Hurricane forecasters call for very active Atlantic season

A very nasty Atlantic hurricane season is on tap in 2007, according to the latest seasonal forecast issued today by Dr. Bill Gray and Phil Klotzbach of Colorado State University (CSU). The Gray/Klotzbach team is calling for 17 named storms, 9 hurricanes, and 5 intense hurricanes. An average season has 10 named storms, 6 hurricanes, and 2 intense hurricanes. The forecast calls for a much above normal chance of a major hurricane hitting the U.S., both along the East Coast (50% chance, 31% chance is normal) and the Gulf Coast (49% chance, 30% chance is average). The Caribbean is also forecast to have an above normal risk of a major hurricane.

The forecasters increased their hurricane activity numbers from their December forecast, citing the rapid dissipation of the winter El Ni�o event, a forecast of neutral or weak-to-moderate La Ni�a conditions for the coming hurricane season, plus a continuation of above average sea surface temperatures in the Atlantic.

How accurate are the April forecasts?
I would have liked to have seen mentioned in today's forecast in big bold letters, "our past April forecasts have shown no skill in predicting Atlantic hurricane activity." Don't get me wrong--the CSU team are very skilled scientists, and I like the fact that they are trying to make useful seasonal hurricane forecasts. However, the skill of these April forecasts when compared to climatology is near zero, and they should be stating that in very clear terms in their April forecasts. In fact, CSU April forecasts from 1995-2006 have shown slightly negative skill. Negative skill means that a forecast of the normal climatology of 10 named storms, 6 hurricanes, and 2 intense hurricanes for the period 1995-2005 performed better than the CSU forecast. CSU Forecasts from the past five years have shown some improvement, and have a slight positive skill. The CSU team has posted an Excel spreadsheet of their forecast errors (expressed as a mathematical correlation coefficient, where positive means a skilled forecast, and negative means they did worse than climatology). You can see from their numbers that the December and April forecasts have near zero skill, but the June 1 forecasts have substantial skill. To rectify their poor April forecast skill, the CSU team is trying a new scheme for this year's April forecast. They found that the February-March SST (30-45�N, 10-30�W), February-March SST (30-45�N, 10-30�W), and February-March sea level pressure (20-45�S, 100-160�W) could be used to explain about 55% of the ups and downs of hurricane activity over the period 1950-2004. Hopefully the new scheme will show positive skill forecasting upcoming hurricanes seasons, and not just "hindcasting" the past ones. For now, you're best off just paying attention to their June 1 forecast, which has been quite skillful over the past ten years.

2007 Atlantic hurricane season forecast from Tropical Storm Risk, Inc.
The British private forecasting firm Tropical Storm Risk, Inc. (TSR), issued a 2007 Atlantic hurricane season forecast today as well. TSR has almost the same forecast as the CSU team--17 named storms, 9 hurricanes, and 4 intense hurricanes. I like how they put their skill level right next to their forecast numbers: 9% skill at forecasting the number of named storms, 15% skill for hurricanes, and 14% skill for intense hurricanes. That's not much better than flipping a coin, but it is better than the slightly negative forecast skill of the Gray/Klotzbach April forecasts. However, TSR doesn't mention the fact that part of their skill may be due to the fact that they issue forecasts of fractional storms--i.e., the numbers they will verify for this April's foreacst are 16.7 named storms, 9.2 hurricanes, and 4.2 intense hurricanes. If we round these numbers to whole storms, the TSR skill numbers may decrease.

TSR projects that five named storms will hit the U.S., with two or three of these being hurricanes. In the Lesser Antilles Islands of the Caribbean, TSR projects two named storms, one of them being a hurricane (50/50 chance of being a major hurricane). TSR cites two main factors for their forecast of an active season: above normal Sea Surface Temperatures (SSTs) are expected in August-September 2007 across the tropical Atlantic, as well as slower than normal trade winds July-September. Trade winds are forecast to be 0.9 meters per second (about 2 mph) slower than average, which would create greater spin for developing storms, and allow the oceans to heat up due to reduced evaporational cooling. SSTs are forecast to be about 0.16 degrees C above normal. TSR gives an 85% chance that the 2007 Atlantic hurricane season will rank in the top third of active seasons observed since 1957.


Figure 1. Accuracy of long-range forecasts of Atlantic hurricane season activity performed by Bill Gray and Phil Klotzbach of Colorado State University (colored squares) and TSR (colored lines). The CSU team's April forecast skill is not plotted, but is near zero. The skill is measured by the Mean Square Skill Score (MSSS), which looks at the error and squares it, then compares the percent improvement the forecast has over a climatological forecast of 10 named storms, 6 hurricanes, and 2 intense hurricanes. TS=Tropical Storms, H=Hurricanes, IH=Intense Hurricanes, ACE=Accumulated Cyclone Energy, NTC=Net Tropical Cyclone Activity. Image credit: TSR.

Landmark greenhouse gas emissions ruling by the Supreme Court
The Supreme Court yesterday ruled that the EPA has the authority to regulate CO2 as a pollutant. An analysis of this ruling is presented by Dr. Ricky Rood today in our Climate Change blog. This could well prove to be the Supreme Court's most important environmental decision ever.

Jeff Masters