Hurricane Laura put on a phenomenal show of rapid intensification prior to landfall, increasing in strength by 65 mph in just 24 hours on August 26, 2020. That ties Hurricane Karl of 2010 for fastest intensification rate in the Gulf of Mexico on record. In the 24 hours prior to landfall, Laura’s winds increased by 45 mph, and the mighty hurricane made landfall in western Louisiana as a category 4 storm with 150 mph winds – the strongest landfalling hurricane in Louisiana history, and the fifth-strongest hurricane on record to make a continental U.S. landfall.
Laura’s rapid intensification was a disturbing déjà vu of what had happened just two years earlier.
As Hurricane Michael sped northwards on October 9, 2018, towards a catastrophic landfall on Florida’s Panhandle, the mighty hurricane made an exceptionally rapid intensification. Michael’s winds increased by 45 mph in the final 24 hours before landfall, taking it from a low-end category 3 hurricane with 115 mph winds to catastrophic category 5 storm with 160 mph winds. And Michael’s performance echoed what had happened in 2017, when Hurricane Harvey rapidly intensified by 40 mph in the 24 hours before landfall, from a Category 1 storm with 90 mph winds to a Category 4 storm with 130 mph winds.
Human-caused climate change causing more rapidly intensifying Atlantic hurricanes
Unfortunately, not only is human-caused climate change making the strongest hurricanes stronger, it is also making dangerous rapidly intensifying hurricanes like Laura and Michael and Harvey more common.
According to research published in 2019 in Nature Communications, Atlantic hurricanes showed “highly unusual” upward trends in rapid intensification during the period 1982 – 2009, trends that can be explained only by including human-caused climate change as a contributing cause. The largest change occurred in the strongest 5% of storms: for those, 24-hour intensification rates increased by about 3 – 4 mph per decade between 1982 – 2009.
Led by hurricane scientist Kieran Bhatia of NOAA’s Geophysical Fluid Dynamics Laboratory – and titled “Recent increases in tropical cyclone intensification rates” – the study used the HiFLOR model to simulate intense hurricanes. HiFLOR is widely accepted as the best high-resolution global climate model for simulating intense hurricanes.
Dangerous scenario – rapidly intensifying hurricane making landfall
Rapidly intensifying hurricanes like Michael and Harvey that strengthen just before landfall are among the most dangerous storms, as they can catch forecasters and populations off guard, risking inadequate evacuation efforts and large casualties. A particular concern is that intensification rate increases are not linear as the intensity of a storm increases – they increase by the square power of the intensity.
Lack of warning and rapid intensification just before landfall were key reasons for the high death toll from the 1935 Labor Day hurricane in the Florida Keys, the most intense hurricane on record to hit the U.S. That storm intensified by 80 mph in the 24 hours before landfall, and it topped out as a Category 5 hurricane with 185 mph winds and an 892 mb pressure at landfall. At least 408 people were killed, making it the eighth-deadliest hurricane in U.S. history.
Another rapidly intensifying hurricane at landfall, Hurricane Audrey in June 1957, tracked on nearly the same course as Hurricane Laura. Audrey was the seventh deadliest U.S. hurricane, killing at least 416. Its winds increased by 35 mph in the 24 hours before landfall near the Texas/Louisiana border as a Category 3 hurricane with 125 mph winds. Lack of warning and an unexpectedly intense landfall were cited as key reasons for the high death toll.
With today’s satellites, radar, regular hurricane hunter flights, and advanced computer forecast models, the danger that another Audrey or 1935 Labor Day hurricane could take us by surprise is lower.
But all of that sophisticated technology didn’t help much for 2007’s Hurricane Humberto, which hit Texas as a Category 1 storm with 90 mph winds. Humberto had the most rapid increase in intensity, 65 mph, in the 24 hours before landfall of any Atlantic hurricane since 1950. A mere 18 hours before landfall, the National Hurricane Center (NHC) in 2007 had predicted a landfall intensity of just 45 mph, increasing its forecast estimate to 65 mph six hours later. It’s fortunate that Humberto was not a stronger system, as the lack of adequate warning could have led to serious losses of life.
Historical records show that since 1950, the eight storms have intensified by at least 40 mph in the 24 hours before landfall. It is sobering to see three of those storms, below in bold face, occurred in the past four years:
Humberto, 2007 (65 mph increase);
King 1950 (60 mph increase);
Eloise 1975 (60 mph increase);
Danny 1997 (50 mph increase);
Laura 2020 (45 mph increase);
Michael 2018 (45 mph increase);
Harvey 2017 (40 mph increase); and
Cindy 2005 (40 mph increase).
Extreme rapid intensification rates just before landfall to become more common
In a 2016 study – “Will Global Warming Make Hurricane Forecasting More Difficult?” from the Bulletin of the American Meteorological Society – MIT hurricane scientist Kerry Emanuel used a computer model that generated a set of 22,000 landfalling U.S. hurricanes between 1979 and 2005. Emanuel then compared their intensification rates to a similar set of hurricanes generated in the climate expected at the end of the 21st century.
For the future climate, he assumed a business-as-usual approach to climate change – the path we are currently on. Emanuel found that the odds of a hurricane intensifying by 70 mph or more in the 24 hours just before landfall were about once every 100 years in the climate of the late 20th century. But in the climate of the year 2100, these odds increased to once every 5 – 10 years.
What’s more, 24-hour pre-landfall intensifications of 115 mph or more, essentially nonexistent in the late 20th-century climate, would occur as often as once every 100 years by the year 2100. Emanuel found that major metropolitan areas most at risk for extreme intensification rates just before landfall included Houston, New Orleans, Tampa/St. Petersburg, and Miami.
Eight-fold increase in ultra-intense hurricanes predicted
The same HiFLOR high-resolution global climate model for simulating intense hurricanes referenced above produced some rather startling findings detailed in a 2018 paper, Projected Response of Tropical Cyclone Intensity and Intensification in a Global Climate Model.
The scientists who authored that paper forecast a dramatic increase in the global incidence of rapid intensification as a result of global warming, and a 20% increase in the number of major hurricanes globally.
For the Atlantic, the model projected an increase from three major hurricanes per year in the climate of the late 20th century, to five major hurricanes per year in the climate of the late 21st century.
The HiFLOR model also predicted a highly concerning increase in ultra-intense Category 5 tropical cyclones with winds of at least 190 mph – from an average of about one of these Super Typhoon Haiyan–like storms occurring once every eight years globally in the climate of the late 20th century, to one such megastorm per year between 2081 to 2100 – a factor of eight increase.
Even more concerning was that the results of the study were for a middle-of-the road global warming scenario (called RCP 4.5), which civilization will have to work very hard to achieve. Under the current business-as-usual track, the model would be expected to predict an even higher increase in ultra-intense tropical cyclones.
One technique for computing hurricane damage uses ICAT’s damage estimator to review all contiguous land-falling U.S. hurricanes between 1900 – 2017. That technique computes the amount of damage they would do currently and corrects for changes in wealth and population. It finds that while Category 4 and 5 hurricanes made up only 13% of all U.S. hurricane landfalls during that period, they caused 52% of all the hurricane damage.
Given that assessment, it’s very concerning that the HiFLOR model, the best model for simulating current and future behavior of Category 4 and 5 hurricanes, is predicting a large increase in the number of these destructive storms. Even more concerning is the model’s prediction of a global factor of eight increase in catastrophic Category 5 storms with winds of at least 190 mph by the end of the century – and that under a moderate global warming scenario.
All of which leads to the regrettable conclusion that the prospects for quickly intensifying storms as they approach landfall are likely to increase in a warming world.