(Bloomberg) — London-based storm forecasters Tropical Storm Risk have predicted North Atlantic hurricane activity could reach as much as 75 percent above the historical average — with at least four of the nine predicted becoming “major” storms.The forecasters estimated 17 tropical storms will form, with nine reaching hurricane force and four of those becoming major hurricanes, with winds topping 111 mph.

Weather systems are classified as tropical storms and given names once winds reach 39 mph. Hurricanes have winds of more than 74 mph.

Warm water in the Atlantic basin and the Caribbean Sea will provide heat and moisture to power the storms, while changes in trade-wind patterns due to La Nina conditions will cause cyclones to form at the centre of the storms, said Mark Saunders, lead scientist for TSR.

La Nina occurs when Pacific Ocean temperatures are lower than normal and can affect global weather patterns.

During the 2006 hurricane season, nine Atlantic weather systems reached tropical-storm strength, and five became hurricanes, with two strengthening to major status.

On average, 10.6 named storms and 5.9 hurricanes were recorded annually in the Atlantic basin from 1965 to 2004, according to the US National Oceanic and Atmospheric Administration.

Tropical Storm Risk, founded in 2000, includes climate scientists and statisticians from University College London and the Met Office, the UK government’s forecasting arm.

The project is sponsored by Benfield Group Plc., a UK reinsurance broker; Royal & Sun Alliance Insurance Group Plc., Britain’s second-largest non-life insurer; and Crawford & Co., which helps manage claims for insurance companies.