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A man clings to the top of a vehicle after Hurricane Katrina.
After Hurricane Katrina, a man clings to a van in Louisiana in September 2005.

Photograph by Rob Galbraith, Reuters

Christine Dell'Amore

National Geographic News

Published June 11, 2010

It's already been forecast to be "extremely active," but the 2010 Atlantic hurricane season may be shaping up to be something even worse: a replay of 2005, the most active and destructive Atlantic basin hurricane season in history.

The warm ocean temperatures and weak winds recorded this past May were similar to those of May 2005—the year that spawned Hurricane Katrina.

Such patterns are "definitely ominous and foreboding," said Chris Hebert, lead hurricane forecaster for the private forecasting company ImpactWeather, based in Houston.

For instance, the similarities to 2005 means there's an increased risk of hurricane impact across the northern Caribbean islands, the Florida Peninsula, and the northeast Gulf Coast, from southeastern Louisiana to Florida, he said.

Even so, there's no guarantee 2010 will be the new 2005.

Meteorologists are also still stumped about "what made 2005 so special," Hebert noted. "We don't fully understand why the 2005 hurricane season, with 28 named storms, was more than twice as active as a typical season," Hebert said.

But Hebert cautioned, the similarities between 2005 and 2010 do not mean there will be 28 named storms this year. (A U.S. National Weather Service forecast released May 27 predicted as many as 23 named storms in 2010.) Instead, the comparison is noteworthy because the 2010 storm-development patterns and wind currents resemble those of 2005, he said.

Likewise, he noted, other years may show even more similarities with 2005, such as 1998.

In general studying past hurricane seasons with spring conditions similar to those of the current year "can reliably tell us if the upcoming season will be more or less active than normal," he said.

However, comparing two seasons alone is not a "perfect predictor," Hebert noted.

"There are other factors that can impact the number of named storms during the season, and these other factors are not always either fully understood or able to be reliably predicted months in advance."

Weak Winds: Calm Before the Storms?

One key factor to hurricane formation and power is vertical wind shear—the storm-disrupting winds that can whip from the ocean's surface to heights of about 8 miles (13 kilometers)—according to Chris Landsea, a meteorologist at the National Hurricane Center in Miami.

In spring 2005—as now—the wind shear was weaker than usual, giving hurricanes ample opportunity to gather strength. Ironically, Landsea said, "hurricanes like a quiet atmosphere."

To measure low-level wind shear, meteorologists compare the difference in atmospheric sea-level pressure between two permanent pressure systems, called the Icelandic Low and the Azores-Bermuda High, ImpactWeather's Hebert said. This relationship is called the North Atlantic Oscillation.

A negative oscillation value means that the Icelandic Low is stronger and farther south than usual and that the Azores-Bermuda High is weaker and located farther to the east than normal. Because the Azores-Bermuda High is responsible for much of the hurricane-suppressing winds, a negative value implies calmer wind conditions—and greater hurricane threat.

Right now the oscillation value is -1.49, compared with -1.25 in 2005, Hebert said.

In 2009, a quiet hurricane year, the value was 1.68, because the Azores-Bermuda high remained strong all year, hampering hurricane formation. Last year unusually warm Pacific waters—part of an El Niño event—caused the jet stream, a high-altitude wind current, to shift southward into Atlantic regions where storms typically form.

(See "2009 Hurricane Season Quietest in Decades.")

But the El Niño dissipated earlier this year—just as it did before the 2005 season, ImpactWeather's Hebert noted.

Warm Waters to Heat Up Hurricane Formation?

Water temperatures too are looking forebodingly similar to those of 2005, Hebert said.

Warm water is a crucial hurricane ingredient, and the waters in the tropical and eastern Atlantic were warmer in May than they've ever been in recorded history—about 0.9 degree Fahrenheit (0.5 degree Celsius) above average.

In spring 2005, Atlantic temperatures were warmer than average too, by about 0.6 degree Fahrenheit (0.3 Celsius), according to the U.S. National Oceanic and Atmospheric Administration (NOAA).

(Test your ocean extremes IQ.)

Those warmer waters are strongly linked to calm wind conditions. With few strong winds to stir up deep, cold waters, the ocean's surface—the part that matters to hurricanes—heats up, Hebert added.

Katrina Conditions Would Aggravate Gulf Cleanup

Among those intently watching and waiting this hurricane season, which began June 1 and ends November 30, are people working to clean up the Gulf of Mexico's Deepwater Horizon oil spill—and residents in its potential path.

A hurricane, for example, could send spilled oil in the Gulf surging inland, experts say.

Ron Kendall, chair of the Department of Environmental Toxicology at Texas Tech University in Lubbock, cautioned in May: "You put a major hurricane in there, you're liable to have oil in downtown New Orleans."

A hurricane in the Gulf of Mexico may also hinder the joint federal-industry Deepwater Horizon response team.

Last week BP, the energy company holding the lease to the leaking well, placed a containment cap on an open pipe above the wellhead, funneling much of the leaking oil to a ship at the surface. So far that method has collected about 3 million gallons (12 million liters) of oil, Reuters reported.

But if a hurricane comes through, the ship will have to disconnect the wellhead, leaving the oil to spew at full force at least until the storm is over.

Busy or Quiet, Hurricane Season Can Be Tricky

On the bright side, the National Hurricane Center's Landsea pointed out that an active Atlantic hurricane season doesn't always portend a pummeling of the U.S. coast.

For instance, it's possible several powerful hurricanes could make a fuss out at sea without ever making landfall.

In the same vein, a quiet-season forecast doesn't mean people can relax, either. A "perfect example" is 1992, when Hurricane Andrew—one of only four hurricanes that year—devastated much of Florida, Landsea said.

"If that one strong hurricane hits you," Landsea said, "it's a busy year for you."

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