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Devising hurricane predictions is a difficult, highly complex exercise that has improved over the years but still remains an educated guess, observers say.
It involves research, global observations and computer models, according to the U.S. National Oceanic & Atmospheric Administration, which has predicted an above-normal hurricane season this year.
Peter Dailey, director of atmospheric science at AIR Worldwide Corp., the Boston-based weather modeling firm of the Insurance Services Office Inc., said two primary factors that influence hurricane activity levels are the sea surface temperature, which is a measure of the warmth of the ocean, and wind shear, which is a measure of the difference between the weaker surface winds and the stronger winds in the upper atmosphere.
One important element in wind shear is what is known as the El Niño/Southern Oscillation. Warming of equatorial Pacific Ocean temperatures, or El Niño, leads to more wind shear and fewer Atlantic hurricanes. The cooling La Niña has the opposite effect.
Forecasters try to anticipate how ocean temperatures and wind shear conditions will develop. But they can be wrong, which was the case last year when a widely predicted busy hurricane season never materialized. Observers say El Niño and the Sahara desert were the main reasons 2006 had a light hurricane season.
Colorado State University based its forecast on incorrect information it received about El Niño, said Philip J. Klotzbach, research associate at CSU's Tropical Meteorology Project in Fort Collins, Colo., and chief author of the forecast. "Nobody saw it coming," he said.
"Until Aug. 1, (2006,) the conditions in the sea surface temperatures and the surrounding atmosphere looked very similar to what was going on in 2005," said Steven Drews, Chicago-based senior lead meteorologist and associate vp at Aon Re Global.
El Niño can change entirely in 20 to 30 days' time for unknown reasons, which affects the quality of researchers' long-term predictions, said Thomas Larsen, senior vp of Oakland, Calif.-based weather modeling firm EQECAT Inc.
The Sahara also played a role last year, say observers. What is known as the Saharan air layer, which is increasingly being recognized as an important factor in hurricanes, can lift and transport dust over the Atlantic. It has been hypothesized that this reduces hurricane activity partly because of its drying effect on the atmosphere.
That was the case last year, Aon's Mr. Drews said. "What ended up happening is a bunch of dry air and dust moved from the Sahara desert out to the central Atlantic, where hurricanes typically start to form" and prevented development of what could result of a tropical storm or hurricane, he said.
Forecasting is getting better but still is not foolproof, experts say.
Mike Halpert, head of forecasting operations for NOAA's Climate Prediction Center in Camp Springs, Md., said, "It's complex, and it won't ever be perfect, but I think there's still room for improvement."
Hurricane forecasting is improving incrementally, said EQECAT's Mr. Larsen. "We'll have a much better understanding of what to expect" in 20 years. However, "we're never going to know" with absolute precision in advance that a Category 4 hurricane is going to make landfall in Miami on a particular date.
Scott Clark, risk and benefits officer for Miami-Dade County Public Schools in Miami, said while the predictions have improved, "I don't know they're ever going to be able to absolutely ascertain both severity and frequency because I think it's much more of an art than it is a science."