MIAMI, Aug 6 (Reuters) - A London-based weather research team ratcheted up its forecast for the 2008 Atlantic hurricane season on Wednesday, saying it now expects 18 tropical storms, with nine strengthening into hurricanes.

Tropical Storm Risk said four or five of the hurricanes would be "intense," with top sustained winds in excess of 111 miles per hour (170 kph).

Two of the five storms that have already formed this year -- Bertha and Dolly -- reached hurricane strength with winds in excess of 74 mph (119 kph) before fading over the open Atlantic and coming ashore in south Texas, respectively.

Tropical Storm Risk noted that as recently as last month, it was still forecasting that the number of hurricanes in the Atlantic basin and making landfall in the United States in 2008 would be just 35 percent above the long-term norm.

But if its new forecast proves accurate, it said the 2008 Atlantic hurricane season would be 90 percent more active than the long-term norm.

An average Atlantic storm season has around 10 storms, of which six become hurricanes. The record-busting 2005 season, which included Hurricane Katrina, spawned 28 storms.

Tropical Storm Risk said a key factor behind its new forecast was an expected drop in westward blowing trade winds over the Caribbean and North Atlantic region through September.

"In 2008 these winds are likely to be weaker than normal, thereby increasing cyclonic vorticity and helping more and stronger storms to spin up," the research team said.

They said sea water temperatures between West Africa and the Caribbean were also expected to be slightly warmer than normal in August and September, providing extra heat and moisture to help power the development of storms. (Reporting by Tom Brown, Editing by Michael Christie)

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