BOSTON — THE Atlantic Basin hurricane season opened this month with a promise of good behavior.
Veteran forecaster William Gray predicts hurricane activity will be well below the 42-year average. His prediction of a similarly quiet hurricane season last year was substantially correct.
Dr. Gray, a professor of atmospheric science at Colorado State University in Fort Collins, has developed a forecasting scheme that he says can "make surprisingly skillful forecasts of Atlantic tropical cyclone activity" up to half a year in advance. He issued a preliminary prediction for a mild 1992 season last December. Nothing has happened in the ocean or atmosphere since then to change his mind.
In his recently released forecast, he expects eight tropical storms strong enough to receive names, four of which will be hurricanes. He expects one hurricane to be "major" - that is with winds of at least 111 miles an hour. That compares with the 42-year average of 9.9 named storms, including 5.9 hurricanes with 2.5 of them being "major."
Gray also forecasts 35 named storm days, including 15 hurricane days, and a hurricane destruction potential (HDP) of 35. A storm "day" is four six-hour periods during which the storm is observed to exist. Destruction potential is a measure of the season's potential for wind and storm surge damage. The 42-year averages are 47.2 storm days, 23.8 hurricane days, and an HDP of 74.5.
Gray bases his predictions on several factors:
* The direction of stratospheric winds that circle the globe above the equator and reverse direction every 12 to 16 years. Their influence is neutral this year.
* Western African rainfall and air pressure and temperature patterns. West African drought last fall indicates a decrease in this year's tropical storm activity.
* Presence of El Nino - a warming of eastern tropical Pacific water. El Nino suppresses Atlantic tropical storm activity. Gray notes that the El Nino now under way is weakening. But he says he thinks it should remain strong enough to keep the lid on Atlantic hurricanes for this season, which ends Nov. 30.
Gray will update this forecast Aug. 3, when he has better data on El Nino. That's just before the most intense part of the hurricane season. Gray warns that "this seasonal forecast is a statistical scheme which will fail in some years." It is less reliable for the Gulf of Mexico, where local factors play a larger role in hurricane activity than in the rest of the Atlantic basin. And the scheme doesn't forecast when any particular storm will develop or where it will track.
Gray expects a cooling of eastern tropical Pacific water next year and thinks the stratospheric winds will favor hurricanes. He says: "There is a good possibility that Atlantic basin hurricane activity in 1993 will be considerably greater than last year or as this year is expected to be."