A “black swan” hurricane—a storm so extreme and wholly unprecedented that no one could have expected it—hit the Lesser Antilles Islands in October 1780. Deservedly called The Great Hurricane of 1780, no Atlantic hurricane in history has matched its death toll of 22,000. So intense were the winds of the Great Hurricane that it peeled the bark off of trees--something only EF5 tornadoes with winds in excess of 200 mph have been known to do. However, hurricanes even more extreme than the Great Hurricane of 1780 can occur in a warming climate, and can be anticipated by combining physical knowledge with historical data. Such storms, which have never occurred in the historical record, can be referred to as “grey swan” hurricanes, according to research published in Nature Climate Change in 2015 by Kerry Emanuel of MIT and Ning Lin of Princeton University (press release here.) Using a detailed hurricane model embedded within six different global climate models, the scientists showed that the risk of extreme “grey swan” hurricanes for three specific locations--Tampa, Florida; Cairns, Australia; and the Persian Gulf--may increase by up to a factor of fourteen by the end of the century, thanks to our changing climate. We'll focus in this post on the results for Tampa.
Figure 1. Damage to Tampa’s Bayshore Boulevard after the 1921 Tampa Bay hurricane.
Figure 2. Track of the Tampa Bay Hurricane of 1921, one of only two major hurricanes ever recorded to hit the city. This Category 3 storm with 115 mph winds brought a storm tide of 10 - 11.5 feet (3 - 3.5 meters) to Tampa Bay.
Tampa’s hurricane history: only two major hurricanes since 1848 Tampa Bay doesn't get hit very often by hurricanes. This is because the city faces the ocean to the west, and the prevailing east-to-west trade winds at that latitude make it uncommon for a storm to make a direct hit on the west coast of Florida from the ocean. This is fortunate, since the large expanse of shallow continental shelf waters offshore from Tampa Bay (less than 300 feet deep out to 90 miles offshore) is conducive for allowing large storm surges to build. In a worst-case scenario, with a powerful hurricane traveling north-northwestwards at just the right speed parallel to the coast, the geometry of the coast creates a unique additional rise in the water level due to a phenomenon known as a coastally trapped Kelvin Wave.
Figure 3. Two possible tracks of the Great Gale of 1848, the most violent hurricane in Tampa's history, a Category 3 or 4 hurricane with 115 - 135 mph winds. A 15-foot storm surge (4.6 meters) was observed in what is now downtown Tampa.
The last time Tampa suffered a direct hit by any hurricane was 1946, when a Category 1 storm came up through the bay. The Tampa Bay Hurricane of October 25, 1921 was a the last major hurricane to make landfall in the Tampa Bay Region. This low-end Category 3 storm with 115 mph winds at landfall brought a storm tide of 10 - 11.5 feet (3 - 3.5 meters), causing severe damage ($10 million in 1921 dollars.) The only other major hurricane to hit the city occurred on September 25, 1848, when the Great Gale of 1848, the most violent hurricane in Tampa's history, roared ashore as a Category 3 or 4 hurricane with 115 - 135 mph winds. A 15-foot storm surge (4.6 meters) was observed in what is now downtown Tampa, and the peninsula where St. Petersburg lies, in Pinellas County, was inundated, making St. Petersburg an island. A large portion of what few human structures were then in the area were destroyed.
Figure 3. Predicted height above ground of the water from a worst-case Category 4 hurricane in the Tampa Bay region, as computed using NOAA's SLOSH storm surge model. Downtown Tampa Bay would be inundated by more than 20 feet of water, and St. Petersburg would become an island, as occurred during the 1848 hurricane. Image taken from our storm surge inundation maps.
Tampa fury: global warming may make grey swan hurricanes up to 14 times more likely The risk of grey swan hurricanes increased in the models used in the study due to an increase in both the frequency and intensity of hurricanes in the future climate. While an increase in the frequency of hurricanes due to global warming is something many models run by other hurricane scientists do not show happening, there is widespread agreement among hurricane scientists that the strongest storms should get stronger, and a number of studies have shown that Cat 4/5 storms already make up an increasing proportion of tropical cyclones, at least in some basins (though the quality of our observations adds considerable uncertainty to this finding.) Stronger storms are to be expected, because hurricanes are heat engines that extract heat energy from the oceans and convert it to wind energy, and any increase in ocean heat energy due to global warming can be expected to increase the maximum potential intensity a hurricane can reach. An additional factor not considered: potential intensity theory suggests that hurricanes will increase in size; storms that have a larger diameter push up a larger and more damaging storm surge since strong winds are blowing over a larger ocean area. Also, the study did not consider the impact of sea level rise, which will steadily increase storm surge damage in the coming decades. A 2007 study by Tufts University, Florida and Climate Change, found that a 2.25 foot increase in sea level--which many sea level rise scientists expect will happen by the end of the century--would put 152,000 people in Pinellas County (where St. Petersburg is located) under water at high tide.
A ferocious upper-end Category 3 or stronger hurricane, capable of delivering a storm surge of 20 feet (6 meters) or higher, had about a 1-in-10,000 year recurrence interval in the historical climate of 1980–2005, the scientists estimated. In other words, such a storm had a 0.01 percent chance of occurring in any given year, or a 0.3 percent chance when summed up over a 30-year period. However, in a business-as-usual global warming scenario, the six climate models showed that such a storm might become between four and fourteen times more likely by the end of the century—a 1-in-2,500 to 1-in-700 year event. Summed up over a 30-year time period, a 1-in-700 year event has a 4 percent chance of occurrence—something disaster planners should definitely think about.
