Hurricane Irma: A close shave for Florida
With wind speeds of up to 300 km/h, Hurricane Irma will go down in history as a storm of superlatives. What is more, if its track had shifted only slightly, losses in Florida would have been substantially higher. The unexpectedly low level of damage in the state may also have been due to improved building regulations. Even so, initial estimates put overall losses at US$ 67bn and insured losses at US$ 32bn, making Irma the fourth-costliest hurricane of all time.
Tropical cyclone Irma developed on 28 August 2017 from a tropical wave in the Atlantic that originated in Africa. As it moved west, it intensified with the help of exceptionally warm ocean waters to become a Category 5 hurricane on the Saffir-Simpson scale, with peak wind speeds of 300 km/h. Irma thus holds the record for the most powerful cyclone ever measured over the Atlantic, not including the Caribbean and the Gulf of Mexico.
After making initial landfall on 6 September on the Lesser Antilles at peak strength, Irma proceeded to devastate numerous islands in the Caribbean. Later, as it tracked westwards, the storm passed over extremely warm ocean waters with temperatures of up to 34°C. Because of its relatively high forward speed, the storm quickly crossed a large area with high ocean temperatures, helping it to intensify rapidly. Hurricanes that track more slowly restrict their intensity more because they remain longer over sea surfaces that have been mixed by the storm and are thus cooler.
Irma next grazed the coast of Cuba before veering north and hitting the Florida Keys as a Category 4 hurricane. Several hours later, when it finally struck Marco Island on the south-west coast of Florida, it had weakened slightly to a Category 3 hurricane.
Destructive path difficult to predict
The track a hurricane takes is mainly determined by the wind direction in the surrounding area. Irma moved in a westerly direction at the southern edge of a subtropical area of high pressure rotating in a clockwise direction. At the western edge of the high, Irma came under the influence of a trough approaching from the west, whose (counterclockwise) winds steered it northwards towards Florida.
The predicted track is based on the forecasted intensity of the high- and low-pressure areas. Even small variations can have serious consequences when a cyclone makes landfall on a narrow landmass such as Florida. For example, a cyclone along the east coast of Florida, with the enormous concentration of values in Miami, Cape Canaveral and Jacksonville, could have produced many times the losses of a storm further west. But in the west of Florida there are also sizeable cities like Fort Myers, Naples and Tampa. These centres would potentially be affected by the right, strong-wind side of a cyclone moving over the sea along the west coast. Uncertainty among those responsible for disaster management, and among the general population of Florida was correspondingly high.
The actual track the storm finally took was extremely fortunate in as much as it released its greatest power over the sparsely populated interior of Florida, where it was also cut off from its source of energy, the warm ocean.
Damage and impact
Photos and satellite images showed severe devastation on Barbuda, on the French and Dutch Caribbean islands of Saint Martin/Sint Maarten and Anguilla, and throughout the British Virgin Islands and the two northern American Virgin Islands, St. Thomas and St. John. Irma’s impact was also felt on Puerto Rico, the Dominican Republic, Haiti, Turks and Caicos, the southern Bahamas and in Cuba. Even though a strip of the Florida Keys was devastated by the storm and storm surge, there were unexpectedly few reports of damage from the mainland and from most of the Florida Keys. In all probability, this is due to a tightening of building regulations after Hurricane Andrew. In 1992, the latter made landfall in Florida near Miami as a Category 5 hurricane, and moved across the southern part of the state with devastating wind speeds.
Despite the relatively low level of damage, extensive evacuation measures in Florida led to chaotic conditions on the peninsula. Most of the population, alarmed by the potentially record-breaking size and intensity of the hurricane, and by media reports on the devastation it had caused on islands in the Caribbean, responded to the official call to evacuate. This led to overcrowded airports, gridlocked roads, long queues at filling stations, fuel shortages, and looting in cities that were virtually deserted.
The fact that Irma’s landfall in Florida was forecast more than five days in advance reflects the high quality of the weather forecast. But even in the future it will remain impossible to predict the track of hurricanes precisely, so those responsible for disaster management will need to weigh up whether or not to order an evacuation each time an extreme event occurs. The probabilities of alternative scenarios provided by meteorologists in so-called ensemble forecasts can help decision-making here.