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FORT COLLINS--The Colorado State University hurricane forecast team led by William Gray, a nationally recognized expert on tropical storms, is calling for a 1999 season as active as the one that just ended.
During the June 1-Nov. 30, 1999, season, Gray and his colleagues are predicting that 14 named storms, nine hurricanes and four intense hurricanes will form in the Atlantic Basin.
In addition, the forecast suggests the U.S. Atlantic coast has about twice the chance of being hit by a major storm with winds of 111 mph or above compared with long-term averages, while the Gulf Coast faces a 150 percent greater probability of landfall by a major hurricane. The probability of a Caribbean Basin major storm landfall is similar to that of the U.S. East Coast.
(A more detailed description of these landfall probabilities for the U.S. Atlantic and Gulf coasts for 1999 will be listed on the Web at the site given above on Dec. 10.)
"We expect a season nearly as active as this year's (1998)," Gray said. "Of particular importance is our prediction that four intense hurricanes will form. Major hurricanes, on a statistical basis, cause the majority of hurricane-spawned destruction.
"We feel the 1999 season will be comparable to the one just past and not too much weaker than the 1995 and 1996 seasons, both of which were very busy," Gray said. "Climatic evidence strongly suggests that we are embarking on a new era of enhanced major hurricane activity."
The 1999 season also should produce more low-latitude storms, between 10 and 23 degrees north, that will affect the Caribbean. In active seasons such as the one that Gray's team is projecting, there is a higher occurrence of low-latitude major hurricanes.
The predicted 14 tropical storms, nine hurricanes and four intense hurricanes compare to 14, 10 and three that occurred in 1998. Long-term statistical averages yield 9.3 tropical storms, 5.8 hurricanes and 2.2 intense hurricanes annually.
The four-year period from 1995-98 was the most active four consecutive years of hurricane activity on record, Gray said, yielding 53 named storms, 33 hurricanes and 15 major hurricanes. Gray and his associates believe this signals the beginning of a "new era" of more major hurricane activity and more intense-storm landfalls along the East Coast and in the Caribbean Basin.
The periods 1900-25 and 1970-94 were relatively quiescent in terms of major hurricane activity, Gray said, while seasons from the early 1930s through the late 1960s generally were more active and more intense storms lashed the Atlantic coast. He attributes this to a phenomenon called the Atlantic Ocean thermohaline circulation system, or Atlantic conveyor belt, which moves waters northward from the vicinity of the Caribbean to an area east of Greenland. There, the current sinks to deep levels, moves southward and flows into the South Atlantic Ocean and beyond.
Warm water and high salinity in the conveyor belt strengthen it, producing more active hurricane seasons and more major landfalling storms (Saffir-Simpson category 3-5 storms with winds of 111 mph or above) along the eastern seaboard, Gray said.
"This ocean circulation, a northbound current that sinks and then moves southbound, tends to go through multi-decadal changes," Gray said. "We believe the Atlantic conveyor belt became stronger between 1994 and 1995, as we interpret the data.
"This in turn has led to a number of Atlantic Ocean and global oceanic circulation changes that have brought on more major storms during the past four years. The pattern is very similar to what was occurring in the 1930s, 1940s and 1950s."
Gray pointed out that while the predicted 1999 season would be nearly twice as active as the long-term average in terms of major hurricanes, projected intense storm activity is even higher than this when compared with the relatively quiet quarter-century beginning in 1970.
The actual number of "weaker" storms - named storms and those in Saffir-Simpson categories 1 and 2, with winds up to 110 mph - aren't likely to increase dramatically, Gray said, but the number of intense storms is expected to rise because a stronger Atlantic conveyor belt is now in place.
"Our concern is that, because of the population buildup along the U.S. East Coast, property damage could be severe, since Saffir-Simpson category 3, 4 and 5 storms comprise only about one-quarter of all landfalling named storms but cause more than 80 percent of all property damage," Gray said. "People should take precautions, buy hurricane insurance and, of course, listen to emergency management instructions. This country's warning system is good enough to prevent loss of life if residents of affected areas take those warnings seriously and follow instructions."
Gray and co-authors Chris Landsea, Paul Mielke, Kenneth Berry and other project colleagues use a variety of climatic factors in their forecasts. Those positive indications dominating the 1999 season appear to be stratospheric winds, continuing warm North Atlantic sea surface temperatures and persistent La Niña conditions.
* The Quasi-Biennial Oscillation consists of winds high in the atmosphere over the earth's equatorial regions that reverse themselves approximately every two years. When these stratospheric winds blow from the west, as they are projected to do for 1999, an enhancing effect on hurricane activity, especially major hurricanes, occurs.
* Conditions are favorable for a continuation of cold-water conditions known as La Niña in the western equatorial Pacific, a situation likely to promote an active hurricane season in 1999.
* The strongest pattern in three decades of relatively high North Atlantic sea surface temperatures and increased salinity suggests that the changes observed beginning in 1995 signal a continuance of a strong Atlantic conveyor belt circulation, bringing with it the chance for more intense hurricanes along the Atlantic coast. There is a chance this enhanced period could continue for two or more decades, Gray believes.
Other factors and their expected effects are a ridge of barometric high pressure called the Azores High, which now is neutral; warmer Atlantic sea-surface temperatures that will enhance 1999 hurricane activity; two measures of west African rainfall which are neutral; and mid-latitude oceanic wind patterns, which combine to suggest an enhanced effect on Atlantic hurricanes in the coming year.
Named Storms (9.3)* 14
Named Storm Days (46.9) 65
Hurricanes (5.8) 9
Hurricane Days (23.7) 40
Intense Hurricanes (2.2) 4
Intense Hurricane Days (4.7) 10
Hurricane Destruction Potential (70.6) 130
Maximum Potential Destruction (61.7) 130
Net Tropical Cyclone Activity (100%) 160
* 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.