The American Meteorological Society, on 5 June 2007, issued a policy statement that outlines the state of science of hurricane forecasting in the United States, including hurricane-related hazards, observations, forecasting skills, and continuing challenges. The AMS is a professional body that is not under the U. S. National Weather Service, and it has an independent and authoritative voice. The AMS statement on U. S. hurricanes makes several important points, some of which are applicable to tropical cyclones in general, and are therefore noteworthy for countries like India, which are regularly visited by such storms.

1. Track forecasts: The most heartening aspect of the AMS statement is that there has been a significant improvement in U. S. hurricane track forecasting in recent years. During the 5-year period 2001-2005, the hurricane track forecasts issued by the National Hurricane Center (NHC) of the U. S. National Weather Service had an average error of 65 nautical miles (120 km) for the 24-hour forecast and 118 nautical miles (269 km) for the 48-hour forecast. These errors are about half of what they used to be in 1990.

2. Intensity forecasts: The AMS statement, however, admits that forecasting of hurricane intensity still remains a challenge to forecasters. During the past 30 years, there has not been any noticeable improvement in the forecasts of storm intensity. Large errors typically occur when storms strengthen or weaken rapidly.

3. Warning and over-warning: The statement notes that although the forecasting of hurricane tracks has improved, hurricane warnings continue to be issued for large coastal areas of the U. S. However, the average length of the coastline warned has come down to 510 km in the current decade from 730 km in the preceding one. While only one-fourth of the warning area may actually experience hurricane conditions, some over-warning is justified in the interest of safeguarding life and property. Over-warning ensures that unexpected rapid increases in storm strength prior to landfall or unanticipated changes in the distribution of damaging winds, do not take under-prepared populations by surprise.

4. Rainfall: Prediction of rainfall from landfalling hurricanes is yet another illusive factor, particularly because of the effects of terrain.

5. Storm surge: The highest loss of life due to a hurricane continues to be attributed to the storm surge, which can be as high as 6 m when a strong hurricane strikes a coastline with shallow water offshore. The statement notes that in recent decades, large losses of life due to storm surge had become less frequent in the U. S. However, the rapid growth of the coastal population and related infrastructure, and the increasing complexity of evacuation have led to a greater vulnerability of the coastal communities. This became evident in the case of Hurricane Katrina in 2005, in which the loss of life in Louisiana and Mississippi was estimated to be 1700.

6. Storm surge modelling: Given a sufficiently accurate forecast of the hurricane’s track and surface wind structure, and the required topographic and bathymetric data, numerical models should be able to make a precise prediction of the inundation by storm surge. However, the AMS statement cautions that because of the uncertainty in hurricane forecasts, evacuation decisions should not be made on the basis of individual runs of a single storm surge model.

7. Seasonal prediction: The statement acknowledges the low confidence associated with seasonal predictions of hurricane activity in the Atlantic basin, particularly when applied to smaller areas of the basin.

8. Preparedness measures: The statement says that in order to build up a higher resilience to the hurricanes, there is a need for greater involvement of other disciplines including engineering, ecology, biology, the social, behavioral, and economic sciences, and public policy. A comprehensive framework is also essential for ensuring public understanding of the hurricane threat and the ability to take appropriate action to mitigate the loss of life and property that links the entire process — from data collection to forecast to communication of the societal impact.

9. New technology and modelling: According to the statement, the forecasts would benefit from continuing improvements in the observational systems such as Doppler radars and satellites that can effectively observe the details of the storm core, data assimilation techniques and modelling capabilities on both the global and regional scale, particularly in ocean–atmosphere interactions. More effective remote sensing of ocean surface winds can also improve the initial detection of storms and their further analysis and forecasting.

The complete text of the AMS statement is available online at http://www.ametsoc.org/POLICY/2007hurricaneforecasting.html.  

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