Note to Editors: Forecast totals are in the attached chart. The complete hurricane forecast and related research and press releases are available on the Web at typhoon.atmos.colostate.edu. An initial forecast for the 2002 hurricane season is scheduled for posting on the Web on Dec. 7, 2001.
For the third consecutive year, Colorado State University hurricane forecaster William Gray and his colleagues were on target with their predictions for the hurricane season.
This was an unusual year in that most of the activity occurred September through November. The first hurricane did not form until Sept. 8 – the latest-forming first hurricane since 1984. Gray attributes the late start to high August Atlantic basin surface pressure and stronger than usual west Atlantic subsidence and dryness.
Other unusual occurrences included one of the most active periods during the last 50 years during October and November, with four hurricanes and two major hurricanes. In addition, two hurricanes happened simultaneously in November, the first time that has happened since 1932.
In early June and early August 2001, Gray and his colleagues predicted this season would have 12 named storms, seven hurricanes and three intense hurricanes for the season. As of today, this year’s hurricane season (which officially ends Nov. 30) has seen 14 named storms, eight hurricanes and four intense hurricanes. In an average year, there are 9.3 named storms, 5.8 hurricanes and 2.2 intense hurricanes.
Gray and his team were close to the mark in predicting other season activity indicators. For example, the team predicted correctly that this season would have five intense hurricane days. An intense hurricane day consists of 4 six-hour periods in which winds exceed 110 miles per hour. The team called for 60 named storm days; this season saw 59 named storm days with sustained winds greater than 39 miles per hour. The team predicted there would be 30 hurricane days, but there were only 24 such days where winds exceed 73 miles per hour. Net Tropical Cyclone Activity (NTC), a measure of the season’s overall activity, was predicted at 120 and measured at 132.
The appearance of four Atlantic basin major hurricanes this year (Erin, Felix, Iris and Michelle) supports Gray and his colleagues’ contention that there has been a strong, multi-decade upswing of major hurricane activity that began in 1995. This upswing will likely continue for two or three more decades, and due to the large increase in coastal population in the U.S. and Caribbean, the increased activity will bring unprecedented hurricane-spawned destruction.
While some may attribute the increased Atlantic basin activity to global warming, Gray points out that there actually have been fewer hurricanes in the Pacific and Indian Oceans during the past seven years. The net global tropical cyclone activity has actually experienced a small reduction during the last seven years. Gray and his team believe that there is no objective way to attribute the sharp upturn of Atlantic basin hurricane activity since 1995 to human induced global warming.
Gray and his colleagues instead believe that long-term (25 to 50 year) shifts in the Atlantic Ocean thermohaline circulation pattern is the primary cause for the increase in Atlantic hurricane activity since 1995. After a quarter-century quiet period between 1970-94 in which the Atlantic Ocean thermohaline circulation was weak, measurements such as sea surface temperature and salinity in the North Atlantic indicate that this circulation system rapidly picked up strength in mid-1995 and has been running strong since that time.
"The upturn we’ve seen in Atlantic hurricanes and activity the last seven years has been remarkable," said Gray.
As a result, 1995-2001 are the seven most active consecutive years on record, with 93 named storms, 57 hurricanes and 27 major hurricanes. Gray points out, however, that of the 27 Atlantic basin major hurricanes (Saffir/Simpson Category 3-4-5 hurricanes) during this seven year period, only three (Opal, Bret and Fran) have crossed the U.S. coastline. During the last century, 73 of 218 major hurricanes (one in three) made landfall. By the usual landfall to Atlantic basin numbers, the U.S. should have experienced about nine major hurricane landfall events in the past seven years. "We’ve been fortunate that an upper-air trough has been located along the U.S. East Coast during a substantial part of the last seven seasons," Gray said. "The fortuitous frequent location of the upper-level East Coast trough has caused a large number of the northwest moving major hurricanes to be recurved to the north before they reach the U.S. coastline. But climatology will eventually right itself and we must expect a great increase in landfalling major hurricanes in the coming decades."
For a second consecutive year, there was not a U.S. hurricane landfall event. This has not occurred since the consecutive seasons of 1981-82. However, three Gulf of Mexico tropical storms (Allison, Barry and Gabrielle) made landfall on the U.S. coast. Barry and Gabrielle were just 6 mph short of being classified as hurricanes and caused significant coastal destruction.
"Our forecast of an above average probability of U.S. hurricane landfall did not materialize this year," Gray said. "Landfall probability is a different type of forecast, which requires a four to five year period or active vs. inactive years to judge skill. For the second year in a row, a separate August forecast was made. We anticipated below-average August activity but above-average September and October activity. But we could not foresee that the early (June-August) vs. the late season activity differences would be as large as they were."
Now in his 18th year of forecasting Atlantic Basin storms, Gray and his colleagues, Chris Landsea, Eric Blake, John Sheaffer and Philip Klotzbach, have shown that recent ongoing research indicates that there are indeed meaningful multi-month precursor signals for the prediction of Atlantic basin hurricane activity and U.S. landfall probability. They believe they are continuing to develop a better understanding of our country’s hurricane problem through the insights derived from making these forecasts.
"We feel our ongoing forecast research will allow us to continue to improve our predictive skill," Gray said. Gray and his team will be issuing their first prediction for the 2002 season on Dec. 7 with planned updates in early April, early June and early August 2002.
Last year, the team predicted 11 named storms, seven hurricanes and three intense hurricanes. That 2000 season concluded with 14 named storms, eight hurricanes and three intense hurricanes.
VERIFICATION OF THE GRAY RESEARCH TEAM HURRICANE FORECAST FOR 2001 SEASON
|Dec. 2000||April 7||June 7||August 7||Actual 2001|
|Named Storms (9.3)*||9||10||12||12||14|
|Named Storm Days (46.9)||45||50||60||60||59|
|Hurricane Days (23.7)||20||25||30||30||24|
|Intense Hurricanes (2.2)||2||2||3||3||4|
|Intense Hurricane Days (4.7)||4||4||5||5||5|
|Hurricane Destruction Potential (70.6)||65||65||75||75||73|
|Maximum Potential Destruction (61.7)||60||60||70||70||83|
|Net Tropical Cyclone Activity (100%)||90||100||120||120||132|
* 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.