Hurricane Outlook Downgraded

Forecaster expects fewer big storms than he first predicted, but says season will still be active. STORM WATCH

By , Staff writer of The Christian Science Monitor

FOR those who live in the Atlantic hurricane zone, storm forecaster William Gray has good news and a warning. The Colorado State University meteorologist says he now expects the hurricane season to be somewhat less intense than he originally forecast. However, he warns that he still expects it to be a rather active season although it has gotten off to mild start.

Dr. Gray is unique among hurricane forecasters in predicting the overall characteristics of a hurricane season rather than trying to anticipate when a specific storm may arise, where it will go, and how strong it may be. He makes an initial forecast for the official hurricane season that runs from June 1 through Nov. 30. He updates his projections in August as the most intense period of the season gets under way.

That update has dropped his forecast of seven hurricanes down to six. Two of these - rather than three as originally forecast - should become major storms ranking three, four, or five on the 1-to-5 hurricane intensity scale. But Gray still expects a total of 11 named storms - storms that reach at least tropical storm intensity.

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Gray also projects a total of 50 storm days, including 25 hurricane days. That's down from the 55 storms days and 30 hurricane days of his June 5 forecast. A storm day is any part of a day when a storm is observed. Gray has dropped his prediction of Hurricane Destruction Potential (HDP) from 90 to 75 as well.

Gray considers the HDP to be a measure of the potential for destruction from both wind alone and from the wind-driven storm surge of water that comes ashore when a hurricane hits land. A single storm's HDP is the sum of the squares of the maximum wind speeds for each six-hour period of the storm's life. Gray adds up the HDPs for all the predicted storms to get the HDP for the season.

The physical destruction a hurricane can cause seems to be related more directly to the square of the wind speed than simply to the speed itself. This means that destruction potential grows much faster than the winds themselves as a storm intensifies.

In short, with fewer storm days, lower destruction potential, and one less hurricane, Gray now predicts an average hurricane season, where the term ``average'' is based on the statistics of the past 40 years.

Gray bases his forecast on five factors that involve upper-air winds, sea-level air pressure in the Caribbean Basin, and the El Nino warming in the equatorial Pacific Ocean. An El Nino, for example, tends to suppress Atlantic hurricane activity. There is no El Nino warming this year.

Gray successfully forecast hurricane seasons from 1984 through '88. He says he failed last year because he didn't consider West African rainfall. The break in the West African drought favored a more intense season than he predicted. He now includes this factor in his forecast.

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