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Catastrophe Updates
2007 Hurricane Forecasts
Opening day for the 2007 Atlantic Basin hurricane season has passed (June 1 is the demarcation for the beginning of the Atlantic hurricane season). Several research bodies have recently updated their forecasts and they are all roughly in line with expectations of an above-average hurricane season with from 3 to 5 intense hurricanes this year.
The May/June hurricane season forecasts have shown to be the most reliable of the forecasts, but as was demonstrated in 2006 they sometimes go wrong. All of the forecasters note that their forecasts are highly dependent upon ENSO measurement of water temperatures in the Pacific Ocean. The absence of an El Niño condition is a strong predictor for above average hurricane activity. In 2006 the Pacific slipped from a La Niña condition to an El Niño condition late in the season which contributed to the low-activity seen last year.
The primary hurricane activity parameter being estimated by these researchers is the ACE (Accumulated Cyclone Energy) Index, an index that measures “the collective intensity and duration of Atlantic named storms and hurricanes occurring during a given season”. The median historic ACE index is 100, the average (reference: NHC) is 92. An ACE index greater than 100 implies a stormy season, less than 100 implies a quiet season.
EQECAT review of the ACE Index shows a very weak correlation between the ACE Index and expected hurricane losses onshore. The other parameters forecasted (number of named storms; number of intense storms; etc) prove to have an even weaker correlation to expected losses to people and property along the Gulf and Atlantic Coasts of the United States. The basis for this weak correlation lies in both the parameter being estimated (all of these measures are for storms at sea, not necessarily making landfall) and the fact that there are only a few large urban centers along the US Coast that have the potential to generate extreme “super-CAT” losses.
The US National Hurricane Center (NHC) is noting that the current persistent state of the AMO is also a strong contributor to their forecast of higher than average hurricane activity. The state of the AMO is the key factor in the EQECAT Near Term hurricane risk model.
Summary of Publicly Available Atlantic Hurricane Season Forecasts
Dr. Gray's Tropical Storm Forecast: June, 2007
(most recent forecast: 31 May 07)
Dr. Gray has been issuing tropical storm forecasts since 1984, his forecasts are available at http://typhoon.atmos.colostate.edu/forecasts/. The most recent forecast is for 5 intense hurricanes to be formed in 2007, with 11 intense hurricane-days. The ACE index is expected to be around 170 for the season.
We expect Atlantic basin Net Tropical Cyclone (NTC) activity in 2007 to be about 185 percent of the long-term average.
Currently, neutral ENSO conditions are observed. We expect either cool neutral or weak-to-moderate La Niña conditions to be present during the upcoming hurricane season. Tropical and North Atlantic sea surface temperatures remain well above their long-period averages.
As has been discussed extensively in previous forecasts, El Niño conditions during the summer and fall tend to decrease Atlantic hurricane activity by increasing vertical wind shear across the area where Atlantic tropical cyclones develop (e.g., Gray 1984a).
Benfield UCL Hazard Research Centre, UCL (University College London), UK
http://tsr.mssl.ucl.ac.uk/
Most Recent Forecast (issued 4 July 2007): The forecast calls for 3.5 intense hurricanes and an ACE index of 140.
“Atlantic basin and US landfalling tropical cyclone activity are forecast to be about 40% above the 1950-2006 norm in 2007. There is a high (~72%) likelihood that activity will be in the top one-third of years historically.”
Prior Forecast (issued 31 May 2007)
Based on current and projected climate signals, Atlantic basin and US landfalling tropical cyclone activity are forecast to be about 65% above the 1950-2006 norm in 2007. There is a high (~84%) likelihood that activity will be in the top one-third of years historically.
National Hurricane Center
Most Recent Forecast: 22 May 07. The forecast calls for 3-5 “major storms” and an ACE index in the range of 125 to 210.
http://www.cpc.noaa.gov/products/outlooks/hurricane.shtml
NOAA’s 2007 Atlantic hurricane season outlook indicates a very high 75% chance of an above-normal hurricane season, a 20% chance of a near-normal season, and only a 5% chance of a below-normal season.
The prediction for an above-normal 2007 hurricane season reflects the expected combination of two main climate factors: 1) the continuation of conditions that have been conducive to above-normal Atlantic hurricane seasons since 1995, and 2) the strong likelihood of either ENSO-neutral or La Niña conditions in the tropical Pacific Ocean.
WSI Corporation
WSI is a private corporation that provides seasonal forecasts for hurricanes. The most recent forecast (25 July 2007) calls for 3 intense hurricanes, with no comment on ACE index or number of hurricane days. The next forecast is scheduled for 14 August.
Researchers
Dr. Lian Xie, professor of marine, earth, and atmospheric sciences at North Carolina State University, and graduate student Elinor Keith have produced a model for forecasting hurricane activity. Their current prediction is for the higher-than-average chances for 2-3 hurricanes forming in the Gulf, 1-2 of which may make landfall. The credentials of this model include the fact that their estimate for the 2006 hurricane season was for 5-6 hurricanes to occur, and the official count was 5. Unfortunately, there is no citable reference beyond media interviews discussing the model and its forecasts.
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