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Hurricane Season – An inexact science - Soundings Online

Hurricane Season – An inexact science

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Though hurricane predictions are much improved from a decade ago, the ocean and atmosphere interact in such a complex and intricately choreographed dance that it’s not always easy to predict what kind of hurricane season we’re going to have, says Steve Letro, meteorologist in charge of the National Weather Service office in Jacksonville, Fla.

Though hurricane predictions are much improved from a decade ago, the ocean and atmosphere interact in such a complex and intricately choreographed dance that it’s not always easy to predict what kind of hurricane season we’re going to have, says Steve Letro, meteorologist in charge of the National Weather Service office in Jacksonville, Fla.

“People say to me, ‘There’s a lot of technology out there. Why can’t you do a better job of forecasting these stupid things?’ ” he says, speaking at a March Hurricane Preparation Symposium in Orlando, Fla.

A lot goes on globally in the ocean and atmosphere that affects whether and where hurricanes form and where they go, he says. For instance, 2006’s quiet hurricane season — with nine named storms, five hurricanes and just two major ones — has been attributed to El Niño, and that certainly was a factor. “There was a rapid development of an El Niño in the Pacific,” says Letro. “It came out of nowhere.” But Letro, a hurricane expert and forecaster for 30 years, says other conditions also came into play.

El Niño and La Niña episodes are caused by warm and cool water sloshing back and forth (oscillating) between the eastern and western Pacific, and this sloshing isn’t always predictable. When the warm water’s in the eastern Pacific, we have an El Niño and a less active hurricane season. When it’s in the western Pacific, we get a La Niña and a more active season.

Warm water in the eastern Pacific generates westerly winds across the tropics, and shear from those winds inhibits hurricane formation. Letro says El Niño was in a development stage during much of the 2006 hurricane season but didn’t strengthen into a full-blown El Niño until well after the peak hurricane months of August and September. He thinks the early-season lull was due to Saharan storms blowing dry, dusty air into the Atlantic and Caribbean, which reflected solar energy and deprived hurricanes of the heat and moisture they need to form.

Letro says those storms began to blow dry air into the Atlantic during the busy 2005 hurricane season, when a record 28 named storms formed. That year’s worst storms — Katrina, Rita and Wilma — formed close to the U.S. coast instead of far at sea or deep in the Caribbean, because so much of that dry air was in the western Atlantic, he says.

The size, strength and location of the Bermuda High, a summertime high pressure system that hangs off the southeast U.S. coast, also is a big factor in the tracks that hurricanes take. From 1995 to 2003, the Bermuda High was weak and located well east, near the Canary Islands. A lot of the hurricanes spawned during that period tracked up the Atlantic coast west of the high. In 2004 the high strengthened and moved west, pushing storms into Florida. In 2005 the ridge moved farther west, shifting storm formation into the southern Caribbean and resulting in Gulf Coast landfalls — Katrina at New Orleans, Rita at Houston, and Wilma on Florida’s west coast, according to Letro. In 2006 the Bermuda High weakened and moved east again, allowing the storms that did form to trail off harmlessly into the North Atlantic, he says.

Letro says the jury is still out in March on the 2007 hurricane season. “There are good indications that El Niño is just about played out,” he says. “We could start seeing La Niña conditions [favorable for hurricane formation] by the start of hurricane season.”

Letro says the Bermuda High fluctuates with warm and cold water oscillations in the Atlantic — five of them altogether. “Do we know exactly what’s going on there?” he asks. “No, we don’t.”

Hurricane guru Dr. William Gray in April predicted a “very active” 2007 hurricane season of 17 named storms, nine hurricanes and five majors, based in large part on the rapid dissipation of El Niño. The ideal scenario, if El Niño is in fact played out, is lots of fall cold fronts rolling into Florida and strong low pressure off the East Coast, Letro says. That would send storms north up the central Atlantic.

“It’s not important how many [hurricanes] there are,” he says. “It’s whether they hit anybody.”