NOAA’s updated 2015 Atlantic Hurricane Season Outlook indicates that a below-normal hurricane season is very likely. The outlook calls for a 90% chance of a below-normal season and a 10% chance of a near-normal season, with no realistic expectation that the season will be above-normal. This 90% probability of a below-normal season is the highest given by NOAA for any such season since their seasonal hurricane outlooks began in August 1998.
The likelihood of a below-normal hurricane season has increased for three main reasons. First, El Niño has strengthened as predicted, and NOAA’s latest prediction calls for a significant El Niño to continue through the remainder of the hurricane season. Second, atmospheric conditions that are exceptionally non-conducive to tropical storm and hurricane formation are now present in response to El Niño. These conditions, which include strong vertical wind shear and enhanced sinking motion, are predicted to continue through the peak months (August-October, ASO) of the hurricane season across the Atlantic hurricane Main Development Region (MDR, which spans the Caribbean Sea and tropical Atlantic Ocean between 9oN-21.5oN; Goldenberg et al. 2001). Third, sea-surface temperatures (SSTs) across the MDR are predicted to remain below average, and to also remain much cooler than the rest of the global tropics. Cooler Atlantic SSTs are associated with stronger trade winds, and further reduce the ability of storms to form and gain strength in the MDR.
Based on the current and expected conditions, combined with model forecasts, we estimate a 70% probability for each of the following ranges of activity during the 2015 hurricane season:
- 6-10 Named Storms, which includes the three named storms to date
- 1-4 Hurricanes
- 0-1 Major Hurricanes
- Accumulated Cyclone Energy (ACE) range of 25%-70% of the median.
The seasonal activity is expected to fall within these ranges in 70% of seasons with similar climate conditions and uncertainties to those expected this year. These ranges do not represent the total possible ranges of activity seen in past similar years.
These expected ranges are centered well below the official NHC 1981-2010 seasonal averages of 12 named storms, 6 hurricanes, and 3 major hurricanes.