MIAMI, Nov 30 (Reuters) - U.S. weather experts posthumously upgraded Tropical Storm Karen to a hurricane as the 2007 Atlantic storm season drew to a close on Friday, making the year a near-average one for hurricane activity.

The U.S. National Hurricane Center, in a post-season analysis of Karen, said the storm briefly reached hurricane intensity on Sept. 26, with winds of 65 knots, equal to 74.8 mph (120 kph) or just over the threshold at which tropical storms become hurricanes.

The upgrade of Karen took the 2007 season's hurricane toll to six, bang on the long-term average. The 14 named storms that formed exceeded the long-term average of around 10 for a six-month Atlantic hurricane season.

But most of the storms were short-lived, meaning the so-called accumulated cyclone energy index, which measures the collective strength and duration of storms and hurricanes, reached only about 82 percent of the 1951-2000 median, "the lowest observed since 2002," the hurricane center said.

Only one hurricane -- Humberto -- hit the United States this year. It was relatively weak and a far cry from the destruction wrought by hurricanes Katrina, Rita and Wilma in the record-setting season of 2005.

Two of the current season's hurricanes did reach maximum Category 5 strength on the five-step Saffir-Simpson scale of intensity.

Hurricane Dean in August slammed into Mexico's Yucatan Peninsula and killed around 27 people throughout the Caribbean, while Hurricane Felix in September plowed into Nicaragua, killing more than 100.

The season's deadliest storm was also its last, Hurricane Noel, which doused the Dominican Republic and Haiti and triggered floods and mudslides that killed more than 150 people in the neighboring countries. (Reporting by Michael Christie; Editing by John O'Callaghan)

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