With Hurricane Irma dominating social media, it seems like forecasts are inundating our feeds more than anything else. In an age where everyone’s competing to get information out first, it’s become more important than ever to understand how forecasts are prepared, and what all of the numbers, indexes, colors and everything else mean.
For that reason, we’ve put together a summary of what you need to know to make sure you’re keeping one step ahead of Mother Nature as we near the peak of hurricane season.
The cone of uncertainty
You can’t get through hurricane season without encountering the infamous “cone of uncertainty,” which is the graphic that shows where the National Hurricane Center (NHC) thinks the center of the storm could reasonably go. It is plastered on every television screen, not to mention shared on social media thousands of times. But what really is the cone?
Most people think the cone simply encompasses the range of possible tracks that a hurricane or tropical cyclone could take from different models. The wider the cone, the bigger the envelope of possible tracks. A narrower cone, and forecasters must have a better idea where the system’s heading.
But that’s not how it works.
The cone only takes into consideration one forecast — the official Hurricane Center forecast. The Hurricane Center issues forecast advisories on tropical depressions, storms and hurricanes every six hours, and intermediate products in between if conditions warrant. The plotted 24, 48, 72, 96 and 120-hour positions mark the spot that the Hurricane Center predicts a storm will be at that time. Essentially, the “official forecast” is just one big game of “connect the dots.”
So what does the width of the cone represent? Contrary to popular belief, it’s not a storm-specific indicator of confidence levels or a direct reflection of model forecasts. Instead, the width of the cone at a given time corresponds to the Hurricane Center’s average track error that far in advance, based on the accuracy of past forecasts (calculated by a five-year running mean of NHC forecast accuracy). The width of the cone at a given point in time is the same for every storm and every forecast all year long.
The cone’s radius is designed to be 67 percent (two-thirds) the width of the historical track error at a given point in time. In other words, if the Hurricane Center is typically “off” by 300 miles five days out, then the cone will extend outward 200 miles on either side.
To that end, residents along the coast shouldn’t panic or relax prematurely just because they may or may not be in the cone. While the cone is an indicator of forecast confidence, it is based on the accuracy of past forecasts. It may or may not be an adequate reflection of current forecast uncertainty.
It’s also important to note the cone does not encompass the entire geographic area that may deal with a storm’s effects, including heavy rain and strong winds — which can occur up to a few hundred miles from the storm center.
For a better gauge of the present degree of uncertainty in a hurricane track forecast, it’s useful to inspect what are known as spaghetti models. We can all picture the clustering of colorful lines that race westward and diverge with time. They’re extremely useful to meteorologists, but it’s important to have a clear understanding of what they represent.
Usually, each “strand of spaghetti” is not representative of a unique model. Instead, the entire bowl of noodles is data churned out by the same weather model. Each noodle is called an ensemble member. When a weather model produces a forecast, it has to initialize the data, taking in current conditions to get an idea of what the atmosphere is already doing on a three-dimensional grid.
However, there may be a “gap” in the data, and models are required to interpolate, guessing what the environment is like in between. Sometimes, the model does a good job of estimating. Other times, not so much.
The role of an individual ensemble member is to guess a little bit differently than every other ensemble member. That way, meteorologists can see how the atmosphere may behave under slightly different initial conditions.
Errors magnify over time, which is why the ensemble members spread out as the forecast progresses.
The spaghetti plots you’ll most frequently see originate from the American model (GFS, for the Global Forecast System) and European model (sometimes referred to as “the Euro” or ECMWF, for the European Centre for Medium-Range Weather Forecasts).
Ensemble members are a valuable tool in the meteorological arsenal, and you’ll probably see them a lot more this hurricane season.
(Note that while spaghetti plots most commonly present ensemble members from the same parent model, some combine ensemble members from several different models. And some spaghetti plots will just show one model run from a bunch of different models, rather than ensemble members from the same model. Finally, some show all of the above.)
Meteorologists rank hurricanes on the Saffir-Simpson Hurricane Wind Scale, which solely assigns strength based on maximum wind speeds. As Harvey just proved, this is a flawed system, as wind is just one of a multitude of hazards associated with tropical cyclones — storm surge, inland flooding from excessive rainfall and tornadoes all accompany landfalling hurricanes.
Moreover, the maximum sustained winds in the eyewall are what lead to a category designation, and most of the time these winds extend outward only a few dozen miles.
Many people who have endured the fringe of a Category 4 hurricane underestimate the destructive power that would be encountered if they were closer to the center. Never judge a hurricane by your experience. Each and every event is different.
Likewise, even areas well away from a storm’s center are subject to collateral damage. Monitor every facet of the storm, and know how to handle each threat that may come your way.