Weather.com published an article noting that the two Cat-4 hurricane strikes this year (Harvey and Irma) is a new record. Here’s a nice graphic they used showing both storms at landfall.
But the statistics of rare events (like hurricanes) are not very well behaved. Let’s look at this new record and compare it to the 11-year-plus period of no major hurricane strikes that ended when Harvey struck Texas.
The Probability of Two Cat 4 Strikes in One Year
By my count, we have had 24 Cat-4 or Cat-5 landfalls in the U.S. between 1851 and 2016. This gives a probability (prior to Harvey and Irma) of one Cat-4+ strike every seven years. It also leads to an average return period of two Cat-4+ strikes of about 50 years (maybe one of you statisticians out there can correct me if I’m wrong).
So, since the average return period is once every 50 years, we were overdue for two Cat-4+ strikes in the same year over the entire 166-year period of record. (Again, for rare events, the statistics aren’t very well behaved.)
The Probability of the 11-Year “Drought” in Major Landfalling Hurricane
In 2015, a NASA study was published which calculated how unlikely the (then) nine-year stretch with no major hurricane landfalls was. They came up with a 177-year return period for such an event.
I used that statistic to estimate what eventually happened, which was 11 years with no major hurricane strikes.
I get a return period of 560 years!
Now, which seems more unusual and potentially due to climate change: something that should happen only once every 50 years, or every 560 years?
Maybe global warming causes fewer landfalling major hurricanes.
Read more at Dr. Roy Spencer’s blog
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