Hurricane Update Predicts Average Season

June 05, 1998

FORT COLLINS--In a report released today, Colorado State University hurricane forecaster William Gray said a fading El NiÒo will help produce more hurricane activity this year than in 1997.

However, storm totals should only constitute what is considered average for the past 50 years, he said. The faster-than-expected disappearance of the strongest El NiÒo on record will eliminate a factor that works against hurricane formation. The El NiÒo phenomenon caused last year's season to be unusually quiet.

Gray's third report of the 1998 season calls for 10 tropical storms to form in the Atlantic Basin through Nov. 30. Six tropical storms will evolve into hurricanes and two will go on to become intense hurricanes with winds of 111 mph or more.

Gray and his team initially predicted nine tropical storms, five hurricanes and two major or Andrew-type intense hurricanes in their initial report last December and increased their forecast total of storms and hurricanes to 10 and six, respectively, in an early April update.

On average, 9.3 tropical storms, 5.8 hurricanes and 2.2 intense hurricanes form annually.

"The basis of our forecast is that the El NiÒo, the strongest one we've ever had by nearly a factor of two, is fading out fast and should be dissipated by the start of the active part of the hurricane season in mid-August," Gray said. "We may even have some slightly cool water in the eastern equatorial Pacific by the height of the hurricane season in September. So we don't think El NiÒo will be a significant inhibiting factor this year."

Last year's hurricane activity was suppressed by what turned out to be the strongest El NiÒo ever recorded during the peak hurricane months of August to October. In today's update, Gray and his colleagues predict that the fading of El NiÒo, plus other climate changes favorable to storm formation, will combine to produce a season with more hurricane activity than last year.

"We think we'll see a slightly above-average number of named storms, slightly above average for the hurricanes and slightly below average for major storms," Gray said. "So I would characterize it as an average season in terms of the last 48 years."

However, Gray said, taken in light of the "remarkable downturn" in storms evident in the period from 1970 to 1994, this year's numbers represent activity busier than what has been seen during most years in recent decades.

While the El NiÒo phenomenon sometimes is followed by a condition in the eastern Pacific called La NiÒa, Gray said data suggest that if the cold water does appear in the eastern equatorial Pacific,it will probably do so too late to have much effect on the hurricane season, which ends by November.

Several climatic factors beside the fading of El NiÒo appear favorable to formation of storms this year. These include above-average sea surface temperatures in the tropical north Atlantic and a ridge of eastern Atlantic air near the Azores Islands in the North Atlantic that has remained weak since last October. This reduced high pressure causes weaker East Atlantic trade winds and is more favorable for hurricane development in the following season, according to Gray.

One of the uncertainties in the 1998 forecast involves rainfall in the Western Sahel region of Africa. When this region is wetter than normal, it typically promotes hurricane formation and the season's net hurricane activity will be increased. When dryer than normal conditions are present in this region, hurricane activity typically is inhibited.

Although information on rainfall in the Sahel region of West Africa isn't clear, "We're pretty sure it's not going to be dry this year," Gray said. Recent drier-than-average conditions in this region were brought on by El NiÒo and therefore should not be considered an indication that 1998 hurricane activity will be greatly reduced, according to the team's report.

As of the end of May, the only negative factor for hurricane formation is the Quasi-Biennial Oscillation, the equatorial stratospheric winds at 68,000 to 75,000 feet that are expected to blow from an easterly direction this summer. This easterly flow tends to inhibit hurricane development. When the QBO blows in a westerly direction, there is typically 50 percent to 75 percent more hurricane activity, according to Gray.

Throughout the season Gray and research team members Chris Landsea at the National Oceanic and Atmospheric Administration Hurricane Research Division in Miami, Fla., and Colorado State faculty members Paul Mielke and Kenneth Berry also take into account temperature and pressure readings in West Africa, Caribbean Sea-level pressure readings, temperature readings above Singapore at about 54,000 feet and tropospheric winds at 40,000 feet.

Gray's hurricane forecasts - issued annually in early December, April, June and August - do not predict landfall and apply only to the Atlantic Basin, the area encompassing the Atlantic Ocean, Caribbean Sea and Gulf of Mexico.

Although El NiÒo suppressed hurricane activity last year, statistics show that the period between 1995-1997 still was the busiest three-year period for hurricane activity on record. The three-year span generated 39 named storms, 23 hurricanes (12 of which were intense) and 107 hurricane days.

Based on that record, Gray theorizes that the Atlantic Basin is entering an era that will see many decades of increased hurricane activity and which will include particularly intense or major hurricanes.

Colorado State University

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