane to hit the U.S.

Andrew storm total rainfall, 23-26 Aug 1992.

Figure 4: Total rainfall from Hurricane Andrew, from 12 UTC August 23rd to 12 UTC August 26th, 1992.

Irma storm total rainfall, 8-12 Sept 2017.

Figure 5: Total rainfall from Hurricane Irma, from 12 UTC September 8th to 12 UTC September 12th, 2017.

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).

Maximum 24-hour Average Recurrence Interval (ARI) for Irma.

Figure 6: Maximum 24-hour Average Recurrence Interval (ARI) values for Hurricane Irma.

Mass curve illustrating the accumulation of rainfall at the location of maximum precipitation from Hurricane Irma.

Figure 7. Rainfall mass curve plot for the point of highest precipitation of the analyzed area during Hurricane Irma.

Had Irma made direct impact on Miami-Dade County, like many models were predicting, she would have likely been a far deadlier, far costlier natural disaster. While the worst impacts of Irma in Florida were mostly relegated to less densely populated areas on the southwest side of the state, that’s no consolation to the communities spanning from the Florida Keys to Naples, which could take months or years to rebuild. Or to the many islands in the Caribbean that suffered catastrophic damage.

For more information on other extreme precipitation events across the U.S., please continue to monitor MetStat’s Extreme Precipitation Blog or contact us at info @ metstat.com.