By Joseph D’Aleo, CCM, AMS Fellow
What a hurricane season! It started very early with Arlene in April but the real action held off until the last week of August when Hurricane Harvey flooded Texas and Louisiana.. Harvey was the first hurricane to make landfall in Texas since Ike in 2008 and Category 4 hurricane in Texas since Hurricane Carla in 1961.
Irma, the 11th strongest Atlantic storm on record (using central pressure, the most reliable measure) had major impacts on Islands like Barbuda and St. Martin, the Virgin Islands, the Turks and Caicos and southern Bahamas. Then crossing northern Cuba it curled back into Florida. It was the first landfalling hurricane and major hurricane in Florida since Wilma in 2005.
Jose too became a major hurricane, but never made landfall though it created large swells along the eastern seaboard and pounded southeastern New England, Cape Cod and the islands with tropical storm winds and coastal flooding as it stalled for days.
Maria was the third major Hurricane, the 10th strongest Atlantic storm, crossed the northern Leeward Islands and plowed through Puerto Rico, doing catastrophic damage to the island. It then moved north into the Atlantic, close enough to pound the Atlantic coast with large swells from Florida to New Jersey.
And then Hurricane Nate avoided another ‘Katrina moment for New Orleans but produced storm surge damage to southeast Louisiana, Mississippi, Alabama and the Florida Panhandle.
Before the landfall of two major storms on the U.S. we had gone just short of 12 years without a major hurricane landfall, the longest such lull since the 1860s.
The quiet period came after three big years. Isabel made landfall on the Mid Atlantic in 2003, Charley, Frances, Ivan and Jeanne in 2004 and Dennis, Katrina, Rita and Wilma in 2005 all made landfall on the mainland. Emily in 2005 was another major hurricane but turned west into Mexico. 2005 holds the record for 5 category 4 or greater and 4 category 5 impact storms. Some speculated this was the new norm for the Atlantic before nature gave us that 12-year break.
So what causes long quiet spells and then big years like 2004 and 2005 and now 2017?
Nothing is new in weather. Great Colonial hurricanes in the northeast with storm surges up to 20 feet occurred in 1635 and 1675. A Katrina like storm made landfall in Louisiana in 1722 with major flooding and damage in Louisiana. The Great Chesapeake storm in 1769 like Isabel in 2003 brought major flooding to North Carolina and Virginia. In the Caribbean, the Great Hurricane of 1780 killed an estimated 27,500 people while ravaging the islands of the eastern Caribbean with winds estimated to top 200 mph. It was one of three hurricanes that year with death tolls greater than 1000.
1893, had at least 10 hurricanes. Of those, 5 became major hurricanes. Two of the hurricanes caused over two thousand (2000) deaths in the United States; at the time, the season was the deadliest in U.S. history.
1886 came close with at least 10 hurricanes, 7 making landfall. 4 of the hurricanes were major hurricanes.
The Galveston Hurricane in 1900 killed at least 8,000 people with some estimates as high as 12,000, making it the deadliest natural disaster in U.S. history.
Ok, major hurricanes have occurred even during cold periods but is there a trend in the modern record?
ACCUMULATED CYCLONE ENERGY INDEX A MEASURE OF SEASONAL TROPICAL ACTIVITY
The Accumulated Cyclone Energy Index which takes into account the number, duration and strength of all tropical storms in a season. The ACE index is a wind energy index, defined as the sum of the squares of the maximum sustained surface wind speed (knots) measured every six hours for all named storms while they are at least tropical storm strength.
The Accumulated Cyclone Energy Index for the Atlantic shows a cyclical behavior with no long term trend but with spikes in 1893, 1926, 1933,1950 then again in 1995, 2004 and 2005. 2017 ranks 8th now with still weeks to go this season.
So what causes long breaks and then big years like 2004 and 2005 and now 2017?
OCEAN TEMPERATURE AND PRESSURE PATTERNS
The North Atlantic like the Pacific undergoes multi-decadal changes in ocean temperature and pressure patterns. It has long been known, when the Atlantic is in what is called its’ warm mode, there are more storms. Since 1995, when the current warm Atlantic mode began, we have average 14.6 named storms per year, more than 5 more than the long-term 1851-2017 average.
