Predicting Irene

Hurricane prediction is by no means a perfect science. Supercomputers crunch vast amounts of data to model a hurricane’s path, taking information from sensors around the world, including balloons, aircraft, ships, and satellites, but it’s a patchwork collection at best. And there’s a practical limit to how much data the models can consider—a simulation that takes more time to calculate than the storm takes to advance isn’t very practical.

That’s why hurricane forecasts aren’t perfect. But they keep getting better. Five years ago the two-to-three-day forecast of Hurricane Rita’s path showed it hitting Houston; hundreds of thousands of people fled in a chaotic evacuation that turned out not to be necessary—Rita missed Houston entirely.

But today, the 48-hour forecast of a hurricane’s track is as good as the 24-hour forecast was 10 years ago—and that’s pretty good. Researchers are also starting to get a handle on predicting the intensity of hurricanes—that’s something they had very little skill at 10 years ago. Operational models, the ones used for day-to-day weather forecasting, have gotten better; research models are taking in data they heretofore only dreamed of getting. And all this prediction capability is being thrown at Hurricane Irene. (For more about how researchers are working to improve the models that predict hurricane tracks , see It's Hurricane Season: Do you know where your storm is? For more information about how researchers predict how a whole season of hurricanes will evolve, see Satellites and Supercomputers Say 6 to 10 Hurricanes Coming)

One key advance: New numerical forecast systems that started being tested this summer on supercomputers in Boulder, Colo., run fast enough to use in real time, explained Robert Gall, head of the developmental test bed for the University Corporation for Atmospheric Research (UCAR). So, with Irene, as many as half a dozen research models are being combined with the current operational models to zero in on the most likely track, says Frank D. Marks Jr., director of the Hurricane Research Division at the National Oceanic and Atmospheric Administration’s (NOAA's) Atlantic Oceanographic Meteorological Laboratory.

And the models have access to more information than ever before. While NOAA has for years sent planes into hurricanes to take measurements, until recently it used those planes to provide data about current conditions only; but now the data collected by the planes is going immediately into the models that provide active hurricane forecasts. When I talked to Marks, he was about to leave on one of those flights, getting ready to collect data on temperature, wind speed, air pressure, and other parameters. More significantly, the plane he was due to travel on was going to be carrying a Doppler radar, which provides valuable insights into storm development, Marks said. This Doppler radar data, Marks says, “is showing the most promise with helping us make models better. We can use it as a benchmark to judge the satellite data we use, because the satellites can’t see through clouds or rain.” (You can follow these hurricane flights on the Hurricane Research Division’s Blog or on twitter @HRD_AOML_NOAA.)

Putting all these tools together means that the National Hurricane Center’s forecasts, at least those only two to three days out, are likely to prove true for most hurricanes these days, but particularly for Irene.

“Irene’s been a good storm for predictability,” says Marks. Global weather models spotted Irene’s formation nearly two weeks ago, he said, and predicted that it would cause trouble around Haiti and Puerto Rico. Irene, Marks said, is a “classic behemoth,” easier to forecast than scrappy faster forming storms like Cindy, Harvey, and Gert, he said, that wandered all over the place.

So Marks says he’s confident about what the models say will happen in North Carolina, that is, that Hurricane Irene will reach the Outer Banks of North Carolina on Saturday. It’s harder, he says, to say with certainty what will happen when Irene reaches New Jersey, New York, or Boston—the track is fairly clear, with Irene having about a 70 percent chance of staying within the cone on the picture above; the intensity less so.

“People want to know, come Sunday, how big the storm surge will be in New Jersey and New York, what the wind speed will be within a knot or two, how much rain they’ll get, how fast the hurricane will be traveling," Marks says. "And we’re not just there yet, though some models this year are showing a lot of promise."

Top Image: Hurricane Irene’s predicted track. The solid white area depicts the track forecast uncertainty for days 1–3 of the forecast, while the stippled area depicts the uncertainty on days 4–5. Source: National Hurricane Center.

Lower Image: a Doppler radar image of Hurricane Irene taken earlier today from a plane flying through the storm. Source: Hurricane Research Division

(updated 8/26)
 

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