Forecast Summary

TSR predicts an active Atlantic hurricane season in 2013. However, the uncertainties at this extended range are large. The precision of TSR's December outlooks for upcoming Atlantic hurricane activity between 1980 and 2012 is low.

The TSR (Tropical Storm Risk) extended range forecast for Atlantic hurricane activity in 2013 anticipates above-average activity. Based on current and projected climate signals, Atlantic basin tropical cyclone activity is forecast to be about 30% above the 1950-2012 long-term norm but slightly below the recent 2003-2012 10-year norm. The forecast spans the period from 1st June to 30th November 2013 and employs data through to the end of November 2012. TSR’s two predictors are the forecast July-September trade wind speed over the Caribbean and tropical North Atlantic, and the forecast August-September 2013 sea surface temperatures in the tropical North Atlantic. The former influences cyclonic vorticity (the spinning up of storms) in the main hurricane track region, while the latter provides heat and moisture to power incipient storms in the main track region. At present TSR anticipates both predictors to have a small enhancing effect on activity

There is a 59% probability that the 2013 Atlantic hurricane season ACE index will be above-average (defined as an ACE index value in the upper tercile historically (>121)), a 27% likelihood it will be near-normal (defined as an ACE index value in the middle tercile historically (72 to 121) and a 14% chance it will be below-normal (defined as an ACE index value in the lower tercile historically (<72)). The 63-year period 1950-2012 is used for climatology.

Key Predictors for 2013
The key factors behind the TSR forecast for a slightly above-average hurricane season in 2013 are the anticipated small enhancing effects of the July-September 2013 forecast trade wind at 925mb height over the Caribbean Sea and tropical North Atlantic region (7.5oN – 17.5oN, 45oW – 85oW), and of the August-September 2013 forecast sea surface temperature for the Atlantic MDR (10oN – 20oN, 20oW – 60oW). The current forecasts for these predictors are 0.36±0.82 ms-1 weaker than normal (1980-2012 climatology) and 0.27±0.28oC warmer than normal (1980-2012 climatology). The July-September 2013 trade wind prediction is based on an expectation of weak negative ENSO conditions in August-September 2013 as forecast by an in-house multi-ensemble extension of the Knaff and Landsea (1997) ENSO-CLIPER model (Lloyd-Hughes et al, 2004). The forecast skills for these predictors at this lead are 27% and 33% respectively. However, it should be stressed there are large forecast uncertainties in both these predictors at this extended lead.

The Precision of Seasonal Hurricane Forecasts
The figure on the next page displays the seasonal forecast skill as a function of lead time for predicting the number of North Atlantic hurricanes. Skill is displayed for the most recent 10-year period 2003-2012 and is shown for three forecast centres: TSR, NOAA (National Oceanic and Atmospheric Administration) and CSU (Colorado State University).
Forecast precision is assessed using the Mean Square Skill Score (MSSS) which is the percentage improvement in mean square error over a climatology forecast. Positive skill indicates that the model performs better than climatology, while a negative skill indicates that it performs worse than climatology. Two different climatologies are used: a fixed 50-year (1950-1999) climatology and a running prior 10-year climate norm.
It should be noted that NOAA does not issue seasonal hurricane outlooks before late May and that CSU stopped providing quantitative extended-range hurricane outlooks from the prior December in 2011. It is clear from the figure that there is little skill in forecasting the upcoming number of hurricanes from the previous December. Skill climbs slowly as the hurricane season approaches with moderate-to-good skill levels being achieved from early August.

In terms of recent seasonal forecast successes and failure, TSR correctly predicted the tercile (lower, middle, upper) of the North Atlantic hurricane seasons in 2004, 2005, 2008, 2010, 2011 and 2012 from the previous December. In contrast, the TSR extended range forecasts for the 2003, 2006, 2007 and 2009 hurricane seasons were less impressive.

Further Information and Next Forecast
Further information about TSR forecasts and verifications may be obtained from the TSR web site http://www.tropicalstormrisk.com. The first TSR forecast update for the 2013 Atlantic hurricane season will be issued on Friday 5th April 2013.