Note to Editors: Forecast totals are in chart form attached. Professor William Gray’s complete forecast plus related research and press releases are available on the World Wide Web at http://tropical.atmos.colostate.edu/forecasts/index.html
This year’s record El Niño is expected to dissipate before the most active part of the 1998 hurricane season but other global climate factors could produce slightly below-average hurricane activity, Colorado State University’s noted hurricane researcher William Gray announced today (Dec. 5).
In the first forecast issued for the 1998 hurricane season, Gray and his colleagues predict nine tropical storms will form in the Atlantic Basin between June 1 and Nov. 30. From those storms, five hurricanes will evolve and two will go on to become intense hurricanes with sustained winds of 111 mph or greater. On average, 9.3 tropical storms, 5.8 hurricanes and 2.1 intense hurricanes form annually.
The team’s 1998 forecast comes on the heels of a hurricane season in which activity was flattened by the strongest El Niño event on record–but the Atlantic Basin still managed to produce seven tropical storms, three hurricanes and one intense hurricane.
"Even though El Niño negatively influenced our 1997 hurricane forecast, it is our belief that this event will die before or shortly after the 1998 hurricane season begins," Gray said. "The most difficult aspect of the 1998 forecast is to determine whether residual effects from El Niño will have any impact on overall hurricane activity. We will have to wait and see."
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 cold water sometime in the early spring or summer. These cooler water temperatures, or La Niña conditions, help to promote hurricane activity.
Although the strength or weakness of El Niño is a major influence on hurricane activity, Gray says that other global climate conditions offer a "mixed bag" for the upcoming hurricane season.
Negative factors for hurricane activity include the Quasi-Biennial Oscillation, equatorial stratospheric winds at 68,000-75,000 feet, which are expected to blow from an easterly direction. This easterly flow tends to prevent hurricane development. When the QBO blows in a westerly direction–as it did this year–there is typically 50 to 75 percent more hurricane activity, according to Gray.
Favoring hurricane development in 1998 are above-average sea surface temperatures in the North-, East- and tropical Atlantic. When these regions are warmer during the summer and fall as they are this year, it typically helps to promote hurricane formation the following year. Supporting this is another 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, which causes weaker East Atlantic trade winds and is more favorable for hurricane development in the following season.
An uncertain factor in the 1998 forecast is below-average rainfall in the Western Sahel region of Africa and along the Gulf of Guinea. When these two regions are drier than normal, it indicates that global conditions are such they could weaken the season’s net hurricane activity. When wetter than normal conditions are present in these two regions, it typically enhances hurricane activity.
Gray and his team believe that the drier-than-average conditions in these two regions this year was brought on by El Niño and therefore should not be considered an indication that 1998 hurricane activity will be greatly reduced.
"El Niño is still hanging over our heads to some degree, because it produced some weather anomalies like the dry conditions in the Western Sahel and Gulf of Guinea," Gray said. "As we get closer to the beginning of hurricane season, 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 NOAA 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–to be issued in 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 the 1997 hurricane season was below average, Gray’s statistics show that the period between 1995-1997 was still 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–particularly intense storms.
"El Niño was just a one-year interruption in a long cycle of increased hurricane activity," Gray said. "In the past three years, we have seen a major shift of Atlantic Ocean surface temperatures, where surface temperatures in the North and the tropical Atlantic have become considerably warmer than in the 25-year period between 1970-1995. This shift promotes major hurricane activity and signals that we may be entering a dangerous and heightened period of hurricane activity."
GRAY RESEARCH TEAM HURRICANE FORECASTS 1998 SEASON
1. Named Storms (9.3)* 9
2. Named Storm Days (46.6) 40
3. Hurricanes (5.8) 5
4. Hurricane Days (23.9) 20
5. Intense Hurricanes (2.3) 2
6. Intense Hurricane Days (4.7) 3
7. Hurricane Destruction Potential (71.2)** 50
8. Net Tropical Cyclone Activity (100) 90
’97 ACTUAL 8/97 6/97 4/97 12/96
1. Named Storms (9.3) 7 11 11 11 11
2. Named Storm Days (46.6) 28 45 55 55 55
3. Hurricanes (5.8) 3 6 7 7 7
4. Hurricane Days (23.9) 10 20 25 25 25
5. Intense Hurricanes (2.3) 1 2 3 3 3
6. Intense Hurricane Days (4.7) 2.2 3 5 5 5
7. Hurricane Destruction Potential (71.2) 26 60 75 75 75
8. Net Tropical Cyclone Activity (110) 54 100 110 110 110
( ) 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.