The Three Category 5 Tropical Cyclones of 2017

January 15, 2018, 4:00 PM EST

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Above:  The strongest storm of 2017, Hurricane Irma, as seen in infrared by the VIIRS instrument on NOAA’s Suomi satellite at 1:35 am EDT Wednesday, September 6, 2017. At the time, the island of Barbuda was in the eye, and Irma was at peak strength--a Category 5 storm with 180 mph winds. Image credit: UW-Madison/CIMSS.

We don't yet know what 2018 will bring our planet in the way of tropical cyclones, but a striking feature of 2017 was the relative lack of Category 5 storms globally—only three. This was the lowest total since 2012, and was well below the unusually high activity of 2014 - 2016, when eight or nine Category 5s appeared each year. Earth averaged 5.1 Category 5 storms per year between 1990 - 2017, according to ratings made by NOAA's National Hurricane Center and the U.S. Navy's Joint Typhoon Warning Center. The record number occurred in the El Niño year of 1997, which had twelve Category 5 storms--ten of them in the Northwest Pacific, where most of Earth’s Cat 5s occur. The Northwest Pacific averaged 2.9 Category 5 storms per year from 1990 - 2017, including five such storms in both 2015 and 2016. UPDATE: in post-season re-analysis, Cyclone Ernie of April 2017, northwest of Australia, was re-analyzed to be a Category 5 storm, while Super Typhoon Noru was re-analyzed to have only become a Category 4 storm. Thus, the total number of Cat 5s in 2017 remained at three. Ernie is now analyzed as having a 95-kt (110 mph) increase in winds in 24 hours, which is tied with Hurricane Wilma (2005) for second place behind Hurricane Patricia (2015) for fastest 24-hour intensification rate on record. Thanks go to Jasper Deng for this info.

Cat 5s
Figure 1. Global Category 5 tropical cyclones from 1990 - 2017, as rated by NOAA's National Hurricane Center and the U.S. Navy's Joint Typhoon Warning Center. The quality of the database rating Cat 5s is too poor and the time series of decent data on these storms is too short to make definitive conclusions about how climate change may be affecting these most fearsome of storms. Climate change is expected to make the strongest storms stronger, and we should expect to see an increase in Category 5 storms in the coming decades.

UPDATE: in post-season reanalysis, Noru was downgraded to a Cat 4, meaning that the Northwest Pacific had no Category 5 storms in 2017.

During 2017, just one Cat 5 was observed in the Northwest Pacific—Super Typhoon Noru of July, which topped out with 160 mph winds. Stronger-than-usual surface trade winds and unfavorable upper-level winds worked to keep 2017's typhoons weaker than usual, despite the presence of sea surface temperatures (SSTs) that were more than 1°C (1.8°F) warmer than average over much of the Northwest Pacific. The other two Cat 5s of 2017 were both in the Atlantic: Hurricane Irma (180 mph peak winds) and Hurricane Maria (175 mph peak winds). Below is a "rogue's gallery" of all three Category 5 storms from 2017:

Super Typhoon Noru
Cat Five #1, Northwest Pacific. The Northwest Pacific typhoon season got off to an unusually late start in 2017, with Noru becoming the first typhoon of the year on July 23 in the waters about 1000 miles southeast of Tokyo, Japan. Noru was an unusually long-lasting storm, remaining at tropical storm or typhoon strength for fifteen days. Noru intensified into a Category 5 storm at 18 UTC July 30, topping out with 160 mph winds in the waters about 800 miles southeast of Japan. Noru then headed northwest towards Japan during the first week of August, and weakened to a Category 1 storm with 75 mph winds by the time it made landfall in Japan’s Wakayama Prefecture on August 7. Noru killed two and did approximately $100 million in damage to Japan. Above: Noru at 0415 UTC July 31, 2017. At the time, Noru was a Category 4 super typhoon with 150 mph winds. Image credit: NASA.
Irma satellite

