Above Average Hurrican Season Likely  

Tropical Weather Underground reports that an above average hurricane season is likely for 2008.

An above average Atlantic hurricane season is on tap for 2008, according to today's seasonal forecast issued by Dr. Bill Gray and Phil Klotzbach of Colorado State University (CSU). The Gray/Klotzbach team is calling for 15 named storms, 8 hurricanes, 4 intense hurricanes, and an Accumulated Cyclone Energy (ACE) index 150% of normal--unchanged from their April forecast. These numbers are also the what the Atlantic has averaged since we entered a period of above-average hurricane activity in 1995.

An average season has 10-11 named storms, 6 hurricanes, and 2 intense hurricanes. The Klotzbach/Gray forecast calls for an above normal chance of a major hurricane hitting the U.S., both along the East Coast (45% chance, 31% chance is normal) and the Gulf Coast (44% chance, 30% chance is average). The Caribbean is also forecast to have an above average risk of a major hurricane.

The forecasters cite the continuation of the above-normal hurricane activity period that began in 1995, the expected lack of an El Niño event, the continuation of above average sea surface temperatures and below average pressure in the eastern Atlantic, and slower trade winds (which result in reduced evaporative cooling of the ocean), as the justification for their forecast of an above average hurricane season. It's of interest to look at the activity maps for the four analogue years that had similar atmospheric and oceanic conditions in May: 1951, 1961, 2000, and 2001. The 1961 season was the nastiest of these four seasons, with two Category 5 hurricanes. One of those storms, the notorious Hurricane Carla, hit Texas as a mighty Category 4 hurricane with a 22-foot storm surge. The other three seasons had no hurricanes that hit the U.S.
How good are the CSU forecasts?

The CSU forecast team has been making seasonal hurricane forecasts since 1984. Based on predictions of a below average, average, or above average season, they have done pretty well over the past nine seasons. Eight of the past nine forecasts have been correct. Their only failure occurred in 2006, when they called for a very active season, and it was a normal year with 10 named storms, 5 hurricanes, and 2 intense hurricanes. A more rigorous way of determining forecast skill is to compute the mathematical correlation coefficient. A correlation coefficient of 1.0 is a perfect forecast, and 0.0 is a no-skill forecast. The late May CSU forecasts have a respectable correlation coefficient of 0.57 for predicting the number of named storms (1984-2006). This decreases a bit to 0.46 and 0.42 for number of hurricanes and intense hurricanes, respectively. These are respectable correlation coefficients, and the late May/early June CSU forecasts are worth paying attention to. This is in contrast to the December and April CSU forecasts, which have had a correlation coefficient near zero (and thus no skill).

The British private forecasting firm Tropical Storm Risk, Inc. (TSR), issues monthly 2008 Atlantic hurricane season forecasts. Their June 2 forecast calls for a 53% chance of an above average season. This compares to the 65% chance of an above average season given by NOAA in their May 22 forecast.