Sep. 21, 1999 FORT COLLINS--Coastal dwellers could be in for as many storms during the rest of the hurricane season as they've seen so far, if Colorado State University's hurricane forecaster William Gray's predictions for 1999 are on the mark.
But there also may be some good news--Gray's predictions for this year called for four major storms, and four have already occurred. The hurricane season officially runs from June 1 though Nov. 30, but the real heart of the season is from mid-August through October.
Gray, who has been issuing hurricane forecasts for more than a decade, predicted an active year in 1999, with 14 named storms, nine hurricanes and four intense hurricanes. As of today, just past the halfway mark for the season, seven named storms, five hurricanes and four intense hurricanes have formed. The long-term average for a season is 9.3 tropical storms, 5.8 hurricanes and 2.2 intense hurricanes each year, based on an analysis of 1950-1990 storm activity.
"Our forecast for this season is based on the future being like the past," Gray said. "Similar atmospheric and ocean patterns as this year occurred in 1950, 1955, 1961, 1964 and 1995. All these were very active seasons. If we don't get an active year in 1999, it means the atmosphere for some strange reason has stopped behaving as it has in the past. We don't expect that to happen."
As part of their research, Gray and his team also forecast the probability of hurricane landfall along the U.S. coastline. For 1999 the team has predicted a roughly 54 percent chance that one or more intense storms (with wind speeds of 110 mph or above) will make landfall along the U.S. East Coast, including Florida. The Gulf Coast has an approximately 40 percent chance that one or more intense storms will make landfall. For the Caribbean and Bahamas land areas, the rough probability of one or more major storm landfalls is 72 percent and for Mexico the probability is 28 percent, according to the landfall forecast.
To date, Hurricanes Dennis and Floyd are the only storms to make landfall along the U.S. Coast.
Hurricanes are rated on the Saffir/Simpson intensity scale, which ranges from 1-5. The scale reflects a hurricane's wind- and ocean-surge intensity. Hurricanes of Category 3 or higher are considered intense storms and have maximum sustained winds of 110 miles per hour or greater.
To issue his seasonal forecasts, Gray and his team rely on "climate signals," or measures of the global oceanic and atmospheric circulation system. These signals have remained both consistent throughout the year and, in all but one case, are favorable for hurricane formation. Factors promoting hurricane formation include:
- La Niña, a mass of cold water in the eastern equatorial Pacific. Gray notes that while La Niña is an important indicator that more storms will form, it is far from the only one the team considers in its calculations. El Niño, the better-known converse situation, occurs when a mass of warm water forms in this same region.
- Stratospheric equatorial winds, which are currently blowing from the west. From that direction the winds tend to generate 50 to 100 percent more storms than when the winds are easterly.
- Warmer-than-normal sea surface temperatures in almost all of the North Atlantic Ocean.
- West African rainfall, which began increasing in July and now is anticipated to be above average for this summer.
- Equatorial winds at 40,000 feet above the earth, blowing from the east off the African continent. These winds, occurring between five and 20 degrees north latitude, combine with easterly trade winds to create less vertical wind shear (less difference between wind speeds at different heights in the atmosphere) and so cause less disruption to hurricane formation.
The period from 1995-98 was the most active, four consecutive years of hurricane activity on record, yielding 53 named storms, 33 hurricanes and 15 major hurricanes. This and certain other climate signals suggest to Gray and his associates that a period of more major hurricane activity and more intense-storm landfalls along the East Coast and in the Caribbean Basin is now underway.
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, with more intense storms lashing the Atlantic coast. He attributes this to a phenomenon called the Atlantic Ocean thermohaline circulation system, or Atlantic conveyor belt, which moves waters north from the vicinity of the Caribbean to an area east of Greenland. There, the current sinks to deep levels, moves south 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 along the eastern seaboard, Gray said.
"This ocean circulation, a northbound current that sinks and then moves southbound, tends to go through decades-long changes," Gray said. "Our interpretation of climate data suggests that the Atlantic conveyor belt became stronger between 1994 and 1995, and this has led to more major storms since that time."
The seasonal forecast, now in its 16th year, is prepared by Gray and co-authors Chris Landsea, Paul Mielke, Kenneth Berry and other project colleagues.
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