Tropical Storm Risk of London has increased its forecast for Atlantic hurricane activity this year.
Based on current and projected climate signals, TSR’s March forecast predicts Atlantic basin and US landfalling hurricane activity to be about 75 per cent above the 1950-2006 norm in 2007, rising from 60 per cent above norm (TSR long-range forecast issued December 2006). This is the highest March forecast for activity in any year since the TSR replicated real-time forecasts started in 1984.
According to TSR, whose long-range outlooks for the exceptionally active 2004 and 2005 hurricane seasons and active 2003 hurricane season proved accurate, it is 86 per cent likely that US land falling hurricane activity in 2007 will be in the top one-third of years historically. The prediction includes:
• An 86 per cent probability of an above-normal Atlantic hurricane season, a 11 per cent probability of a near-normal season and only a 3 per cent chance of a below-normal season
• 17 tropical storms for the Atlantic basin as a whole, with nine of these being hurricanes and four intense hurricanes
• An 85 per cent probability of above-normal US land falling hurricane activity, a 12 per cent likelihood of a near normal season and only a 2 per cent chance of a below-normal season
• Five tropical storm strikes on the US, of which two will be hurricanes
• Two tropical storm strikes on the Caribbean Lesser Antilles, of which one will be a hurricane
The two main climate factors influencing the TSR hurricane forecast for 2007 are the expected values in August and September for the speed of trade winds, which blow westward across the tropical Atlantic and Caribbean Sea and the temperature of the sea waters between West Africa and the Caribbean where many hurricanes develop. The former influences cyclonic vorticity – the spinning up of storms – while the latter provides heat and moisture to power incipient storms. The key factors behind the TSR forecast for an above-average season in 2007 are the anticipated moderate enhancing effect of the July-September forecast trade winds over the Caribbean Sea and North Atlantic region, and of the August-September forecast sea-surface temperature for the Atlantic.
Commenting on the forecast upgrade, 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 said: ‘The El Niño conditions present since September 2006 dissipated rapidly during February. This has increased the expectation since last month that weak La Niña conditions will occur during the summer. As a result, the July to September Caribbean trade wind anomalies are expected to be weaker than thought previously. This factor will increase cyclonic vorticity and cause more storms to be spun up. The sudden El Niño dissipation is the main reason for the TSR forecast for hurricane activity in 2007 rising from 60 per cent above-norm in our long-range forecast last December, and also in our January and February updates to 75 per cent above-norm in our March forecast.’
TSR’s April forecast update will be issued on 5 April. TSR forecasts may be accessed through the website www.tropicalstormrisk.com.
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