Tampa's "Category 6" hurricane: a mind-boggling 830 mb storm with 233 mph winds The researchers found that extreme Category 5 hurricanes capable of delivering a storm surge of 26 - 36 feet (8 -11 meters) to Tampa had an extremely low or negligible probabilities in the climate of the late 20th Century, but are projected to happen as 1-in-5,000 to 1-in-150,000-year events in the late twenty-first century. We might need to invent a "Category 6" designation for the 1-in-150,000 year storm that came up in their simulations--a run of the HADGEM climate model that showed an unimaginably intense hurricane with a central pressure of 830 mb, top sustained winds of 233 mph, traveling parallel to the coast along just the right track to generate a titanic 36-foot storm surge. Even accounting for the 15% reduction in winds that would occur due to friction over land, the winds from such a “Category 6” hurricane would be like those of the EF5 tornado that leveled Joplin Missouri--except that EF4 to EF5 damage would be along a swath 22 miles wide, instead of a few hundred yards wide! A July 2016 paper by Columbia University hurricane scientist Adam Sobel and colleagues in Science, Human influence on tropical cyclone intensity, stated that we should expect to see about a 2.2 mph (1 m/s) per decade increase in the winds of the strongest hurricanes, or about 19 mph by the year 2100. If we assume the 215 mph sustained winds of 2015’s Hurricane Patricia (off the Pacific coast of Mexico) as the current maximum potential intensity that a hurricane can reach, a 235 mph hurricane by the end of the century is definitely a possibility.
Other estimates of Tampa Bay's hurricane risk Extreme "grey swan" hurricanes are difficult to model, since they are such rare and extreme events. Thus, we should consider all the estimates of the potential return periods of such storms as highly uncertain.
In 2010, the Tampa Bay Regional Planning Council put out the Tampa Bay Catastrophic Plan, a scenario where a Category 5 "Hurricane Phoenix" hits downtown Tampa with 160 mph winds and a 26-foot storm surge. The study projected that the city would see about 2,000 deaths and nearly $250 billion dollars in damage.
Dr. Peter Sousounis, a meteorologist for risk modeling company AIR Worldwide, told me in an email that their maximum loss-causing event for Tampa Bay is a Category 5 storm with sustained winds of 220 mph that hits Manatee County with a central pressure of 887 mb. This nightmare storm generates $220 billion in insured losses in a four county region (Manatee, Pinellas, Pasco and Hillsborough counties)--a level of loss for those counties that has a 1-in-10,000 year recurrence interval. Since uninsured losses from a hurricane strike are usually roughly as much as the insured losses, the total damage from this storm might top $400 billion dollars.
An August 2015 report by Karen Clark & Company, Most Vulnerable US Cities to Storm Surge Flooding, cited Tampa/St. Petersburg as the most vulnerable metropolitan area in the U.S. to storm surge damage. Their 1-in-100 year storm--a strong Category 4 hurricane with 150 mph winds--could be expected to cause $175 billion in damage just from the storm surge.
Two mass evacuations in Tampa in the past 35 years Two hurricanes have prompted mass evacuations of more than 300,000 people from the Tampa Bay area over the past 35 years. The first was Hurricane Elena of 1985, a Category 3 hurricane that stalled 80 miles offshore for two days on Labor Day weekend, bringing a 6 - 7 foot storm surge, wind gusts of 80 mph, and torrential rains. On August 13, 2004, another mass evacuation was ordered for Hurricane Charley. Thanks to a late track shift, Charley missed Tampa Bay, and instead hit well to the south in Port Charlotte as a Category 4 storm with 150 mph winds. More limited evacuations of low-lying areas and mobile homes in the 4-county Tampa Bay region were ordered for three other hurricanes in the past twenty years--Hurricane Georges of 1998, Hurricane Frances of 2004, and Hurricane Jeanne of 2004.
Figure 4. A sensible hurricane awareness effort: Hillsborough County, Florida got a $30,000 grant to post 30 of these signs around the Tampa Bay area to show how high a storm surge from a major hurricane might reach. This sign by a McDonalds at 19th Ave and Highway 41 is thirteen feet above ground level. Image credit: photonews247.com.
Tampa Bay's vulnerability to hurricanes When the 1921 hurricane hit Tampa Bay, there were 160,000 residents in the 4-county region, most of whom lived in communities on high ground. Today there are 2.8 million residents in the region, and that number is growing by about 50,000 people per year. Most of the population in the 4-county Tampa Bay region lives along the coast in low-lying areas; about 50 percent of the population lives at an elevation less than ten feet. Over 800,000 people live in evacuation zones for a Category 1 hurricane, and 2 million people live in evacuation zones for a Category 5 hurricane, according to the 2010 Statewide Regional Evacuation Study for the Tampa Bay Region. Given that only 46% of the people in the evacuation zones for a Category 1 hurricane evacuated when an evacuation order was given when 2004's Category 4 Hurricane Charley threatened the region, the potential exists for hundreds or even thousands of people to die when the next major hurricane hits.
References Barnes, J., 1999, Florida’s Hurricane History. The University of North Carolina Press.