The noted team, led by Professor William Gray, released an updated forecast today for 1997, that, if it holds true, would make this year the third consecutive active hurricane season. In fact, if this forecast bears out, the period from 1995-97 would become the most-active, three-year hurricane span in history. That directly contrasts 1991-94, the most inactive four-year period in history.
Gray says this recent active trend could signal the start of a return of more tropial storms and hurricanes, particularly more intense hurricanes.
"More and more it's looking like we have left a period of lessened hurricane activity and we could be seeing a basic change in the long-term global circulation patterns that could lead to more hurricanes," Gray said. "What's particularly disturbing is this change would likely increase the number of major hurricanes. This is an area we need to look at to help determine what might be in our future."
In this forecast--the second of four the team will issue for the 1997 season--Gray and his research team predict 11 tropical storms will form. Of those, the team says seven will become hurricanes, and of those, three will become intense, or major, hurricanes.
These major storms, which cause 75 percent of all storm-spawned damage, are characterized by sustained winds of at least 111 mph. Major storms fall into Category 3,4 and 5 hurricanes on the Saffir-Simpson scale. Hugo and Andrew were both Category 4 hurricanes when they made landfall.
In comparison, storms categorized simply as hurricanes have sustained winds of 74 mph or greater.
In the research team's first forecast (Dec. 1996) for the 1997 hurricane season, Gray and his colleagues predicted that a total of 11 tropical storms would form between June 1 and Nov. 30, 1997. From those storms, seven hurricanes will evolve and three will become major hurricanes. On average, 9.3 tropical storms, 5.8 hurricanes and 2.1 major hurricanes form annually.
Gray's hurricane forecasts--issued in December, April, June and August--do not predict landfall and apply only to the Atlantic Basin, the area including the Atlantic Ocean, Caribbean Sea and Gulf of Mexico.
Gray and his research team base their forecasts on a number of global weather features. In today's forecast, Gray continues to point to two factors that indicate above-average activity.
First, Gray and his team predict there will be no El Nino--a period of warmer than normal water temperatures off the coast of Peru. When El Nino is in place, it produces upper-level westerly winds at 40,000 feet in the tropical Atlantic Ocean. This helps shear off hurricane development. Gray predicts La Nina conditions will be in place--meaning temperatures off the coast of Peru will be slightly cooler than average--which will promote hurricane activity.
In addition, the equatorial stratospheric winds at 68,000-75,000 feet--known as Quasi-Biennial Oscillation--will blow from a relative westerly direction. This tends to promote the formation of hurricanes--particularly major hurricanes. Gray also relied on these factors heavily in issuing his forecast in December.
Gray and his colleagues also look at other weather signals around the globe. For example, Gray says that temperature readings of air above Singapore at the 100 millibar level--or about 54,000 feet--continue to be colder than usual, which is a precursor for above-normal hurricane activity for the Atlantic Ocean in the six-month period.
Gray and research team members Chris Landsea, a Colorado State graduate and researcher at the NOAA's Hurricane Research Laboratory in Miami, Fla., and John Knaff, Paul Mielke and Kenneth Berry, all of Colorado State, rely primarily on the following factors to issue their forecasts: the strength or weakness of El Nino; the direction of equatorial stratospheric winds at 68,000-75,000 feet; rainfall in the West African Sahel region; temperature and pressure readings in West Africa; Caribbean Sea-level pressure readings; Atlantic sea-surface temperature readings; and tropospheric winds at 40,000 feet. The research team also added pressure readings in the northeast Atlantic to the forecast.