Colorado State's Last Seasonal Hurricane Forecast Update For 1998 Calls For Near-Normal Number Of Named Storms, Hurricanes
Friday, August 7, 1998
Note to editors: Forecast totals are in the attached chart.
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
FORT COLLINS--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.
HURRICANE FORECASTS--1998 SEASON
9/98 6/98 4/98 12/97
1. Named Storms (9.3)* 10 10 10 9
2. Named Storm Days (46.9) 50 50 50 40
3. Hurricanes (5.8) 6 6 6 5
4. Hurricane Days (23.7) 25 25 20 20
5. Intense Hurricanes (2.2) 2 2 2 2
6. Intense Hurricane Days (4.7) 5 4 4 3
7. Hurricane Destruction Potential (70.6)** 75 70 65 50
8. Net Tropical Cyclone Activity (100%) 110% 100% 95% 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 (46.9) 28 45 55 55 55
3. Hurricanes (5.8) 3 6 7 7 7
4. Hurricane Days (23.7) 10 20 25 25 25
5. Intense Hurricanes (2.2) 1 2 3 3 3
6. Intense Hurricane Days (4.7) 2.2 3 5 5 5
7. Hurricane Destruction Potential (70.6) 26 60 75 75 75
8. 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.
This page © 1997-1998 World Wide Express, Inc. All Rights Reserved. Many news stories on RamLine.com come from the Colorado State University Public Relations Office. You can get copies of the news releases directly by filling out this form.