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U.S. Agency Predicts Active 2007 Hurricane Season

Scientists project chance of three to five major Atlantic hurricanes

Scientists at the U.S. National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center projected a 75 percent chance that the Atlantic hurricane season will "above normal," predicting three to five major hurricanes for the 2007 season.

An average Atlantic hurricane season brings 11 named storms, with six becoming hurricanes, including two major hurricanes. The Atlantic hurricane season is June 1 through November 30, with peak activity occurring August through October.

"NOAA scientists predicted 13 to 17 named storms," NOAA Administrator Conrad Lautenbacher said during a May 22 release of NOAA's 2007 Atlantic Hurricane Season Outlook, "with seven to 10 becoming hurricanes, of which three to five could become major hurricanes of Category 3 strength or higher."

Hurricane categories are determined according to the 1-5 intensity rating of the Saffir-Simpson Hurricane Scale. The scale is used to estimate the potential property damage and flooding expected along a coast from a hurricane landfall. Wind speed is the determining factor in the scale.

Major hurricanes - Category 3 and higher - have winds of 178 kilometers per hour to 209 kilometers per hour. Storm surge can be 2.7 meters to 3.6 meters above normal, and low-lying residences within several blocks of the shoreline might have to be evacuated.

Climate patterns responsible for the expected above-normal hurricane activity are the ongoing multidecadal signal (a set of ocean and atmospheric conditions that arose in 1995 and spawn increased Atlantic hurricane activity), warmer-than-normal sea surface temperatures in the Atlantic Ocean, and the El Niņo/La Niņa cycle.

In this naturally occurring cycle, also called the El Niņo/Southern Oscillation, El Niņo is the warm phase and is sometimes called a Pacific warm episode. El Niņo originally referred to an annual warming of sea-surface temperatures along the west coast of tropical South America. The cool phase is La Niņa, a periodic cooling of surface ocean waters in the eastern tropical Pacific.

Both conditions affect weather patterns around the world.

The El Niņo/la Niņa Cycle

In 2006, NOAA's seasonal hurricane predictions proved too high when an unexpected El Niņo rapidly developed and created a hostile environment for Atlantic storms to form and strengthen.

El Niņo typically suppresses hurricane activity by increasing vertical wind shear over the Caribbean Sea region. Vertical wind shear is the magnitude of wind change with height. Hurricanes need low values of vertical wind shear between the surface and the upper atmosphere. High vertical wind shear can disrupt a tropical cyclone trying to form by literally tearing it apart.

When storms developed during the 2006 season, steering currents kept most of them over open water and away from land.

A La Niņa weather pattern has the opposite effect on hurricane activity.

"There is some uncertainty this year as to whether or not La Niņa will form, and if it does how strong it will be," said Gerry Bell, lead seasonal hurricane forecaster at the NOAA Climate Prediction Center.

According to the center, a La Niņa could form in the next month to three months. If the La Niņa develops, storm activity likely will be in the upper end of the predicted range, or higher depending on how strong the La Niņa becomes.

"Even if La Niņa does not develop," Bell said, "conditions associated with the ongoing active hurricane era still favor an above-normal season."

African Dust and Hurricanes

African winds and dust conditions also influence the birth of hurricanes in the Atlantic Ocean, Bell said, and scientists from NOAA, NASA, universities and international agencies are engaged in a project called the African Monsoon Multidisciplinary Analyses, aimed at improving knowledge and understanding of the West African monsoon and its variability.

How the dust influences Atlantic hurricanes still is being studied, Bell said.

"Some people argue that it affects the radiation balances and can therefore affect atmospheric stability," he said. "Others say the dust is simply a manifestation of extremely dry air coming off the Sahara. We see African dust outbreaks a lot, and it's not clear if it actually affects seasonal [hurricane] activity or not."

The Atlantic Hurricane Seasonal Outlook is an official forecast product of the NOAA Climate Prediction Center.

Instituted in 1998, the outlook is produced in collaboration among NOAA scientists at the NOAA Climate Prediction Center, NOAA National Hurricane Center, NOAA Hurricane Research Division and the NOAA Hydrometeorological Prediction Center.

The NOAA Climate Prediction Center will issue an updated seasonal forecast in August just before the season's historical peak.

The 2007 Tropical Eastern North Pacific Hurricane Outlook is available on the NOAA Web site.

More information about hurricanes is available at the NOAA National Hurricane Center Web site.

NOAA's Hurricane Portal is available at the NOAA Web site.

Source: U.S. Department of State