An important factor that affects whether hurricanes affect the United States is El Nino and La Nina. When El Ninos develop, more storms develop in the eastern and central Pacific threatening Mexico, Hawaii and sometimes in weakened forms Arizona and California.
These storms enhance high-level winds that cross into the Atlantic. These winds produce shear that disrupts developing storms causing them to weaken or dissipate and/or turn harmlessly north into the North Atlantic. Storms can still develop near the coast where the water is warm like in the Gulf and near the Gulf Stream off the southeast coast.
Image courtesy of climate.gov based on originals by Gerry Bell
When La Ninas develop there are usually fewer storms in the eastern Pacific and less shear to disrupt the Atlantic storms.
Image courtesy of climate.gov based on originals by Gerry Bell
In warm Atlantic years, that means trouble as the storms can track the entre Basin with more time to turn into major hurricanes. Even the east coast is more vulnerable to a landfalling hurricane. We had 8 high impact east coast hurricanes from 1938 to 1960 and 9 from 1988 to 2012.
The last important La Nina stretch was in 2010/11 to 2011/12. We avoided a major hurricane hit, though major hurricanes at sea made final landfall in the NYC metro – Irene (as a tropical storm) in 2011 and Sandy in 2012 (as a post tropical cyclone). They caused massive flooding (from rains with Irene in upstate NY and Vermont and from a storm surge with Sandy in New York City and New Jersey).
We are still in the latest Atlantic warm period. This year, a spring attempt at an El Nino failed and La Nina like conditions developed. Had El Nino succeeded we may have had Harvey, which developed near the Texas coast and Nate which came out of the bath water in the western Caribbean but maybe Irma and Maria would have been weakened or deflected. But with La Nina conditions developing, no shear and warm Atlantic water we saw a return to big storms just as we saw in 2004 and 2005.
It may not be over as in 2005, we had Wilma come out of the Caribbean in late October.
At Weatherbell, Joe Bastardi led the team in the tropical outlook and correctly called for a big season. When the water in the Main Development Regions warmed further, Joe upped the ACE forecast and the team began alerting that come mid-August, big things would happen.
So when we get a year like 2017 or back-to-back bad years like 2004 and 2005, we have to accept that is how the weather works. Permadroughts ended with record wet years for Texas and California this decade. The record nearly 12 year major hurricane ‘drought’ ended with 2017.
Joe D’Aleo is currently a Senior Co-chief Meteorologist with WeatherBELL Analytics. Joe is a CCM, Fellow of the AMS, former chair of the AMS Committee on Weather Analysis and Forecasting. He was a college professor of Meteorology/Climatology, the co founder and first Director of Meteorology at The Weather Channel and Chief Meteorologist with 3 companies the last 30 years. He is the Executive Director of Icecap.us since 2007.
Dr. Roy Spencer
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.
Left: Hurricane Harvey makes landfall near Rockport, Texas, on Aug. 25, 2017 | Right: Hurricane Irma makes its first landfall at Cudjoe Key, Florida, on Sept. 10, 2017 (graphic: Weather.com).
But the statistics of rare events (like hurricanes) are not very well behaved. Let’s look at this new record, and compared it to the 11+year 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 Cat4+ strike every 7 years. It also leads to an average return period of two Cat4+ 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 Cat4+ strikes in the same year over the entire 166 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) 9-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.
Why the Climate Alarmist Claims About 2017 Hurricanes Are Nonsense
The 2017 North Atlantic hurricane season has the climate change alarmists out in full force.
These activists claim that man-made global warming has made the Gulf of Mexico warmer and the air more humid, thereby making tropical cyclones — called hurricanes in the North Atlantic — more frequent and more intense. They demand we reduce our carbon dioxide (CO2) emissions to lessen the “increasing” hurricane threat.
