MONTE CARLO, Monaco—Halfway through the Atlantic hurricane season, London-based Tropical Storm Risk decreased its hurricane forecast for 2006 to slightly below-normal activity, from an earlier prediction of above-average activity.

The updated outlook anticipates hurricane activity being 10% below the long-term norm, between 1950 and 2005; compared to a forecast of 40% above-norm, which TSR was calling for back in March.

The main factor behind the revised forecast is the "unexpected and influential presence" of dry air and Saharan dust blowing off Africa over the main hurricane development region between the west coast of Africa and the Caribbean, said Professor Mark Saunders, the TSR lead scientist and head of seasonal forecasting and meteorological hazards at the Benfield Hazard Research Centre at University College London.

TSR is a consortium of experts on insurance, risk management and seasonal climate forecasting led by the Benfield center."It appears to be this very dry, dusty air is inhibiting thunderstorms forming as they usual do," Prof. Saunders said Monday in an interview at the 50th annual Monte Carlo Rendez-Vous de Septembre.

"If the dust starts to recede this month, and with the other factors still being favorable, I think activity may well pick up during this month," he said. Other factors still remain fairly conducive for activity, he noted, such as warm sea surface temperatures and also trade wind speeds that are weaker than normal.

"But since we are now halfway through the season, it is unlikely that will be enough activity to make the season above average as a whole now," Prof. Saunders added.

TSR is now expecting 13 tropical storms for the Atlantic basin as a whole, with six of these being hurricanes and two being intense hurricanes. Previously, it forecasted 15 storms, including eight hurricanes and three to four intense hurricanes. To date, there have been six tropical storms, with two of these being hurricanes.

The dry air and dust this year has not been a significant factor in previous hurricane seasons. Prof. Saunders said TSR will look at the models to identify "why this is so important this year and, if so, can we include that in a revision forecast model."

Prof. Saunders said the revision marked the first time TSR had to reduce an earlier forecast by so much.