How should we make sense of the different forecasting models? Why is the storm so unpredictable? And, how bad will it be?
As Hurricane Florence approaches, we’ve received many questions from readers about the science of forecasting hurricanes, and how climate change affects them. Here are some answers.
There are at least 20 computer forecasting models out there, though only five of them are considered accurate by most meteorologists. And for a hurricane like this one, you only need to concern yourself with one: the official model released by the National Hurricane Center. Most local weather services will be taking the center’s lead.
The hurricane center combines the data from the top five models to create its official forecast. Researchers weight the consensus forecast in favor of the models that have performed better (the ones that have recently predicted real-world outcomes).
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In recent years, the so-called European model, developed at the European Center for Medium-Range Weather Forecasts in Reading, England, has gained a reputation as the best. But for Hurricane Florence, the American model, which is officially called the Global Forecast System and is run by the National Weather Service, has been outperforming the European model, said Jeff Masters, director of meteorology at the forecasting service Weather Underground.
Companies like Weather Underground make their own forecasts for routine weather, but for hurricanes they go with what the hurricane center says. The center doesn’t provide forecasts beyond five days, however, so anything you’re seeing in the five-to-10-day window is based on weather companies’ own modeling and projections.
The cone of uncertainty, formally called the National Hurricane Center Track Forecast Cone, shows the area where the center of the storm is likely to go. The eye of the hurricane stays within the cone about two-thirds of the time, though people outside the cone can still experience catastrophic winds, floods and storm surges.
The cone may seem frustratingly wide for people in the path of the storm trying to make what could be life-or-death decisions. But the cone has actually shrunk in recent years, according to Dennis Feltgen, a meteorologist and spokesman for the National Hurricane Center. The cone shrinks as our ability to predict the weather improves. It’s still large because predicting weather is hard in general and predicting hurricanes is even harder.
[Scientists want to clear up these misconceptions about hurricanes .]
We’ve gotten so used to weather predictions being timely and accurate that we forget that weather forecasts that are more right than wrong are a recent phenomenon.
Mr. Feltgen and his colleagues want the public to know that the cone is just the best prediction of where the center of the hurricane will track. “It has nothing to do with the impacts of the hurricane,” he said.
It’s hard to tell ahead of time whether one storm will be worse than another, because how bad a storm is really depends on the human impacts. To that point, many experts avoid using the term “natural disaster.” They say that a disaster is what happens when a natural hazard — hurricane, tornado, blizzard — overlaps with a human population. Absent that, it’s just weather.
It also depends on what you mean by worse. Hurricane Andrew was a memorable storm and one of the most costly on record when it hit Florida in 1992. But according to data from the National Oceanic and Atmospheric Administration, in recent years it’s fallen to sixth place in the United States by that measure, as Hurricanes Katrina, Harvey, Maria, Sandy and Irma have supplanted it when adjusted for inflation. Part of that is because more Americans are now living in the path of storms, and part of that is because climate change has played a role in making the effects of storms more severe.
As for fatalities, the deadliest storm on record in the United States happened in 1900, when surging waters killed more than 6,000 people in Galveston, Tex. This was before modern weather forecasting, however, and many people failed to evacuate the area.
NOAA says to think of warm water as the engine that fuels hurricanes. They only form in ocean waters that are at least 80 degrees Fahrenheit (27 degrees Celsius) to a depth of at least 165 feet, which is why the most intense Atlantic hurricanes tend to happen in late summer and early fall after the ocean has had a summer to warm up.
Because of climate change, the world’s oceans are warmer. Though climate change did not necessarily create Hurricane Harvey last year or Hurricane Florence this month, both of them were fueled by warmer than average waters.
Climate change can also make hurricanes wetter. Air temperatures are warmer, and basic physics tells us that warmer air can hold more moisture, which can translate into more rainfall.
To further exacerbate things, our warming climate seems to be causing hurricanes to stall, or linger longer in place. That’s what happened with Hurricane Harvey, which meandered back out to the Gulf of Mexico after making its first landfall in Texas, and then back in. When a hurricane sticks around, even more rain can fall over a single area, leading to catastrophic flooding. Of the 50 inches of rain that fell over some parts of Houston because of Harvey, 38 percent, according to one study, was attributable to climate change.
Though Hurricane Florence’s circular wind speeds are high, it, too, is predicted to move slowly across the Carolinas.
Finally, climate change also makes sea levels higher, thanks to a mix of thermal expansion (warmer water takes up more space than cold water) and melting glaciers. Higher sea levels make storm surges worse, another fear along the Carolina coast.
A storm surge happens when the high winds of hurricanes and other tropical storms push water onto land. Arthur DeGaetano, director of the Northeast Regional Climate Center at Cornell University, suggested visualizing a snowplow: The hurricane winds function as the plow, and the water “piles up” as it’s pushed toward the shore.
The process also creates large, fast-moving waves that ride in on top of the surge. The end result is rising waters that can flood coastal areas and, by pushing up through rivers and streams, inland communities as well.
The flooding risk associated with storm surge is what prompts the authorities to call for evacuations, along with inland flooding from heavy rainfall. Storm surge is responsible for about half of the deaths in hurricanes, and inland flooding for another quarter.
