Above average hurricane season expected by Dr. Bill Gray's team

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.


Figure 1. Sea Surface Temperature departure from average from May 29, 2008. Caribbean SSTs are near average, but are much above average in the Eastern Atlantic. Image credit: NOAA.

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).

Other forecasts
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.

Last blog entry for a week
This will be my final blog entry until June 11. With the tropics looking quiet for the next week, it's a good time to do some hiking in SW Montana. In my absence, I've arranged for a few "canned" blogs of mine to be posted. If there's anything stirring in the tropics, I've arranged for a guest blogger, Bryan Woods, to jump in. Bryan has done a great job over the past three years blogging on the tropics over at thestormtrack.com. Here's Bryan's bio:

Bryan received his BS in Meteorology from the University of Massachusetts in Lowell, MA in 2005, and his M.Phil. in Geology & Geophysics from Yale University in New Haven, CT in 2007. Bryan is currently a Ph.D. candidate at Yale where he is also the graduate and professional student body president.

Bryan has spent two summers working on a National Science Foundation (NSF) sponsored field micrometeorological research project in Atlanta, GA, studying evapotranspiration rates in urban forest canopies. Currently, Bryan's research is focused on combining wavelet techniques and aircraft data from the NSF/NCAR Gulfstream V to diagnose energy and momentum fluxes from atmospheric gravity waves. Bryan has spent the past three hurricane seasons writing blogs on the tropics for thestormtrack.com.

Jeff Masters