Even scientists who should know better are promoting the hurricane/global warming connection. Dr. Gregory Flato — a Canadian government scientist who is vice chair of the United Nations Intergovernmental Panel on Climate Change working group that reports on the causes of climate change — maintains:
There’s also an expectation that as the sea surface temperatures increase, that the intensity of very intense hurricanes will become larger.
Dr. Michael Mann of Pennsylvania State University has made similar claims, asserting that the potential intensity of hurricanes and other large storms has risen as a result of climate change.
But basic observations and meteorological science do not support their claims.
Meteorologist Dr. Roy Spencer, a principal research scientist at the University of Alabama in Huntsville and an expert on tropical cyclones, explains:
[M]ajor hurricanes don’t really care whether the Gulf [of Mexico] is above average or below average in temperature.
Similarly, in a research paper published in March 2017, M. Mohapatra and V. Vijay Kumar, both of the India Meteorological Department, state:
[T]here is a decreasing trend in the tropical cyclone number over the North Indian Ocean in recent years, though there is an increasing trend in the sea surface temperature.
That ocean temperature rise in the Gulf of Mexico will not increase hurricane frequency has been part of fundamental meteorology since the 1970s. America’s “hurricane guru,” the late Dr. Bill Gray — emeritus professor of atmospheric science at Colorado State University (CSU) and the head of the Tropical Meteorology Project at CSU’s Department of Atmospheric Sciences — showed in peer-reviewed research that the seasonal hurricane frequency is determined by six climatological factors:
1. The rotational tendency, or vorticity, already present in the atmosphere. Low vorticity lessens the likelihood of a hurricane forming.
2. Pressure gradients determined by latitude. Low pressure gradients lessen the likelihood of a hurricane forming. Within 4 to 5 degrees of the equator, pressure gradients are too weak for hurricanes to form.
3. Wind shear, the changes in wind speed and direction that occur between layers in the atmosphere. Low wind shear allows the whole system in a growing hurricane, from sea level up to around 15,000 meters, to turn together. This helps to keep the storm intact and strengthening. High wind shear essentially cuts the top off a cyclone before it can become a hurricane.
4. Ocean thermal energy as indicated by temperatures to a depth of 60 meters. Temperatures in the Gulf and other hurricane formation regions are always above the critical value of 26.7 degrees Celsius necessary to spawn hurricanes at this time of year. Therefore, like trying to put more water into a full bucket, higher ocean temperatures have relatively little effect on the frequency or intensity of hurricanes.
5. The rate of change of temperature with altitude. High rates of change encourage hurricane formation.
6. Relative humidity in the mid-troposphere, a layer centered at about 5 km above the surface.
Of these six factors, only wind shear has been exceptional this year in the Gulf of Mexico.
It was very low wind shear, not temperature, vorticity, humidity, or anything else, that is therefore the main cause of the high activity in this year’s hurricane season. And wind shear is a natural phenomenon that varies across the globe and is determined by a host of meteorological factors, none of which are under human control.
The global warming/hurricane connection completely falls apart when one looks at the observational data.
For example, during the 1945-1977 cooling period, when ocean temperatures worldwide were undoubtedly lower than today, we witnessed stronger hurricanes than now. Hurricane Camille, the second-most intense tropical cyclone to strike the U.S. on record, slammed into Mississippi as a category 5 (the strongest) hurricane on August 18, 1969, producing a storm surge of 7.3 meters. Hurricane Flora struck in September 1963, killing over 6,000 people. This was the highest number of fatalities for a tropical cyclone in the Atlantic Basin since the 1900 Galveston Hurricane, which may have claimed as many as 12,000 lives.
The strongest tropical cyclone on record worldwide was not recent, either. It was the 2,200 km-wide “Super Typhoon” Tip which made landfall in southern Japan on October 19, 1979, immediately following the global cooling period (by contrast, Irma was about 680 km in diameter; Harvey about 400 km).
The biggest storm surges worldwide were 14.5 meters in Australia in 1899 and 13.6 meters in Bangladesh in 1876. During the warmer 21st century, no tropical cyclone was strong enough to generate a surge greater than 10 meters.
