Colorado State University Hurricane Forecast Team Issues Accurate Predictions for Second Consecutive Year

Note to Editors: Forecast totals are in the attached chart. The complete hurricane forecast and related research and press releases are available at http://tropical.atmos.colostate.edu/forecasts/index.html. An initial forecast for the 2001 hurricane season is scheduled for posting on the Web on Dec. 7, 2000.

For the second consecutive year, Colorado State University hurricane forecaster William Gray and his colleagues have come very close to the mark with their forecast for the 2000 hurricane season.

In December 1999, Gray and his colleagues predicted 11 named storms, seven hurricanes and three intense hurricanes for the season. As of today, observed totals are 14 named storms, eight hurricanes and three intense hurricanes. Long-term averages are 9.3 named storms, 5.8 hurricanes and 2.2 intense hurricanes annually.

Other indications of the season’s activity such as named storm days, a measure of duration, also came close to what actually took place. The Colorado State team predicted 55 named storm days while 66 were counted. Thirty hurricane days were projected and 32 counted, while six intense hurricane days were predicted and five and a quarter actually occurred.

Hurricane Destruction Potential was predicted to be 90 in the August forecast update and came out at 85. Maximum Potential Destruction, estimated at 70, was observed at 78. Net Tropical Cyclone Activity, a measure of the season’s overall activity, was predicted at 130 and measured at 134.

"This was an active year, in keeping with our predictions of a new era of increased hurricane activity," Gray said. "We think the forecast was a good one and, following on last year’s accurate forecast that incorporated some new techniques, we believe that our predictive abilities are continuing to improve.

"Our estimate of U.S. landfall by an intense hurricane did not prove correct, but landfall predictions don’t work well in any individual year."

Gray’s team had initially called for a 72 percent chance of landfall along the entire U.S. coastline and 54 percent for the East Coast and Florida Peninsula. Only two named storms hit the mainland, both in north Florida. Hurricane Gordon formed on Sept. 14 but weakened to a tropical storm by the time it made landfall Sept. 17. Tropical Storm Helene reached that status from a tropical depression on Sept. 22 and landed near Fort Walton Beach as a very weak tropical storm the next day. This was the first year since 1994 that there has not been a hurricane making landfall in the United States.

Last year, the team predicted 14 named storms, nine hurricanes and four intense hurricanes. That 1999 season concluded with 13 named storms, eight hurricanes and five intense hurricanes.

Unusual behavior this season included the formation of six named storms within a three-week period in September and no named storms occurring in June, July or November.

The appearance of three major hurricanes (Alberto, Isaac and Keith) supports Gray and his colleagues’ contention that a period of more intense storm formation has been underway since 1995.

While some attribute the increased activity to global warming, Gray points out that there actually have been fewer hurricanes in the North Pacific Ocean during the past six years. The net storm formation for the Pacific and Atlantic basins combined (the Atlantic Basin includes the North Atlantic, Gulf of Mexico and Caribbean Sea) has actually been reduced. He argues that global warming could hardly cause increased storms in one ocean and decreased numbers of storms in another.

Gray and his colleagues instead believe that long-term (25 to 50 year) shifts in the Atlantic Ocean thermohaline circulation system are responsible for the increase in intense storms. After a quiet period between 1970-94 in which the Atlantic thermohaline system ran weakly, measurements such as sea surface temperature and salinity in the North Atlantic indicated the circulation system suddenly picked up strength in 1995.

As a result, the years 1995-2000 are the six most active consecutive years on record, with 79 named storms, 49 hurricanes and 23 major hurricanes. Gray points out, however, that of the 23 Atlantic Basin major hurricanes during this six year period, only three came ashore in the United States. During the last century, 73 of 218 major hurricanes (one in three) made landfall.

"By the standards of the last century, we should have expected seven or eight major U.S. landfall events since 1995," Gray said. "We’ve been lucky, but that luck will not hold.

"The long-term climatology is eventually going to manifest itself, and in the coming decades we will likely see hurricane damage much greater than ever before experienced."

Now in his 17th year of forecasting Atlantic Basin storms, Gray and his colleagues have begun a shift from the use of a statistical regression model to one he calls an "analog" model. He believes that when precursor climate signals from previous years closely resemble those for the forthcoming season, the latter will resemble those earlier, analog years in terms of hurricane activity. The analog years for the 2000 season were 1949, 1956, 1981, 1989 and 1996. The hurricane activity in these years closely resembled what occurred in 2000.

"Since 1995, our statistical regression prediction schemes have shown less accuracy than our actual forecasts," Gray said. "This took place at a time when we entered a new, multi-decadal era of altered atmospheric and oceanic circulation features and significantly more hurricane activity in the Atlantic Basin.

"Recognizing this fall in reliability, we chose to base the 2000 forecast, as we did the 1998 and 1999 forecasts, on our new analog prediction methodology, and we’ve been rewarded with improved accuracy."

GRAY RESEARCH TEAM
HURRICANE FORECAST FOR 2000 SEASON

	December 1999	June 7	August 4	April 7	Actual 2000
Named Storms (9.3)*	11	11	12	11	14
Named Storm Days (46.9)	55	55	65	55	66
Hurricanes (5.8)	7	7	8	7	8
Hurricane Days (23.7)	25	25	35	30	32
Intense Hurricanes (2.2)	3	3	4	3	3
Intense Hurricane Days (4.7)	6	6	8	6	5
Hurricane Destruction Potential (70.6)**	85	85	100	90	85
Maximum Potential Destruction (61.7)	70	70	75	70	78
Net Tropical Cyclone Activity (100%)	125	125	160	130	134

* Number in ( ) represents average year totals based on 1950-1990 data.

** 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.