On-The-Mark Prediction Caps 1999 Season for Colorado State University Hurricane Forecast Team; Five-Year Active Trend Bears Out Theory

Note to Editors: Forecast totals are in the chart at the end. The complete hurricane forecast and related research and press releases are available on the World Wide Web at: http://tropical.atmos.colostate.edu/forecasts/index.html OR http://www.colostate.edu. An initial forecast for the 2000 season is scheduled for posting on the Web site Dec. 8, 1999.

Noted Colorado State University hurricane forecaster William Gray and his colleagues are showing their climatological savvy and skill in the wake of a very good 1999 seasonal forecast.

In December 1998, Gray’s team predicted 14 named storms, nine hurricanes and four intense hurricanes for the 1999 season. As of today, the total stands at 12 named storms, eight hurricanes and five intense hurricanes. The team did not change their numbers through updates in April, June and August.

"This is one of our best forecasts," Gray said. "While we’ve done as well or a bit better on exact numbers of storms in the past, especially 1985, 1986, 1987, 1991 and 1992, our above-average numbers for related parameters–for example, those describing hurricane duration and intensity–worked out very well."

In recent years, the team has included such measures of duration as "named storm days" (the number of six-hour periods in which tropical storms are active) and "hurricane days," which similarly measure hurricane duration. This year’s prediction was 75 named storm days and 40 hurricane days. The actual totals were 77 named storm days and 43 hurricane days. In addition, a measure of damage possible from wind and storm surge termed "hurricane destruction potential" was predicted at 130. The observed hurricane destruction potential was 145.

While the forecast underestimated by one the number of intense hurricanes (those with wind speeds above 110 mph), this year’s five intense hurricanes bear out a theory advanced by Gray and colleagues a number of years ago.

"We believe we are entering a new era of more intense hurricane activity along the U.S. East Coast, the Florida Peninsula and the Caribbean," he said. "In the five-year period from 1990-1994, there were only five such hurricanes in the Atlantic Basin (the North Atlantic Ocean, Caribbean Ocean and Gulf of Mexico). In the five years since, starting with 1995, there have been 20 storms rated Saffir-Simpson Category 3, 4 or 5–a fourfold increase.

"Meanwhile, during the period 1970-1994 when hurricane activity overall was down and we weren’t seeing many of these monster landfalling storms, a lot of construction took place along the southeast U.S. coastline. If this new period of increased landfalling storms is now with us, it could pose serious threats to safety and to property for the country."

The years 1995-99 were, in the Atlantic, the most-active five consecutive years of hurricane activity on record, yielding 65 named storms, 41 hurricanes and 20 major hurricanes. By contrast, hurricane and typhoon activity in the Pacific Ocean has lowered. This upward trend in the Atlantic should not be interpreted as a result of human-induced global warming, Gray said. According to Gray, the Atlantic Ocean thermohaline circulation system, or Atlantic conveyor belt, has strengthened since 1995. This manifests itself in increasing North Atlantic sea surface temperatures and salinity changes that have been observed since that time. This mechanism doesn’t produce many more named storms overall but is responsible for many more intense hurricanes and greatly increased probability of intense hurricane landfall along the Atlantic coast and in the Caribbean. This period could continue for two or more decades, Gray believes.

The Colorado State team called for an active season, similar to last year’s, because oceanic and atmospheric phenomena that Gray calls "climate signals" were in all cases favorable for hurricane formation. It was not surprising that this season was so active, Gray said.

"If the atmosphere behaved this year as it has in the past, then a very active hurricane season was to be expected," he said. "Prior years with similar precursor signals to this year were 1950, 1955, 1961, 1964 and 1995, all very active years."

"Some people think that La Niña (a mass of cold water in the eastern equatorial Pacific) is the only reason for this year’s very active season," Gray said. "Other climate factors also were involved. These include North Atlantic sea surface temperatures, ideally favorable stratospheric winds and favorable Atlantic sea surface pressure and middle level moisture conditions.

"These climate signals, taken together and in tandem with other factors, gave us the season we just experienced."

In issuing his early August update, Gray said the occurrence of only one tropical storm to that point did not indicate a mild season was in store. Bret, a Category 4 storm, was the strongest to strike south Texas since 1970, although it caused relatively little damage. While Bret was the only intense storm that struck the U.S. coast, Floyd, coming ashore in North Carolina as a strong Category 2 storm, caused severe flooding problems along the eastern United States.

Both Floyd and Lenny affected the Caribbean. Lenny, a Category 4 storm, is the most intense late-season storm to have occurred in the Atlantic Basin, Gray said, and no other hurricane or tropical storm is known to have had a longer easterly track through the Caribbean. Trade winds usually keep Caribbean hurricanes moving to the west, he said.

Now in his 16th year of predicting Atlantic Basin hurricanes, Gray said the team was convinced its predictive methods are more accurate than the long term average, but future improvements are possible.

"We see our predictions being adjusted to reflect what we call previous analog years," Gray said.

"We believe the climate has extended range predictive signals with regard to Atlantic seasonal hurricane activity," he said. "If La Nina, easterly stratospheric winds, warm sea-surface temperatures and other climate signals are similar to those of past active hurricane seasons, then it was to be expected that the current year would be active.

"For example, 1995 was an analog year for 1999, and by recognizing that pattern we were able to make a successful forecast."


December 1998 April 7 June 4 August 6 Actual
Named Storms (9.3)* 14 14 14 14 12
Named Storm Days (46.9) 65 65 75 75 77
Hurricanes (5.8) 9 9 9 9 8
Hurricane Days (23.7) 40 40 40 40 43
Intense Hurricanes (2.2) 4 4 4 4 5
Intense Hurricane Days (4.7) 10 10 10 10 15
Hurricane Destruction Potential (70.6) 130 130 130 130 145
Maximum Potential Destruction (61.7) 130 130 130 130 114
Net Tropical Cyclone Activity (100%) 160 160 160 160 193

* 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.