Is 2017 Really the First Year since 1850 with Two Category-4 U.S. Landfall Hurricanes?

Posted: Oct 06, 2017 12:01 AM
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Is 2017 Really the First Year since 1850 with Two Category-4 U.S. Landfall Hurricanes?

Keeping up the (false) drumbeat that manmade global warming is causing more frequent and stronger hurricanes, lots of media outlets, following the spectacular Hurricanes Harvey and Irma, have proclaimed 2017 the first time in the 166 years of records in which two Category-4 hurricanes made landfall on the mainland United States in the same year.

Among them were USA Today, PBS, Forbes, Fortune, Slate, The Daily Beast, Business Insider, New York Magazine, Townhall.com, and even Scientific American

Like them, Weather.com reported on September 10:

Harvey and Irma both made a U.S. landfall as Category 4 hurricanes.

Two Atlantic hurricanes of this intensity have never made landfall during the same year in the U.S.

For the first time in 166 years of weather records, two Atlantic Category 4 hurricanes have made landfall in the United States during the same year. [Emphasis added.]

… The winds for a Category 4 hurricane on the Saffir-Simpson Hurricane Wind Scale range from 130 mph to 156 mph. Winds of that strength are capable of causing catastrophic damage.

Weather.com repeated the claim 10 days later.

And the claim's true—sort of.

But it's really false, because it compares apples and oranges.

Weather.com doesn't mention that in 2012 the National Hurricane Center modified the Saffir-Simpson Hurricane Wind Speed (SSHWS) scale "in order to resolve awkwardness associated with conversions among the various units used for wind speed in advisory products." Specifically, "The change broaden[ed] the Category 4 wind speed range by one mile per hour (mph) at each end of the range, yielding a new range of 130–156 mph."

Why was the change necessary?

Because of the inherent uncertainty in estimating the strength of tropical cyclones, the National Hurricane Center and the Central Pacific Hurricane Center assign tropical cyclone intensities in 5-knot (kt) increments (e.g., 100, 105, 100, 115 kt, etc.). Some advisory products, however, require intensity to be given in units of mph and kilometers per hour (km/h). For these products, the intensity in knots is converted into mph and km/h and then rounded to 5-mph and 5-km/h increments, so as not to suggest that the intensity of the storm can be known to unrealistic precision (e.g., 127 mph!). [Emphasis added.]

Prior to the modification, the SSHWS was:

-Category 1: 74–95 mph / 64–82 kt / 119–153 km/h

-Category 2: 96–110 mph / 83–95 kt / 154–177 km/h

-Category 3: 111–130 mph / 96–113 kt / 178–209 km/h

-Category 4: 131–155 mph / 114–135 kt/ 210–249 km/h

-Category 5: 156 mph / 136 kt / 250 km/h or higher

So under the modified SSHWS scale, both Harvey and Irma were Cat-4 hurricanes at landfall, with maximum sustained winds of 130 mph. Under the pre-modified scale, both were Cat-3.

So in order to compare apples and apples, we need to ask not "Did two Cat-4 hurricanes ever make landfall on the mainland U.S. in the same year before 2017?" but "Did two (or more) Cat-3 hurricanes ever make landfall on the mainland U.S. in the same year before 2017?"

The answer: Yes, in 10 different years: 1852, 1855, 1879, 1893, 1909 (three storms), 1926, 1933, 1985, 2004, and 2005 (four storms).

In short, it's false that, applying the same (pre-modification) SSHWS scale, 2017 was the first year on record in which 2 or more hurricanes of the same category as Harvey and Irma made landfall on the mainland U.S.