(Written by Adeel Hassan and Patrick J Lyons)
Dorian is coming for Florida — that much gets clearer with each bulletin from the National Hurricane Center, even if the forecasters still can’t say just where in the state the storm will crash ashore.
The where is important, but so is the how much. Over the horizon, out in the summer-warm Atlantic, Dorian has been gaining energy since it blew through the Virgin Islands on Wednesday, and is winding up to deliver a devastating punch.
The National Hurricane Center said Thursday that Dorian was expected to hit the east coast of Florida over the weekend as a “major” hurricane, in Category 3 or possibly Category 4.
What do those labels say about how much destruction Dorian may wreak? Here is what you need to know to decipher them.
What do the categories mean?
Powerful winds are what define a hurricane, so they are named and classified based on how hard their winds are blowing. To qualify as a hurricane, a storm must have sustained winds of 74 mph or more.
All hurricanes are dangerous, but some pack more punch than others. So meteorologists try to quantify each storm’s destructive power by using the Saffir-Simpson scale, placing it in one of five categories based on sustained wind speed:
Category 1, 74 to 95 mph: These storms’ winds may knock down some trees and power lines and do a bit of damage to buildings. Dorian was in Category 1 when it blew through the Virgin Islands on Wednesday.
Also Explained | What a skull tells us about human evolution
Category 2, 96 to 110 mph: These storms are likely to uproot many trees, disrupt electric power over wide areas and do significant roof and siding damage.
Category 3, 111 to 129 mph: These are major storms that can take roofs off even well-constructed houses and knock out electric and water systems for days or weeks. Roads will be blocked by falling trees and poles. Dorian is forecast to be at least this strong when it makes landfall.
Category 4, 130 to 156 mph: These major storms do catastrophic damage, felling most trees and power poles and wrecking some buildings. Affected areas may be uninhabitable for days or weeks afterward.
Category 5, 157 mph or more: Storms this powerful are rare, and when they strike, they are immensely destructive. Few structures will come through a direct hit unscathed, and a large percentage of frame buildings will be destroyed. Recovery may take weeks or months.
What are some recent examples of Category 3 storms?
Hurricane Katrina was in Category 3 when it slammed into the Louisiana coast on Aug. 29, 2005, devastating New Orleans and other communities. But it had weakened a bit by then; at its peak over the Gulf of Mexico, Katrina was a Category 5 monster.
Hurricane Sandy in 2012 peaked as a Category 3 storm, but by the time it came ashore to wreak havoc on the New York metropolitan region, its winds had slowed to the point that it was no longer technically a hurricane.
How about Category 4 storms?
Hurricane Maria was a Category 4 storm when it hit Puerto Rico in 2017, ultimately leading to thousands of deaths and blacking out the island for months, with effects that linger today. Tens of thousands of homes on the island still have blue tarpaulins for roofs.
Hurricane Harvey was another Category 4 storm in 2017, making its first landfall in Texas at peak strength, jogging back out into the Gulf of Mexico and then coming ashore again as Category 3.
And Category 5?
Just four Atlantic hurricanes since 1924 have been this powerful when they made landfall in the United States. The most recent, Hurricane Michael, struck the Florida Panhandle last year, causing at least 59 deaths in the United States and about $25 billion in damage.
Before Michael, the country had gone 26 years without a Category 5 landfall, since Hurricane Andrew walloped South Florida in August 1992. Andrew was one of the biggest natural disasters in American history, blamed for 61 deaths and about $50 billion in damage in today’s dollars.
Hurricane Irma, which swept through the Caribbean and Cuba before heading for Florida in 2017, peaked at Category 5 strength. But it had weakened to Category 4 when it hit the Florida Keys and then Category 3 when it reached the mainland.
Is a storm’s category all we need to know about its dangers?
Some experts say the scale is a limited way to assess a storm’s destructive potential because it focuses only on the power of its winds, and not on the surge of seawater that a storm flings ashore, or the flooding caused by its torrential rains. Most hurricane fatalities and property damage tend to be caused by those factors, and not directly by the wind.
For example, it was Katrina’s storm surge that overwhelmed New Orleans’s flood walls and levees and devastated the city. Sandy’s storm surge was responsible for the vast majority of the harm the storm did in the New York area. And when Harvey stalled over Houston, its winds were far below its peak, but its record-setting rains flooded much of the city.
The National Oceanic and Atmospheric Administration said the severity of storm surge will vary greatly from one place to another on a coastline, depending on tides and local conditions, so there was no simple way to factor storm surge into hurricane classification classifications.
The agency’s storm forecasts include warnings about heavy rainfall and threats of flooding, but those, too, can vary greatly from location to location. So rain potential is not an ingredient in the category system, which is meant largely to forecast wind damage.