THERE'S good news for people living in hurricane-prone areas of the North Atlantic basin. Forecaster William M. Gray says he expects the North Atlantic hurricane season, running from June through November, to have below-average activity this year. A professor of atmospheric science at Colorado State University in Fort Collins, Colo., Dr. Gray has forecast hurricane seasons since 1984, generally with good success. His scheme did fail in 1989 when the unsuspected factor of West African rainfall changed from long-term drought to wet. Gray now includes West African rainfall in his reckoning.
Although the forecast doesn't work well for the Gulf of Mexico, it helps public-safety officials anticipate the degree of potential hurricane danger throughout the Caribbean and North Atlantic, up to middle latitudes. Moreover, as Gray's success record grows, so does confidence in his claim to have identified certain global and regional meteorological factors that significantly influence Atlantic hurricane activity.
These factors don't seem to correlate with activity in another hurricane region. Yet they do provide what Gray calls "a surprising 3-to-5-month advance seasonal hurricane predictive signal ... for the Atlantic basin." That basin is a unique laboratory in which to study the relation between Earth's most powerful storms and large-scale atmospheric conditions.
The five factors include:
1. The direction of stratospheric winds circling Earth above the equator. Wind direction oscillates between east and west in a roughly two-year cycle. On average, there's nearly twice as much Atlantic hurricane activity when these winds are westerly. This year, they will change to an easterly direction.
2. Presence or absence of a moderate to strong El Nino warm-water event in the tropical Pacific. Such warm water suppresses Atlantic hurricane activity. Gray expects "a significant El Ni by September or October.
3. Sea-level air pressure in the eastern Caribbean basin. Above-normal April and May pressures suggest a less-active hurricane season.
4. Westerly winds blowing over the lower Caribbean Basin just below the stratosphere. Stronger-than-normal winds are associated with reduced hurricane activity.
5. West African rainfall. Gray says new research shows that "nearly 60 percent of the year-to-year variance in the seasonal number of Atlantic intense hurricane days over the past 42 years can be explained by West African rainfall amounts prior to 1 August." He expects a dry year.
Putting it all together, Gray expects 1991 to have eight tropical storms (including four hurricanes) strong enough to be named. He forecasts 35 storm days (including 15 hurricane days). A "day" is four 6-hour periods during which a tropical cyclone has storm- or hurricane-intensity winds. The 42-year average is 9.9 named storms and 47.2 storm days. Only one hurricane is expected to be "major," compared with 2.5 for the 42-year average. Gray also expects a hurricane destruction potential of 40 - a measur e
of hurricane wind damage for the season. The 42-year average is 74.5.
Gray plans to update his forecast Aug. 3. Stay tuned.