2008 will be 20% higher than standard rather than previous forecast of 35% higher

Atlantic basin and US landfalling hurricane activity in 2008 will be 20% above the long-term norm rather than 35%, according to Tropical Storm Risk, the consortium of experts on insurance, risk management and seasonal climate forecasting.

The consortium, led by the Benfield UCL Hazard Research Centre at University College London, said it represents a ‘slight’ increase in risk, based on norms recorded over a 50-year period from 1957 to 2007. The announcement was made as part of its pre-season outlook for the 2008 hurricane season.

The announcement compares with a 35% above-norm prediction issued on the 7 April 2008. The lowering of the forecast is due to the unexpected rapid waning of La Niña conditions now occurring in the tropical Pacific. However, uncertainties remain and La Niña and other key climate factors will be closely monitored.

TSR’s pre-season hurricane prediction includes:

• A 53% probability of an above-normal Atlantic hurricane season, a 29% probability of a near-normal season and only an 18% chance of a below-normal season

• 14 tropical storms for the Atlantic basin as a whole, with seven of these being hurricanes and three intense hurricanes. This compares to long-term norm values of ten, six and three respectively.

• A 52% probability of above-normal US landfalling hurricane activity, a 33% likelihood of a near-normal season and only a 15% chance of a below-normal season

• Four tropical storm strikes on the U.S., of which two will be hurricanes

• One hurricane strike on the Caribbean Lesser Antilles

Professor Mark Saunders, the TSR lead scientist and head of weather and climate extremes at the Benfield UCL Hazard Research Centre at University College London says there are three main climate factors which will determine how active the Atlantic basin 2008 hurricane season is, while a fourth factor will also influence the level of US landfalling hurricane activity. These factors, which will be closely monitored by scientists at TSR over the next 2-3 months, are:

(1) The speed of trade winds which blow westward across the tropical Atlantic and Caribbean Sea in August and September. These winds influence cyclonic vorticity and vertical wind shear over the main hurricane track region. Cyclonic vorticity either helps or hinders the spinning up of storms depending upon its anomaly sign and magnitude. Vertical wind shear either helps or hinders a vertically coherent storm vortex from developing depending upon its magnitude

(2) The temperature of the sea waters between west Africa and the Caribbean where many hurricanes develop during August and September. Waters here provide heat and moisture to help power the development of storms within the hurricane main development region.

(3) The presence of African dry air and Saharan dust over the hurricane main development region. Dry air and dust inhibits thunderstorm occurrence and tropical storm formation as happened during the start of the 2006 hurricane season.

(4) Tropospheric wind anomalies between heights of 925mb and 400mb over North America, the east Pacific and the North Atlantic. Wind anomalies in these regions in July are indicative of persistent atmospheric circulation patterns that either favour or hinder evolving hurricanes from reaching US shores during August and September.

Commenting on these factors Professor Saunders said: “TSR expects the key trade wind and sea temperature factors to be slightly enhancing for hurricane activity in 2008. However, at this lead it is difficult to make reliable projections for the roles of African dry air and tropospheric wind anomalies in August and September. Climate conditions have changed a lot in the past month and this is also adding to uncertainties.”

Allaying concerns over a repeat of the 2004 and 2005 US hurricane seasons, which saw nine hurricanes strike the US, Professor Saunders added: “A repeat of the devastation seen in 2004 and 2005 is most unlikely. The more likely scenario is that 2008 will produce US landfalling activity close to the 1995-2007 norm level.