Forecaster predicts a 48% probability of above-normal US hurricane activity

Tropical Storm Risk (TSR) has released its outlook for the 2012 Atlantic hurricane season, running from 1 June to 30 November 2012.

The outlook anticipates Atlantic basin and US landfalling hurricane activity being close to their long-term (1950-2010) norm values but 10% to 40% below their norm values for the last ten years (2002-2011).

“At present the main climate indicators point to the 2012 hurricane season being close to norm,” said TSR’s professor Mark Saunders. “However, uncertainties remain and we are overdue US landfalling hurricane strikes.

“Only one hurricane (Irene) has struck the US in the last three years and no major hurricane has struck the US since Wilma in 2005. On average four or five landfalling hurricanes would strike the US in three years and four major hurricanes would strike over six years. Nature has a habit of correcting herself.”

The pre-season outlook includes:

  • A 34% probability of an above-normal Atlantic hurricane season, a 36% probability of near-normal season, and a 30% chance of a below-normal season.
  • 13 tropical storms including six hurricanes and three intense hurricanes. This compares to long-term norms of 11, six and three respectively.
  • An ACE (Accumulated Cyclone Energy) value of 98. The long-term norm is 103.

For US landfalling activity, TSR forecasts:

  • A 48% probability of above-normal US landfalling hurricane activity; a 26% likelihood of a near-normal season, and a 26% chance of a below-normal season.
  • Four tropical storm strikes on the US, including two hurricanes. This compares to long-term norms of three and 1.5 respectively.

Three main climate factors will determine the level of hurricane activity in the Atlantic basin. Occurring in August and September, these are the speed of trade winds over the tropical North Atlantic, sea temperatures in the tropical North Atlantic, and the sign and strength of El Niño Southern Oscillation. 

US landfalling hurricane activity is influenced by July tropospheric wind patterns over the North Atlantic and US, and by the level of hurricane activity occurring at sea.

Despite the success of recent basin forecasts sources of uncertainty remain. Model projections of ENSO (El Niño Southern Oscillation) conditions for August-September currently show a range from neutral ENSO to moderate El Niño. 

There is uncertainty in how warm the tropical North Atlantic ocean will be in August-September. Also variance exists in the level of basin hurricane activity possible from the same August-September climate factors and, as seen in recent years, even larger variance exists in the level of US landfalling hurricane activity possible from the same basin activity.