The number of hurricanes and intense hurricanes expected to form in the Atlantic Basin between now and Nov. 30 will remain near normal, according to the final 1998 update issued by Colorado State University’s hurricane forecast team today.
Despite "a mixed bag of climate signals," the hurricane research team led by William Gray, professor of atmospheric sciences, is continuing to predict that the number of Atlantic Basin hurricanes will remain at six and the number of intense storms (hurricanes packing sustained winds of 111 m.p.h. or more) will total two.
The team continues to forecast 10 named storms this year. Gray’s annual prediction has produced better-than-average results, with accurate or near-accurate forecasts for 10 of the past 14 years.
In their initial 1998 forecast issued in December 1997, Gray and his colleagues called for an average season. That report said nine tropical storms would form, five would evolve into hurricanes and two into intense hurricanes. The forecast was updated to 10, six and two, respectively, in April and remained unchanged in June.
Long-term averages are 9.3 named storms, 5.8 hurricanes and 2.2 intense hurricanes annually.
Factors affecting the early August forecast include a rapidly developing La Niña, an upwelling of cold water in the eastern equatorial Pacific, and a recent decrease in rainfall in the Western Sahel region of Africa, Gray said.
Gray said the unusual speed with which La Niña appeared has all but wiped out the vestiges of last year’s El Niño. La Niña produces, among other things, weak westerly winds in the upper troposphere that do little to shear the tops off developing storms and are a positive factor in hurricane formation.
By contrast, El Niño produces strong westerly winds at 40,000 feet that shear the tops off tropical depressions and lead to the weakening of many potential storms. The El Niño of 1997, the strongest on record by a factor of nearly two, wreaked havoc with the Colorado State forecast for that year and limited the number of hurricanes to three and the number of intense hurricanes to one.
"We think the speedy onset of La Niña will definitely enhance this year’s activity relative to last," Gray said. "This is not a notably strong La Niña, and we don’t think we’ll get the full effect of its cold water until late fall or winter, after the peak hurricane season (August and September) has passed.
"However, its forceful beginning leads us to believe it will encourage the formation of stronger storms in the Atlantic Basin for the rest of the season."
The team believes the cold surface waters in the east equatorial Pacific Ocean later this year will make 1999 a more active hurricane season than this year.
Another key climate signal affecting hurricane formation is rainfall in the Western Sahel region of Africa (including all or portions of Benin, Burkina Faso, Cameroon, Chad, Central African Republic, Ghana, Guinea, Guinea-Bissau, Ivory Coast, Liberia, Mali, Mauritania, Niger, Nigeria, Senegal, The Gambia, Sierra Leone and Togo.)
Wetter conditions in this region tend to promote intense hurricane formation, Gray said. Very dry conditions occurred during June and July this year, one of the factors that Gray said led the team to hold their prediction of the number of intense hurricanes to two.
Other climatic factors favorable to formation of storms this year include above-average sea surface temperature anomalies in the tropical and north Atlantic and a ridge of relatively low barometric pressure near the Azores Islands earlier this year. This reduced high pressure causes weaker East Atlantic trade winds and is more favorable for hurricane development in the following season, according to Gray.
The other major factor present that normally inhibits hurricane formation is the Quasi-Biennial Oscillation, equatorial stratospheric winds at 68,000 to 75,000 feet currently blowing from an easterly direction. This easterly flow often shears the tops off tropical depressions that otherwise could become named storms or hurricanes. (On the other hand, when the QBO blows in a westerly direction, there is typically 50 to 75 percent more hurricane activity, Gray said.)
Other, minor factors such as sea surface temperature anomalies in the South Atlantic or air temperatures some 54,000 feet above Singapore, an indicator for Atlantic Basin hurricane activity in the coming six to 12 months, are negative or neutral.
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, assisted by staff and graduate students, take into account these and other factors.
Gray’s hurricane forecasts – issued annually in early December and updated in April, June and August – apply only to the Atlantic Basin, the area encompassing the Atlantic Ocean, Caribbean Sea and Gulf of Mexico. Beginning with a separate report slated for release August 7 on the team’s Web page (see accompanying press releases), Gray will issue landfall probability reports for 1998 based on a statistical model developed recently. This information will be incorporated into future forecasts and updates.
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, with 39 named storms, 23 hurricanes (12 of which were intense) and 107 hurricane days.
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. His landfall probability model takes into account the "Atlantic conveyor belt" that circulates water from the tropics to the northeast Atlantic and back south along the North American continent and a statistical average of six measures of hurricane activity for a given year called "net tropical cyclone" activity.
Together, these indicators suggest more active seasons, such as those that occurred from the late 1920s through the late 1960s, producing more intense hurricane activity. Gray believes that since 1994, the Atlantic conveyor belt has been strengthening, making conditions more favorable to hurricane formation.
"I think because of the measurements of North Atlantic temperatures, we’ll see a stronger conveyor belt in the coming year," Gray said. "Those temperatures are higher – above average – and they fit the conditions of previous periods when the conveyor belt was strong."
"Other factors that help strengthen the conveyor effect, such as the salinity content of the North Atlantic, are being reported as higher than average, and other, lesser signals are going that way."
"Once these cumulative climate signals become really favorable, the number of landfalls goes up disproportionately," Gray said.
GRAY RESEARCH TEAM
HURRICANE FORECAST FOR 1998 SEASON
|Named Storms (9.3)*||10||10||10||9|
|Named Storm Days (46.9)||50||50||50||40|
|Hurricane Days (23.7)||25||25||20||20|
|Intense Hurricanes (2.2)||2||2||2||2|
|Intense Hurricane Days (4.7)||5||4||4||3|
|Hurricane Destruction Potential (70.6)**||75||70||65||50|
|Net Tropical Cyclone Activity (100%)||110%||100%||95%||90%|
|Named Storms (9.3)||7||11||11||11||11|
|Named Storm Days (46.9)||28||45||55||55||55|
|Hurricane Days (23.7)||10||20||25||25||25|
|Intense Hurricanes (2.2)||1||2||3||3||3|
|Intense Hurricane Days (4.7)||2.2||3||5||5||5|
|Hurricane Destruction Potential (70.6)||26||60||75||75||75|
|Net Tropical Cyclone Activity (100%)||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.