The fact that hurricanes and other tropical cyclones are not caused by — or even significantly enhanced by — man does not give us an excuse to do nothing about them, however.
Florida and Texas need to engage in “vertical evacuation” by building multistory storm shelters that allow residents to take refuge above the storm surge, instead of today’s ineffective “horizontal evacuation,” forcing people to flee the waves on clogged highways. The U.S. should look to India’s storm shelter network for an example of success in this regard — no one need walk more than one kilometer anywhere on India’s Bay of Bengal coast (where the vast majority of India’s tropical cyclones hit) to get to a shelter. If India can afford it, why not the U.S.?
It is time to develop improved early warning systems and more storm shelters. Concerning ourselves with CO2 emission reduction is a wasteful distraction and will have no impact on future hurricane strength or frequency.
Hurricanes Are Not The Result Of Climate Change
A Washington Post-ABC News poll released on Thursday shows that 55% of Americans think that the severity of recent hurricanes is most likely “the result of global climate change.” That people believe this is not surprising—we are told over and over: man-made global warming has made the Gulf of Mexico warmer and the air more humid thereby making tropical cyclones—called hurricanes in the North Atlantic—more frequent and more intense. ‘We must reduce our carbon dioxide (CO2) emissions to lessen the increasing hurricane threat,’ they claim.
But this is completely wrong. Dr. Roy Spencer, a principal research scientist at the University of Alabama in Huntsville explains that “major hurricanes don’t really care whether the Gulf [of Mexico] is above average or below average in temperature.” Similarly, M. Mohapatra and V. Vijay Kumar of the India Meteorological Department state in their March 2017 research paper, “there is a decreasing trend in the tropical cyclone number over the North Indian Ocean in recent years, though there is an increasing trend in the sea surface temperature…”
So, if not temperature, what does make one hurricane season worse than another?
America’s ‘hurricane guru,’ the late Dr. Bill Gray, emeritus professor of atmospheric science at Colorado State University, showed that the seasonal hurricane frequency is determined by six factors:
- The rotational tendency, or vorticity, already present in the atmosphere.
- Pressure gradients determined by latitude.
- Wind shear, the changes in wind speed and direction that occur between layers in the atmosphere.
- Ocean thermal energy.
- The rate of change of temperature with altitude.
- Relative humidity in the mid-troposphere.
Of these six factors, only wind shear has been exceptional this year in the Gulf of Mexico. It was very low wind shear, not temperature, vorticity, humidity or anything else, that is therefore the main cause of the high activity in this year’s hurricane season. And wind shear is a natural phenomenon that varies across the globe and is determined by a host of meteorological factors, none of which are under human control.
The global warming/hurricane connection completely falls apart when one looks at the observational data. For example, it was during the 1945-77 global cooling period, when ocean temperatures worldwide were undoubtedly lower than today, that we witnessed stronger hurricanes than now. For example, Hurricane Camille, the second-most intense tropical cyclone to strike the U.S. on record, slammed into Mississippi as a category 5 (the strongest) hurricane on August 18, 1969, producing a storm surge of 7.3 meters.
The most powerful tropical cyclone on record worldwide was not recent either. Immediately following the cooling period, the 2,200 km-wide ‘Super Typhoon’ Tip, the strongest ever, made landfall in southern Japan on October 19, 1979 (Irma was 680 km across; Harvey 400 km). The biggest storm surges worldwide were 14.5 meters in Australia in 1899 and 13.6 meters in Bangladesh in 1876. During the warmer 20th and 21st centuries, no tropical cyclone was strong enough to generate a surge greater than 10 meters.
The fact that hurricanes and other tropical cyclones are not caused by, or even significantly enhanced by man does not give us an excuse to do nothing about them, however. Florida and Texas need to engage in ‘vertical evacuation’ by building multistory storm shelters that allow residents to take refuge above the storm surge, instead of today’s ineffective ‘horizontal evacuation,’ forcing people to flee the waves on clogged highways. The U.S. should look to India’s storm shelter network for an example of success in this regard—no one need walk more than one kilometer anywhere on India’s coast to get to a shelter. If India can afford it, why not the U.S.?
