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Hurricane Forecast Calls For Continuing Activity After Floyd

September 21, 1999
Colorado State University
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.

FORT COLLINS--Coastal dwellers could be in for as many storms during therest of the hurricane season as they've seen so far, if Colorado StateUniversity's hurricane forecaster William Gray's predictions for 1999 are on themark.

But there also may be some good news--Gray's predictions for this yearcalled for four major storms, and four have already occurred. The hurricaneseason officially runs from June 1 though Nov. 30, but the real heart of theseason 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 fourintense hurricanes. As of today, just past the halfway mark for the season,seven named storms, five hurricanes and four intense hurricanes have formed. Thelong-term average for a season is 9.3 tropical storms, 5.8 hurricanes and 2.2intense hurricanes each year, based on an analysis of 1950-1990 storm activity.

"Our forecast for this season is based on the future being like thepast," Gray said. "Similar atmospheric and ocean patterns as this year occurredin 1950, 1955, 1961, 1964 and 1995. All these were very active seasons. If wedon't get an active year in 1999, it means the atmosphere for some strangereason has stopped behaving as it has in the past. We don't expect that tohappen."

As part of their research, Gray and his team also forecast theprobability of hurricane landfall along the U.S. coastline. For 1999 the teamhas predicted a roughly 54 percent chance that one or more intense storms (withwind 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 thatone or more intense storms will make landfall. For the Caribbean and Bahamasland areas, the rough probability of one or more major storm landfalls is 72percent and for Mexico the probability is 28 percent, according to the landfallforecast.

To date, Hurricanes Dennis and Floyd are the only storms to makelandfall along the U.S. Coast.

Hurricanes are rated on the Saffir/Simpson intensity scale, which rangesfrom 1-5. The scale reflects a hurricane's wind- and ocean-surge intensity.Hurricanes of Category 3 or higher are considered intense storms and havemaximum sustained winds of 110 miles per hour or greater.

To issue his seasonal forecasts, Gray and his team rely on "climatesignals," or measures of the global oceanic and atmospheric circulation system.These signals have remained both consistent throughout the year and, in all butone case, are favorable for hurricane formation. Factors promoting hurricane formation include:

  • La Niña, a mass of cold water in the eastern equatorial Pacific. Graynotes 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, thebetter-known converse situation, occurs when a mass of warm water forms in thissame region.
  • Stratospheric equatorial winds, which are currently blowing from thewest. From that direction the winds tend to generate 50 to 100 percent morestorms than when the winds are easterly.
  • Warmer-than-normal sea surface temperatures in almost all of the NorthAtlantic Ocean.
  • West African rainfall, which began increasing in July and now isanticipated to be above average for this summer.
  • Equatorial winds at 40,000 feet above the earth, blowing from the eastoff the African continent. These winds, occurring between five and 20 degreesnorth latitude, combine with easterly trade winds to create less vertical windshear (less difference between wind speeds at different heights in theatmosphere) and so cause less disruption to hurricane formation.

The period from 1995-98 was the most active, four consecutive years ofhurricane activity on record, yielding 53 named storms, 33 hurricanes and 15major hurricanes. This and certain other climate signals suggest to Gray and hisassociates that a period of more major hurricane activity and more intense-stormlandfalls along the East Coast and in the Caribbean Basin is now underway.

The periods 1900-25 and 1970-94 were relatively quiescent in terms ofmajor hurricane activity, Gray said, while seasons from the early 1930s throughthe late 1960s generally were more active, with more intense storms lashing theAtlantic coast. He attributes this to a phenomenon called the Atlantic Oceanthermohaline circulation system, or Atlantic conveyor belt, which moves watersnorth 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 AtlanticOcean and beyond.

Warm water and high salinity in the conveyor belt strengthen it,producing more active hurricane seasons and more major landfalling storms alongthe eastern seaboard, Gray said.

"This ocean circulation, a northbound current that sinks and then movessouthbound, tends to go through decades-long changes," Gray said. "Ourinterpretation of climate data suggests that the Atlantic conveyor belt becamestronger between 1994 and 1995, and this has led to more major storms since thattime."

The seasonal forecast, now in its 16th year, is prepared by Gray andco-authors Chris Landsea, Paul Mielke, Kenneth Berry and other projectcolleagues.

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Colorado State University. "Hurricane Forecast Calls For Continuing Activity After Floyd." ScienceDaily. ScienceDaily, 21 September 1999. <>.
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