The East Coast of the United States and the Caribbean area are in little or no danger of being struck by a major hurricane this year, according to a new forecast. But the next two decades should see the return of a pattern of more frequent killer storms that prevailed from the late 1940s through the 1960s, the forecasters warned.
The forecast, based on a number of climatic and meteorological factors identified by Dr. William M. Gray of Colorado State University, an expert in tropical cyclones, says that in 1991 there will be no Atlantic hurricanes in categories 3 through 5 of the Saffir-Simpson scale of hurricane intensity.
The majority of damage caused by hurricanes results from a relatively few highly destructive storms in those categories.
Category 3 storms have top winds of 111 mph to 130 mph, category 4 of 131 mph to 155 mph and category 5 top winds of more than 155 mph. The Atlantic hurricane season runs from June through November.
"The 1991 Atlantic hurricane season will likely be substantially less active than have the hurricane seasons of the last three years, " Dr. Gray said in the forecast issued last week.
In addition to predicting no killer storms, the Gray forecast says there will be about three Atlantic hurricanes weaker than category 3 and about 10 days in which hurricanesexist.
The most important factor in making the forecast, Dr. Gray says, is rainfall in the western Sahel region of Africa, just east of the area in the Atlantic where hurricanes are spawned.
When rain is plentiful in the western Sahel, more strong hurricanes develop in the Atlantic and strike the United States, he says. Such a period existed from the years 1947 through 1969. In that stretch, the study found, 13 hurricanes in categories 3 through 5 struck the East Coast and Florida.
When there is drought in the western Sahel, according to the study, fewer strong Atlantic hurricanes develop -- as in the period from 1970 through 1987.
An analysis of conditions through last month indicates that 1991 will be one of the driest years in the western Sahel in the last 43 years, Dr. Gray said in his latest report.
"For this reason we are forecasting no intense hurricane activity for this season," the report said.
In the longer term, Dr. Gray says, the western Sahel has experienced alternate wet and dry periods roughly two decades long. If that pattern holds, he said, a long-term wet period should begin soon, bringing with it an extended stretch of more frequent storms like that of 1947 through 1969.
Last year, Dr. Gray predicted there would be six hurricanes in the Atlantic Basin. There were eight. He predicted there would be 25 days in which hurricanes existed. There were 27.5 such days. And he predicted two hurricanes in category 3 or of greater intensity. There was one, and it did not make landfall.