On August 27th, 2017, two weeks prior to its destructive trek through Florida and Georgia, Hurricane Irma was just another wave moving off the western coast of Africa. Unlike most African waves, this one was immediately identified by the National Hurricane Center as an area of disturbance with a high-likelihood of tropical development. Three days later, the National Hurricane Center upgraded the disturbance from merely a low-pressure system to Tropical Storm Irma, bypassing the intermediate “tropical depression” classification. Irma would go on to not only become one of the strongest storms in the Atlantic Ocean, but one of the most powerful storms on Earth.
Wind speed and atmospheric pressure are the two most common tools used to measure a tropical system’s strength. The Saffir-Simpson Scale uses a storm’s 1-minute sustained surface winds to place it in a category; those categories range from 1 to 5 with a category 1 storm having sustained surface winds between 74 and 95 miles-per-hour (mph) and a category 5 storm having sustained surface winds exceeding 156 mph. Atmospheric pressure is used to gauge strength (the lower the pressure, the stronger the storm), but wind speed is the only factor used in determining a storm’s place on the Saffir-Simpson Scale.
Hurricane, Cyclone, Typhoon…Which is it?
Tropical cyclones are referred to as hurricanes in the Atlantic and eastern Pacific, typhoons in the western Pacific, and cyclones in the Indian Ocean and south Pacific (and ‘medicanes’ in the Mediterranean Sea!). To make things more confusing, the Caribbean Sea and Gulf of Mexico are part of the Atlantic Ocean Basin, but storms in the Caribbean and Gulf are often differentiated from those also in the Atlantic, but outside the Caribbean and Gulf. This is because sea surface temperatures in the Gulf and Caribbean tend to be much higher than those in the surrounding Atlantic waters, which helps produce much stronger storms.
Above is a chart of the wind and pressure records for the Atlantic basin and western and eastern Pacific basins – these three basins historically produce the strongest tropical cyclones on Earth. Hurricanes Irma, Katrina, and Maria are included for comparison purposes. The above data doesn’t necessarily represent the storm’s strength at landfall as most storms weaken prior to coming ashore. For instance, both Hurricanes Katrina and Irma made landfall at 920 mbar, though both had significantly lower pressure at their maximum strength.
Irma’s Record-Setting Intensity
On September 4, 2017, Hurricane Irma exploded into one of the most intense tropical cyclone’s on record. After several days vacillating between a category 2 and 3 hurricane, it strengthened from a 120 mph/944 mbar storm to a strong category 5 storm with winds of 185 mph and a pressure of just 916 mbar.
Irma’s 37 hours spent with 185+ mph winds easily surpassed the previous record of 24 hours held by 2013’s Typhoon Haiyan. Irma’s pressure of 914 mbar broke the record for the lowest pressure of any tropical system in the Atlantic outside of the Caribbean Sea and Gulf of Mexico. The storm’s 185 mph winds are tied for the second highest in the Atlantic (after Allen), though the three other storms it’s tied with (Gilbert, Wilma, and the “Labor Day” storm) occurred in the traditionally warmer waters of the Caribbean and Gulf.
Irma’s burst in intensity is reminiscent of Hurricane Harvey, which struck Southeast Texas in August of 2017. That storm was predicted by the National Hurricane Center and most forecasting models to be no stronger than a Category 1 storm at landfall, though Harvey had intensified to a Category 4 when it rolled into Texas. Hurricane Jose, which followed right behind Irma, was only predicted to possibly reach Category 3 status, though it quickly developed into a Category 4 storm. Similarly, in 2015 Hurricane Patricia intensified from a tropical storm with winds of 65 mph to a powerful category 5 hurricane with winds of 200 mph in just two days. Patricia holds the global sustained wind speed record for tropical cyclones at 215 mph. Had it not weakened prior to landfall, Patricia would have surpassed 2013’s Typhoon Haiyan as the strongest storm to make landfall in history.
Warmer Ocean Temperatures (Possibly) Lead to Stronger Storms
Hurricanes need not only warm water to intensify, but relatively high humidity levels and low wind shear. At a minimum, the sea surface temperatures must be 80°F, though every degree of warming results in four percent more evaporation (which creates more humidity). Temperature, humidity, and low wind shear have to come together to produce storms of great magnitude. It should therefore come as no surprise to see stronger storms in the Pacific and Atlantic if water temperatures in the northern latitudes increase. High wind shear and low humidity levels could still deter tropical development, but the mere presence of warmer-than-usual water gives tropical storms the ability to intensify quickly once they find favorable conditions.
Every degree matters for storms. While humans can hardly tell the difference between 83°F and 85°F, a .5, 1, or 2 degree change in water temperatures can have a dramatic impact on the development of tropical systems. As the chart below shows, sea surface temperatures in parts of the Atlantic basin were 1-5°C (1.8-7.2°F) above average during Irma’s rapid intensification.
As Georgia found out, even a tropical system that doesn’t hit the U.S. as a Category 5 storm can have a major impact. Irma’s path up Florida’s west coast as a quickly-diminishing category 4 hurricane brought significant flooding to Georgia’s coast, caused massive statewide power outages, and closed schools for several days. The shear amount of energy Irma was able to amass from its record-setting time as a Category 5 storm allowed it to exert influence over a large portion of the Southeast. Despite making landfall some 400+ miles away, the amount of water Irma dragged through the Caribbean and its large area of tropical storm force winds resulted in damaging storm surge on the coasts of Georgia and South Carolina.
While the Southeast U.S. got hit hard, much of the Caribbean was decimated. All 1,800 residents of Barbuda were left homeless as 90 percent of the structures on the island were destroyed. While an eventual evacuation of the island would have been necessary given the destruction, those evacuation efforts were expedited as Category 4 Hurricane Jose followed right behind Irma. The US Virgin Islands were completely stripped of all signs of life.
Given This, It Makes Sense to Cut the National Weather Service’s Budget
While communities in Texas and Florida are well on their way to full recovery, Caribbean communities may never recover without massive amounts of foreign aid. Let’s not forget that people in those communities took the brunt of a storm that caused mere power outages for those of us in Atlanta. The African wave that progressed from Invest 91L to Potential Tropical Cyclone 9 to Tropical Storm Harvey to Hurricane Harvey was partially responsible for massive mudslides in Sierra Leone on August 14, 2017 that killed around 1,000 people. Nearly everyone in Puerto Rico (who are US citizens) went several months without power after Hurricane Maria slammed into the island as a strong category 4 storm. Many still didn’t have electricity 6 months after the storm passed.
We officially enter the 2018 hurricane season on June 1, just a couple months after the White House proposed a 23 percent cut to the National Oceanic and Atmospheric Administration’s (NOAA) 2019 budget (NOAA houses the National Weather Service and the National Hurricane Center). This is a perplexing proposal given the intensity and unpredictability of the 2017 hurricane season. While our national forecast and warning systems have made significance gains in accuracy, Hurricanes Irma, Harvey, Maria, and Jose are proof that we still have a ways to go in predicting and forecasting intensity. While such a cut has almost no chance of passing Congress (it increased NOAA’s 2018 budget by 4 percent), this is the exact wrong time to be proposing any cut, let alone such a steep cut, to the budget of the administration tasked with making predictions when our weather and climate have become increasingly volatile.