Atlantic Hurricane Season Ends: Fewer Storms Than Forecast

Two back-to-back Category 5 hurricanes hit Central America and the rapid near-shore intensification of the single U.S. landfalling hurricane - Hurricane Humberto in September - marked the 2007 Atlantic hurricane season. One death has been attributed to Humberto, while total damages are estimated to be near $50 million dollars.

For the first time in recorded history, two Category 5 hurricanes made landfall in the Atlantic Basin during the same season.

Hurricane Dean hit the Yucatan Peninsula near Costa Maya on August 21 with winds of 165 miles per hour, followed by Hurricane Felix on September 2, near Punta Gorda, Nicaragua, with 160 mph winds.

Hurricane Dean blows across Mexico's Yucatan Peninsula as a Category 5 hurricane. (Image courtesy NASA)

"The 2007 Atlantic hurricane season produced the predicted number of named storms, but the combined number, duration and intensity of the hurricanes did not meet expectations," said Gerry Bell, Ph.D., lead seasonal hurricane forecaster at NOAA’s Climate Prediction Center.

As a whole, the 2007 Atlantic hurricane season produced a total of 14 named storms, including six hurricanes, two of which became major hurricanes.

NOAA's August update to the seasonal forecast predicted 13 to 16 named storms - of which seven to nine would be hurricanes, including three to five major hurricanes of Category 3 strength or higher.

An average season has 11 named storms, with six becoming hurricanes, including two major hurricanes, NOAA says.

"The United States was fortunate this year to have fewer strong hurricanes develop than predicted," said Bell. "Normally, the climate patterns that were in place produce an active, volatile hurricane season."

The climate patterns predicted for the 2007 hurricane season - an ongoing multi-decadal signal (the set of oceanic and atmospheric conditions that have spawned increased Atlantic hurricane activity since 1995) and the cooling eastern Pacific ocean temperatures brought by La Niña - produced the expected below-normal hurricane activity over the eastern and central Pacific regions.

La Niña's impact over the Atlantic was weaker than expected, which resulted in stronger upper-level winds and increased wind shear over the Caribbean Sea during the peak months of the season from August through October. This limited Atlantic hurricane formation during that period.

NOAA's scientists are investigating possible climate factors that may have led to this lower-than-expected activity.

All in all, one hurricane, one tropical storm and three tropical depressions struck the United States.

Tropical Depression Barry came ashore near Tampa Bay, Florida, on June 2; Tropical Depression Erin hit southeast Texas on August 16 and Tropical Depression Ten came ashore along the western Florida panhandle on September 21.

In addition, Tropical Storm Gabrielle hit east-central North Carolina on September 9, and Hurricane Humberto hit the upper Texas coast on September 13.

Hurricane Humberto grew from a tropical depression with top winds of 35 mph into a hurricane with winds of 85 mph within 24 hours. Only two other storms - Celia 1970, Arlene and Flora in 1963 - intensified faster during a 24-hour period from below tropical storm strength.

Talia Hubbard looks at her flooded home in El Reno, Oklahoma. She vacated in the middle of the night when flood waters rose rapidly from Tropical Storm Erin. August 20, 2007 (Photo by Marvin Nauman courtesy FEMA)

Also this year, the U.S. was reminded of the dangers of inland flooding "Texas and Oklahoma experienced deadly flooding when Erin dumped up to 11 inches of rain," said Ed Rappaport, acting director of NOAA's National Hurricane Center. "Fresh water flooding is yet another deadly aspect of tropical cyclones."

A closely watched hurricane analysis out of the Colorado State University's Department of Atmospheric Science also over-estimated the 2007 Atlantic Hurricane season.

"Our August-only forecast was quite successful. Our September and October-November forecasts were not successful," wrote Philip Klotzbach and William Gray on Tuesday.

"We predicted September and October-November to be active," they wrote. "September experienced activity at average levels, while below-average activity occurred in October-November."

"Despite a slightly below-average season in 2006 and average activity in 2007, we believe that the Atlantic basin is currently in an active hurricane cycle associated with a strong thermohaline circulation and an active phase of the Atlantic Multidecadal Oscillation," wrote Klotzbach and Gray.

"This active cycle is expected to continue for another decade or two at which time we should enter a quieter Atlantic major hurricane period like we experienced during the quarter-century periods of 1970-1994 and 1901-1925," they predict.

The U.S. landfall of major hurricanes Dennis, Katrina, Rita and Wilma in 2005 and the four Florida landfalling hurricanes of 2004 - Charley, Frances, Ivan and Jeanne - raised questions about the possible role that global warming played in these two unusually destructive seasons, wrote Klotzbach and Gray.

"The global warming arguments have been given much attention by many media references to recent papers claiming to show such a linkage. Despite the global warming of the sea surface that has taken place over the last 3 decades, the global numbers of hurricanes and their intensity have not shown increases in recent years except for the Atlantic," wrote Klotzbach.

The Atlantic has seen a very large increase in major hurricanes during the 13-year period of 1995-2007 (average 3.8 per year) in comparison to the prior 25 year period of 1970-1994 (average 1.5 per year), he explained.

"This large increase in Atlantic major hurricanes is primarily a result of the multi-decadal increase in the Atlantic Ocean thermohaline circulation that is not directly related to global temperature increase," wrote Klotzbach. "Changes in ocean salinity are believed to be the driving mechanism." Thermohaline circulation is the global density-driven circulation of the oceans.

Atlantic hurricanes go through cycles that last for decades. Cycles in Atlantic major hurricanes have been traced back to the mid-19th century through observation, explain Klozbach and Gray, who say that changes in the Atlantic Multidecadal Oscillation have been inferred from Greenland paleo ice-core temperature measurements going back thousand of years."

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