Mark Saunders, lead scientist with Tropical Storm Risk (TSR), a consortium gathering Benfield Hazard Research Centre at University College London and business interests, said it was "unusual but not unprecedented" to have a storm of such magnitude at this stage in the season.

"There was a category five hurricane, Edith, which hit Nicaragua in September 1971, close to where Felix is projected to make landfall," he told AFP by phone from London.

"However, this season so far has had two cat (category) five hurricanes and there's only one season on record that has had more than two storms in this category and that was 2005, the Katrina year, when there were four. No other year on record has more than two."

In Fort-de-France, on the French Caribbean island of Martinique, Meteo-France expert Jean-Noel Degrace said that so far the hurricane season had been "virtually normal" in terms of numbers.

From 1990-2005, there was an average of 12 or 13 hurricanes a year, he said.

So far, a week short of the midway point in the season, which runs from the start of June to the end of November, there have been six, said Degrace.

"However, what constitutes activity is not only the number of storms in a season but also the capability of storms to become intense storms," he said.

"The fact that there have already been two intense storms marks out 2007 as an active year," said Degrace, deputy director for the agency's Caribbean-Guiana region.

Saunders noted that Felix took only 36 hours to brew from a tropical depression to a Category Five hurricane, packing sustained winds of up to 260 kilometers (160 miles) per hour.

"That is surprising, it is rare in the Atlantic for a storm to be intensifying that fast," he said.

Three factors combine to make a hurricane, say scientists. These are: a warm sea surface, whose air convection provides the raw energy for a storm; moisture in the air, to form clouds; and the angle, or shear, of the winds, to spin the system into a cyclone.

Felix is a potentially catastrophic hurricane and forecasts are still predicting several more storms of hurricane strength this year, at least one or two which will strike the US, said Saunders.

related report
Hurricane Felix Weakens Slightly To Category Four
Miami (AFP) Sept 3 - Hurricane Felix on Monday lost some of of its massive power to become a category four storm, but remained extremely dangerous and could regain strength as it barreled toward central America, US forecasters said.

At 1800 GMT, the storm's maximum sustained winds had eased off a little, but still raged at 230 kilometers (145 miles) per hour, with higher gusts, according to the Miami-based National Hurricane Center (NHC.)

"Felix has weakened but is still extremely dangerous and could restrengthen," said NHC forecaster Richard Pasch as the storm swirled 490 kilometers (305 miles) east of Cabo Gracias a Dios on the Nicaraguan-Honduran border.

The weakening brings the storm down one notch from the topmost category five on the Saffir-Simpson intensity scale it had reached on Sunday.