Note to Editors: Forecast totals are in the attached chart. A taped interview with Professor William Gray is available by calling (970) 491-1525. The complete hurricane forecast, plus related research and press releases, are available on the World Wide Web at: http://tropical.atmos.colostate.edu/forecasts/index.html
The much-discussed El Niño will fade in 1998, and that will help lead to an increase in hurricane activity this year compared with last year, Colorado State University hurricane forecaster William Gray said in a report released today.
In this second forecast for the upcoming hurricane season, Gray and his colleagues are increasing their forecast totals from nine to ten tropical storms that will form in the Atlantic Basin between June 1 and Nov. 30. From those storms, the team now predicts six hurricanes will evolve and two will go on to become intense or major hurricanes with sustained winds of 111 mph or greater. That’s a slight increase from the initial forecast, when Gray and his team predicated a season with nine tropical storms, five hurricanes and two intense hurricanes, or Andrew-sized storms. On average, 9.3 tropical storms, 5.8 hurricanes and 2.1 intense hurricanes form annually.
Last year’s hurricane activity was suppressed by what has turned out to be the strongest El Niño in recorded history–a record that has generated considerable media attention for its impact on weather patterns around the globe. In today’s report, Gray and his research team predict that the El Niño will be mostly dissipated by the start of the active part of the hurricane season in mid-August 1998, and that, taken with other favorable climate changes, will work to produce a season with more hurricane activity. Overall, Gray and his team are looking for a season that closely matches what occurs in an average year.
When El Niño is in place, it produces upper-level westerly winds at 40,000 feet in the tropical Atlantic Ocean that help block hurricane development. Gray and other forecasters watching this El Niño believe these warm-water temperatures will be replaced by cool water sometime in the late summer. These cooler-water temperatures, or La Niña conditions, help promote hurricane activity. Although the strength or weakness of El Niño is a major influence on hurricane activity, other global conditions offer a more favorable influence for increased activity in the upcoming hurricane season, Gray and his research team said. The only major negative factor for hurricane activity is the Quasi-Biennial Oscillation, or the equatorial stratospheric winds at 68,000 to 75,000 feet, which are expected to blow from an easterly direction. This easterly flow tends to inhibit hurricane development. When the QBO blows in a westerly direction, there is typically 50 to 75 percent more hurricane activity, according to Gray.
The factors favoring hurricane development in 1998 include above-average sea surface temperatures in the North, East and tropical regions of the Atlantic. When these regions are warmer during the summer and fall, it typically helps to promote hurricane formation the following year.
Another factor working to promote hurricane activity is a condition known as the Azores High, a ridge of high surface pressure located near the Azores Islands in the North Atlantic. This ridge of high pressure was below the long-term average in October and November, and continues below-average in March. This causes weaker East Atlantic trade winds and is more favorable for hurricane development in the following season.
One of the uncertain factors 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, it typically inhibits hurricane activity. Gray and his research team will watch this factor closely for upcoming forecasts in June and August. Gray and his team believe that the drier than average conditions in this region were brought on by El Niño and therefore should not be considered an indication that 1998 hurricane activity will be greatly reduced.
"We’re still watching the El Niño very closely, and we will have to keep a close eye on what the lingering effects of El Niño might be," Gray said. "In the last season, it produced some weather anomalies like the dry conditions in the Western Sahel that had an impact on the hurricane season and on our forecasts.
"In the next two reports in early June and early August, we will likely have a much better picture of how these interconnected global conditions will affect storm activity."
In addition to these factors, throughout the season Gray and research team members Chris Landsea at the National Oceanic and Atmospheric Administration Hurricane Research Division in Miami, Fla.; John Knaff; 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 Niño 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 (13 of which were intense) and 116 hurricane days. Based on that record, Gray maintains his theory that the Atlantic Basin is entering an era spanning many decades of increased hurricane activity and which will include particularly intense or major hurricanes.
GRAY RESEARCH TEAM
HURRICANE FORECAST FOR 1998 SEASON
|Named Storms (9.3)*||10||9|
|Named Storm Days (46.6)||50||40|
|Hurricane Days (23.9)||20||20|
|Intense Hurricanes (2.3)||2||2|
|Intense Hurricane Days (4.7)||4||3|
|Hurricane Destruction Potential (71.2)**||65||50|
|Net Tropical Cyclone Activity (100)||95||90|
|Named Storms (9.3)||7||11||11||11||11|
|Named Storm Days (46.6)||28||45||55||55||55|
|Hurricane Days (23.9)||10||20||25||25||25|
|Intense Hurricanes (2.3)||1||2||3||3||3|
|Intense Hurricane Days (4.7)||2.2||3||5||5||5|
|Hurricane Destruction Potential (71.2)||26||60||75||75||75|
|Net Tropical Cyclone Activity (110)||54||100||110||110||110|
* Number in ( ) represents average year totals based on 1950-1990
** Hurricane Destruction Potential measures a hurricane’s potential for wind- and ocean-surge damage. Tropical Storm, Hurricane and Intense Hurricane Days are four, six-hour periods where storms attain wind speeds appropriate to their category on the Saffir-Simpson scale.