Colorado State Hurricane Forecast Team Maintains Prediction for Active 1999 Season; Landfall Probabilities Above Average

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Colorado State University’s hurricane forecaster William Gray is maintaining his prediction for a 1999 hurricane season similar to last year’s in the Atlantic Basin.

Today’s update calls for 14 tropical storms, nine hurricanes and four intense hurricanes, the same figures issued in Gray’s initial 1999 forecast released last December.

Meanwhile, he anticipates the probability of a major storm landfall at about 200 percent of the long-term average for the East Coast and Florida Peninsula, 146 percent of long-term average for the Gulf Coast and slightly less than twice the long-term average for the Caribbean.

A major storm falls into category 3-5 of the Saffir-Simpson scale and has winds of at least 111 mph. The long-term average is based on landfalls by intense hurricanes along the Gulf Coast and Eastern Seaboard from data of the last century.

(A more detailed description of these landfall probabilities for the U.S. Atlantic and Gulf coasts for 1999, based on a new model, is planned for the late spring.)

"We expect the current La Niña below-average water conditions in the eastern equatorial Pacific to continue through the 1999 hurricane season," Gray said. "This should be an enhancing influence on this season’s hurricane activity."

Gray repeated the warning issued in his prediction team’s initial report: a key factor in the forecast is the call for four major hurricanes with minimum winds of 111 mph. Historically, when such storms have made landfall, he said, they cause a vast majority of hurricane-spawned destruction as determined on a statistical basis.

The April update reiterated its prediction for more low-latitude storms, between 10 and 23 degrees north, that will affect the Caribbean. In active seasons such as the one anticipated, 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.

In addition to La Niña, other climatic factors expected to affect the June 1-Nov. 30, 1999, official hurricane season include westerly stratospheric winds, expected above-average Atlantic sea surface temperatures and anticipated below-average Caribbean Basin sea-level pressure in August and September.

"All these factors are, in addition to La Niña, expected to enhance 1999 hurricane activity," Gray said. "There are no strong suppressing influences for 1999 hurricane activities that we can see at this time."

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 pattern is reminiscent of that of the 1930s, 1940s and 1950s, he said.

The actual number of "weaker" storms – tropical 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 because of the conveyor belt phenomenon, he expects the number of intense storms to rise.

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 include:

  • Westerly stratospheric winds, called the Quasi-Biennial Oscillation, that exist high in the atmosphere over the earth’s equatorial regions and 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.
  • The 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.
  • High North Atlantic sea surface temperatures and increased salinity that suggest 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 in March was below average and an enhancing influence on hurricane activity, and an April 1 prediction of below-average Caribbean Basin sea-level pressure anomaly for August and September 1999, another enhancing seasonal hurricane factor.


Initial 12/98 Forecast April Forecast
Named Storms (9.3)* 14 14
Named Storm Days (46.9) 65 65
Hurricanes (5.8) 9 9
Hurricane Days (23.7) 40 40
Intense Hurricanes (2.2) 4 4
Intense Hurricane Days (4.7) 10 10
Hurricane Destruction Potential (70.6) 130 130
Maximum Potential Destruction (61.7) 130 130
Net Tropical Cyclone Activity (100%) 160 160

* 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 IntenseHurricane Days are four, six-hour periods where storms attain wind speeds appropriate to their category on the Saffir-Simpson scale.