It is time to develop improved early-warning systems and more storm shelters. Concerning ourselves with CO2 emission reduction is a wasteful distraction and will have no impact on future hurricane strength or frequency.
Dr. Madhav Khandekar is a former Research Scientist with Environment Canada. He was an Expert Reviewer for the 2007 climate change documents prepared by the UN climate body, the IPCC. Tom Harris is Executive Director of the International Climate Science Coalition.
Irma illusions – and realities
If human emissions made Irma worse, did they also bring the 12-year lull in Cat 3-5 hurricanes?
Hurricanes Harvey and Irma brought out the best in us. Millions of Americans are giving money, toil and sweat to help victims rebuild. Unfortunately, the storms also highlighted some people’s baser instincts.
Some advanced ideological commitments to campaigns to “keep fossil fuels in the ground,” raise energy costs and reduce living standards. Others hyped Harvey’s record rainfalls, claiming carbon dioxide emissions made the Gulf of Mexico warmer and its air more moisture-laden. A few were just obnoxious.
These storms are a product of “this administration’s climate denial, racism and callousness,”350.org activist Jenny Marienau fumed. “How many once-in-a-lifetime storms will it take, until everyone admits manmade climate change is real?!” Daily Show comedian Trevor Noah fulminated.
Perhaps these newly minted “experts” received mail-order degrees in climatology or meteorology – or recently stayed at a Holiday Inn Express. They should at least take a few minutes to review hurricane and climate history, and talk to real climatologists and meteorologists, before launching tirades.
My geology, ecology and other studies taught me that climate change has been “real” throughout history. I’ve learned to be humble, respectful and vigilant in the face of nature’s power; to recognize that climate shifts can range from beneficial or benign to harmful or unbelievably destructive; and to understand that the sun and other powerful natural forces totally dwarf whatever meager powers humans might muster to alter or control Earth’s climate and weather.
Harvey marked the end of a record 12-year absence of Category 4-5 hurricanes hitting the US mainland. The previous 8-year record was set 1860-1869. NOAA’s Hurricane Research Division counts ten Category 4-5 monsters 1920-1969 (50 years) hitting the USA, but only three 1970-2016 (46 years). This year has brought two more, and the hurricane season isn’t over yet.
If Harvey and Irma were caused or intensified by human greenhouse gas emissions, shouldn’t those gases be credited for the 12-year lull and half-century decline in Cat 4-5 landfalling storms? For Irma’s changed intensity and route as it reached Florida and headed north? Certainly not.
If fossil fuels caused Harvey’s rainfall, were previous deluges like Hurricane Easy (45 inches in Florida, 1950), Tropical Cyclone Amelia (48 inches in Texas, 1978) and Tropical Storm Claudette (a record 43 inches in 24 hours on Alvin, Texas, 1979) the result of lower fossil fuel use back then? Highly unlikely.
Indeed, the National Oceanic and Atmospheric Administration (NOAA) concludes that neither the frequency of North Atlantic tropical storms and hurricanes, nor their energy level, has displayed any trend since 1950. Despite slightly warmer ocean waters in some regions, global Accumulated Cyclone Energy (ACE) levels in recent years have been at their lowest levels since the late 1970s.
When the Pacific Decadal Oscillation is in its cyclical positive phase, the tropics, west coast of North America and our Earth overall get warmer; cooling occurs during the PDO’s negative phase. The Atlantic Multi-decadal Oscillation (AMO) also cycles between warm and cool phases, affecting regional and planetary temperatures, as well as hurricane formation, strength and duration.
Any link between hurricanes and human carbon dioxide/greenhouse gas emissions is nebulous, tenuous and very poorly understood at this time. Asserted links to recent hurricanes are ideological illusions.
Hurricane Irma remained symmetrical and grew in size and intensity into the massive Category 5 hurricane seen in satellite photos, because it remained over warm water for a week as it crossed the Atlantic and Caribbean – and was not pulled apart by mid-altitude wind sheer – weather experts explained. Its encounter with Cuba’s coastal lands and mountains finally reduced its wind speeds and disrupted its symmetry.
