Colorado State hurricane forecast team calls for 'average' 2000 season

December 09, 1999

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FORT COLLINS--The United States and countries in and bordering the Caribbean Basin should experience fewer storms and hurricanes in 2000 than in recent years, according to the Colorado State University hurricane forecast team's initial prediction.

William Gray, a nationally recognized hurricane expert, and his colleagues are calling for a "moderate" season in 2000 with 11 named storms, seven hurricanes and three intense hurricanes predicted.

While above the long-term average, the 2000 forecast calls for fewer storms than occurred in the years 1995, 1996, 1998 and 1999. The team is calling for an above-average probability of U.S. major hurricane landfall.

Their record for the 1999 season was one of the best in the 16 years the seasonal forecasts have been issued. In 1999, the team retained its original forecast throughout the season, predicting 14 named storms, nine hurricanes and four major, or intense, hurricanes. At the end of the June 1-Nov. 30 season, observed totals were 12 named storms, eight hurricanes and five major hurricanes.

Long-term averages based on the period 1950-1990 indicate 9.3 named storms, 5.8 hurricanes and 2.2 intense hurricanes per year. The 2000 season forecast, while closer to average than others of recent years, is still expected to exceed the average season during the relatively quiet period between 1970-94.

A principal reason for the moderate storm numbers is the team's belief that the La Ni a phenomenon, present for the past two years, will reduce in strength by next season. This should weaken its enhancing effect on hurricane formation in the Atlantic Basin (comprising the North Atlantic Ocean, Caribbean Sea and Gulf of Mexico). Gray warned, however, that indications continue to show that intense storms (with wind speeds above 110 mph) will occur.

In terms of the probability of landfall, this year's forecast calls for the U.S. Atlantic Coast, including peninsular Florida, to have a 45 percent chance of being hit by one or more major storms with winds above 110 mph. The long-term (100-year) mean is 31 percent. The Gulf Coast faces a 37 percent probability of one or more landfalling major hurricanes, while the long-term Gulf Coast average is 30 percent. The chances of one or more intense storms coming ashore somewhere along the entire U.S. coast, from Brownsville to the Canadian border, is 66 percent (the long-term average is 52 percent).

"We do not foresee the number of intense storms in Saffir-Simpson categories 3-5 that we saw in the Atlantic Basin in 1995, 1996 and last year," Gray said. "Of the five major hurricanes in 1999, only one--Bret--struck the U.S. coast, in a nearly uninhabited area of south Texas.

"Floyd, another category 4 storm, lashed the East Coast from Florida north before coming ashore as a strong category 2 hurricane in North Carolina, bringing unprecedented amounts of rainfall."

Gray, professor of atmospheric sciences at Colorado State, and his colleagues believe the 2000 season will start earlier than the seasons of 1998 and 1999. Until four hurricanes formed during a brief period in the last third of August of this year, only one tropical storm (Arlene) had occurred in the first two-plus months of the 1999 season.

"Relatively moderate seasons frequently start earlier, which is why we think we'll see more storms earlier in the summer of 2000," Gray said. "In addition, 2000 will be one of these seasons in which equatorial stratospheric winds known as the Quasi-Biennial Oscillation are easterly, which tends to inhibit storm formation at lower latitudes."

Contributing to the anticipated moderate season this year are a number of atmospheric and oceanic phenomena that Gray calls "climate signals." Those include:

*La Ni a, a mass of cold water in the eastern equatorial Pacific. La Ni a was powerful this year and produced anomalous upper tropospheric easterly winds over the Caribbean and tropical Atlantic, a condition that acted to enhance hurricane formation.

Next year, while the mass of water will likely remain cold, Gray and his colleagues expect the temperature to moderate. They do not anticipate an El Ni o condition developing.

*A reversal of the Quasi-Biennial Oscillation, which consists of stratospheric, equatorial east-west winds blowing 16 to 35 kilometers above the earth's surface. These winds typically blow for 12-16 months from the east and then reverse and blow the same length of time from the west with a total period of about 26 to 30 months, or roughly two years. The shift due next season will produce easterly winds, which tend to interfere with the development of easterly (barometric) waves coming off the coast of Africa.

*Continued warm North Atlantic sea surface temperatures. While these temperatures have moderated slightly during the last year, they indicate that the Atlantic Ocean thermohaline circulation system, or Atlantic conveyor belt, continues strong.

A strong Atlantic conveyor belt, Gray and others believe, contributes to the formation of greater numbers of major or intense (Saffir-Simpson category 3-5) storms. This increases the probability of major hurricane landfall on the U.S. East Coast and in the Caribbean.

"I think 2000 will continue the new era of increased Atlantic conveyor belt strength and hence of the probability of more intense hurricanes," Gray said. "Following a period of suppressed major hurricane activity from 1970 to 1994, it is expected that 2000 will continue the upward trend established in 1995."

The years 1995-99 were the most-active five consecutive years of hurricane activity on record in the Atlantic Basin, yielding 65 named storms, 41 hurricanes and 20 major hurricanes. By contrast, the numbers of hurricanes and typhoons in the Pacific Ocean are lower, which Gray takes as an indication that the upward trend in Atlantic storms should not be interpreted as being the result of human-induced global warming.

Gray and co-authors Chris Landsea, Paul Mielke, Kenneth Berry, with the assistance of Todd Kimberlain, Eric Blake and Bill Thorson, use a variety of climatic factors in their forecasts. In addition to stratospheric winds, continuing warm North Atlantic sea surface temperatures and continuing La Niña conditions, they look at a ridge of barometric high pressure called the Azores High, warmer sea-surface temperatures throughout the Atlantic, two measures of west African rainfall and mid-latitude oceanic wind patterns in the Atlantic and Pacific oceans.

Colorado State University

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