The Gulf Coast has been badly battered by hurricanes in recent weeks. Harvey submerged much of Houston, causing destruction that will take years and billions of dollars to undo; Irma roared north through Florida, and with the power still out across much of the state, the damage is still being tallied. The storms elicited a nation of nervous weather-watchers and exposed several myths.
This seems intuitive. The Weather Company has raised the number of named storms that it expects to develop in its latest 2017 Atlantic hurricane season outlook, warning of "higher-than-normal landfall risks in the northeast U.S." Colorado State University's seasonal outlook said that the probability of major hurricanes making landfall in the United States was above normal this year "due to the forecast for an above-average season."
Actually, there is only some correlation between the total number of storms in a season and the number of storms making landfall. In 2004, there were 15 named storms, and eight struck the United States. In 2010, there were 19 named storms but only two U.S. landfalls. (The total in both years was well above the long-term average for the Atlantic.) It only takes one roaring storm for an inactive season to be considered awful: 1992 saw just seven storms, well below the average, but one of them was Hurricane Andrew, at the time the nation's costliest hurricane.
How many storms make landfall depends on tropical currents, such as the trade winds, that steer hurricanes. These systems shift around and can strengthen or weaken. For instance, if the Bermuda High current moves closer to the U.S. mainland, as it did in 2004 and 2005, more storms land on our shores. If the Bermuda High moves closer to Africa, more tend to curve over the ocean instead of striking the United States.
The well-known Saffir-Simpson hurricane intensity scale defines five categories of storms. These levels are based on the maximum sustained wind, which seems like the most meaningful metric. "The lower the pressure, the more intense the hurricane, especially in terms of wind speed and damage," the ABC affiliate in Chicago reported this month. And before Harvey, news outlets suggested that as a Category 4 storm, it would be "stronger" than Katrina.
But size matters, too. Both the size of a storm's wind "footprint" and the strength of its winds control the height of the storm surge. Compare Hurricanes Charley (2004) and Ike (2008). Charley packed 120 mph sustained winds and was very small. Ike was about 10 times larger but considerably weaker, with 95 mph winds. Charley's surge along Florida's west coast was in the six-to-seven-foot range, while Ike devastated the Texas-Louisiana coast with a very broad surge reaching 12 to 17 feet in height. The difference in surge heights largely reflected Ike's much larger size.
Storm rainfall, which can be catastrophic, also has little to do with wind intensity. Some of our worst coastal flooding disasters from tropical cyclones have come from humble tropical storms: Allison, for example, brought inland Texas about 40 inches of rain in 2001.
When hurricanes strike the United States, attention focuses on the coastline, where watches and warnings are issued. According to LiveScience, the big cities at greatest risk are coastal ones: Houston; New Orleans; Mobile, Ala.; Tampa; Miami; Boston; New York and others. "If you live hundreds of miles inland," the automobile site Jalopnik put it in an article on how to escape a storm's path, ". . . you don't need to worry."
It's true that storms lose 50 percent of their wind intensity within 12 to 18 hours of landfall. But hurricane remnants have surprisingly long reach. Many storms throughout U.S. history have projected formidable hazards hundreds of miles inland, sometimes days after landfall. And tropical remnants that join with preexisting weather systems, such as fronts and jet stream disturbances, have new sources of energy to sustain them. Inland hazards have been particularly devastating over the mountainous regions of the East Coast, where steep terrain lifts tropical moisture.
Infamous examples include Camille (1969), which made landfall over Louisiana but killed half its victims days later in a horrific flood in Virginia's Blue Ridge Mountains. Agnes (1972) made landfall in the Florida Panhandle but unleashed its worst flooding in central Pennsylvania . And Ivan (2004) first struck Alabama but spun up a record-breaking swarm of damaging tornadoes across Northern Virginia and central Maryland.
Hurricanes are huge, atmospheric vortices, and they are properly feared for their widespread, high-energy onslaughts of wind. The Weather Channel holds that "wind is responsible for much of the structural damage caused by hurricanes." Floridians understandably see wind as their enemy: Hurricane Andrew was a landmark storm, killing 65, to the tune of $27 billion , most of this because of winds exceeding 175 mph.
But wind is only one of a deadly triad of impacts that hurricanes deliver. The storm surge — a sudden rise in sea level along the coast, where ocean water is pushed inland by the hurricane's strongest winds — causes far more fatalities. The maximum surge is confined to the eyewall of the storm, but its effects can be broad: In 1876, a storm surge killed as many as 400,000 people in a single cyclone along coastal Bangladesh. A recent study shows that, in the United States, the majority of hurricane-related deaths come from water.
Sometimes this means storm surge, but more often people drown in freshwater. Torrential rains often dump six to 12 inches or more, leading to flash flooding. Hurricane Katrina (2005) produced the highest death toll of any hurricane — at least 1,000 — since the Okeechobee hurricane in Florida in 1928, and most of the fatalities were drownings.
But the mission of the National Weather Service, which raised urgent warnings about the latest hurricanes, is to protect property and save lives. And it only makes sense for local news outlets and private forecasters to follow their lead; it is a standard, and responsible, practice for these organizations to post every official watch and warning issued by the hurricane center.
Furthermore, many scientists are cautious about the connection between hurricane activity and climate change. The National Oceanic and Atmospheric Administration, for instance, has said a detectable human influence has not yet been discovered, though it projects increases in hurricane intensity during the coming decades.