Cat 5 #2, Atlantic. The strongest tropical cyclone globally in 2017 was Hurricane Irma, which maintained its peak intensity of 180 mph winds for a remarkable 24 hours over the northeast Caribbean on September 5 – 6, 2017. This is the second longest period on record that an Atlantic hurricane has maintained that intensity, behind the 30 hours of Hurricane Allen of 1980. Irma made landfall in Barbuda, Sint Maarten, and the British Virgin Islands on September 6 with 180 mph winds, putting it in a tie for the second strongest landfalling tropical cyclone ever recorded globally. Irma also made five other destructive landfalls: Little Inagua, Bahamas (160 mph, September 8), Camaguey Islands, Cuba (160 mph, September 8); Cudjoe Key, FL (130 mph, September 10); Marco Island, FL (115 mph, September 10); and Naples, FL (115 mph, September 10). Irma’s Florida Keys’ landfall pressure of 929 mb was tied with the Lake Okeechobee Hurricane of 1928 for the 7th lowest on record for a continental US hurricane. According to insurance broker Aon Benfield, Irma caused $30 billion in damage to the mainland U.S, and $20 billion in damage in the Caribbean. The hurricane was being blamed for at least 124 deaths. Above: MODIS image of Hurricane Irma taken on Tuesday afternoon, September 5, 2017. At the time, Irma was at peak strength, a Category 5 storm with 180 mph winds. Image credit: NASA.

Strongest landfalling hurricanes

Figure 2. Irma’s landfall intensity of 180 mph winds in the Leeward islands is second only to the 1935 Florida Keys Labor Day Hurricane as the highest landfall intensity on record for an Atlantic hurricane. Irma effectively destroyed the island of Barbuda in the Leeward Islands, forcing all 1800 inhabitants to leave, making the island unpopulated for the first time in 300 years. By the end of 2017, only about 15% of the population had returned.


Cat 5 #3, Atlantic. Hurricane Maria got its start as a tropical storm on September 16, a few hundred miles east of the northern Lesser Antilles Islands, and intensified a remarkable 70 mph in 18 hours as it approached the islands, hitting the island of Dominica as a Category 5 hurricane with 160 mph winds. Only Wilma (2005), Felix (2007) and Ike (2008) intensified more rapidly in 18 hours. After hitting Dominica, Maria reached peak intensity  (908 mb pressure, 175 mph winds) over the northeastern Caribbean; this pressure was the lowest on record for a hurricane in the eastern Caribbean (less than 20°N latitude, and between 75 - 60°W longitude). Maria weakened slightly as it powered through the Virgin Islands and Puerto Rico as a top-end Category 4 storm with 155 mph winds, causing catastrophic damage and at least 98 deaths. However, several analyses have shown that the indirect death toll in Puerto Rico alone may have been more than 1000, when deaths that occurred in the weeks and months after Maria’s passage are fully taken into account. This would make Maria only the second hurricane since 1928, along with Katrina of 2005, to have caused at least 1000 deaths in the U.S. or its territories. Some damage estimates for Maria are in excess of $100 billion (insured plus uninsured damage), which would make Maria more costly than any hurricane except Hurricane Katrina, which caused $161 billion in damage in 2005. Above: VIIRS infrared satellite image of Hurricane Maria moving just west of St. Croix while at Cat 5 strength at 2:13 am EDT Wednesday, September 20, 2017. Image credit: NOAA/CIMSS/UM-Madison.


The two almost Cat 5s of 2017. Two tropical cyclones just missed achieving Category 5 status in 2017, topping out with 155 mph winds (157 mph winds are the threshold for a Cat 5). These almost-Cat 5s were Super Typhoon Lan in the Northwest Pacific (155 mph winds at 0 UTC October 21, 2017), and Hurricane Jose in the Atlantic (155 mph winds at 11 UTC September 9, 2017). Above: perhaps the most impressive satellite image of a tropical cyclone in 2017, Hurricane Jose near peak strength as seen on September 9, 2017 by the Sentinel-2 satellite. Image credit: Antti Lipponen.

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