Over Florida, strong north-to-south winds high in the atmosphere clipped the top off the hurricane. That further disturbed Irma’s shape and intensity, and steered the storm westward as it traveled north up the Citrus State. As is usually the case with storms moving north over Florida and parallel to its west coast, Irma’s front wall began to pull in both drier air and upwelling water. The bigger the storm the more it does this, WeatherBELL Analytics chief forecaster Joe Bastardi explained.
All these factor combined to slow whirling winds in the storm’s eyewall still more. It began wobbling on its axis, and Irma gradually became a disorganized tropical storm after it pounded Fort Meyers.
As to Hurricane Harvey, consulting meteorologist Joe D’Aleo notes that “hurricanes entering Texas are almost always very wet and often stall or meander.” This year, a large cool trough trapped Harvey and kept it from moving inland, enabling the Gulf of Mexico to feed it trillions of gallons of water for days, said Bastardi. It was “an unusual confluence of events,” said Weather Channel founder John Coleman, “but it was certainly not unprecedented.”
If there was a “human factor” in Harvey and Irma, climate alarmists need to explain exactly where it was, how big it was and what role it played. They must present hard evidence to show that fossil fuels and carbon dioxide emissions played a significant role amid, and compared to, the hundreds of natural forces involved in these storms. Their loud rhetoric only highlights their failure and inability to do so.
In fact, the Atlantic, Caribbean and Gulf of Mexico are warm enough every summer to produce major hurricanes, says climatologist Dr. Roy Spencer. But you also need other conditions, whose origins and mechanisms are still unknown: pre-existing cyclonic circulation off the African coast, upper atmospheric calm, sea surface temperatures that change on a cyclical basis in various regions, to name just a few.
The combination of all these factors – plus weather fronts and land masses along the way – determines whether a hurricane arises, how strong it gets, how long it lasts, and what track it follows.
Damage from hurricanes has certainly increased over the years. But that is because far more people now live and work in far more expensive communities along America’s Atlantic and Gulf coasts. Since 1920, Greater Houston has grown from 138,000 people to 5.7 million; Miami from 43,000 to 6.1 million; Tampa from 50,000 to 3 million.
Meanwhile, death tolls have declined – at least in countries where fossil fuels, highways and modern technologies enable us to construct stronger buildings, track storms, warn, evacuate and rescue people, and bring in water, food, clothing, and materials to rebuild power lines and buildings in stricken areas.
Over 6,000 people perished in the 1900 Category 4 Galveston Hurricane, 2,500 in the 1928 Okeechobee, Florida Category 4 hurricane and storm surge. More than 1,800 died in Katrina (Category 3), due largely to corrupt and incompetent local and state governments.
Thanks to better preparation, warning and evacuation, overall tragic deaths were kept to 82 from Harvey and 93 from Irma. Incredibly, despite the vicious 185-mph winds that reduced most of Anguilla and Barbuda to rubble, Irma killed only one person on those Caribbean islands.
Even in recent years, cyclones and hurricanes have brought far more death and destruction to poor nations where modern energy and technology are still limited or nonexistent: 400,000 dead in Bangladesh in 1970, 138,000 in Myanmar in 2008, and 19,000 from Hurricane Mitch in Central America in 1998.
It may be fashionable to focus on alleged “social costs of carbon” and asserted fossil fuel contributions to extreme weather events. But it is essential that we never forget the enormous benefits these fuels bring.
Our Earth is a complex, wondrous, resilient planet. But it can unleash incredible fury. Wealthy, technologically advanced nations fueled by oil, natural gas, coal and nuclear power are far better able to avoid, survive and recover from those disasters. We must count our blessings, but always be prepared.
Paul Driessen is senior policy analyst for the Committee For A Constructive Tomorrow (www.CFACT.org), and author of Eco-Imperialism: Green power – Black death and other books on the environment.
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.
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.
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.