Colorado State hurricane forecasters continue to call for active season; Caribbean, Mexican landfall probabilities predicted

June 08, 1999

Note to editors: The complete hurricane forecast and related research and press releases are available on the World Wide Web at:
Taped comments from Gray are available at 970-491-1525.

A news release dated May 26, 1999, incorrectly listed the number of 1999 hurricanes as 10. Prof. Gray has and continues to forecast nine hurricanes this season.

FORT COLLINS--Colorado State hurricane forecaster William Gray and his team are standing by earlier predictions of an active hurricane season similar to last year's, with 14 tropical storms, nine hurricanes and four intense hurricanes expected in 1999.

For the first time, Gray and his associates have issued probability predictions of hurricanes occurring in the vicinity or making landfall in the Caribbean Basin-Bahamas or on the east coast of Mexico.

"We expect this year to be an active season, comparable to the ones in 1996 and 1998, but less busy than the extreme season of 1995," Gray said.

Last year's season saw 14 tropical storms, 10 hurricanes and three intense hurricanes. By comparison, a 1950-1990 baseline has indicated an average of 9.3 tropical storms, 5.8 hurricanes and 2.2 intense hurricanes annually.

The team believes there is a 50 to 75 percent higher probability that a storm will come ashore in the Caribbean or along Mexico's east coast this year compared with the average per-year probability for the past century.

Meanwhile, there is an approximately 72 percent chance that one or more major hurricanes (Saffir-Simpson 3-5) will strike the U.S. coastline between Brownsville, Texas, and the Canadian border, or 44 percent above the 100-year average. Landfall probabilities for the U.S. East Coast, including the Florida peninsula, are about 54 percent for one or more major hurricanes, or 74 percent above the 100-year average, and about 40 percent for one or more major hurricanes making landfall on the Gulf Coast from the Florida panhandle west to Brownsville (33 percent above the long-term average.)

Predictions made in December 1998 have remained the same for the April 7 and current updates, the first time since 1992 that numbers have not changed based on new data.

"The climate signals we saw in December and early April remain the same, indicating that we're likely to see activity quite a bit above the average season," Gray said. "We don't see anything in the new information we have through the month of May that would cause us to alter our forecast."

In fact, Gray said, two climate signals have strengthened his belief that this will be an active season.

"The Atlantic Ocean looks slightly more conducive to hurricane formation as sea surface temperatures have risen and, we believe, will continue to rise," he said.

In addition, "the West Coast from southern Canada to Baja California has unusually cold sea surface temperatures. During similar episodes we've had very active hurricane seasons," Gray said.

The phenomenon is not necessarily related to La Niña, an upwelling of cold water limited to the equatorial Pacific. However, La Niña also contributes to Atlantic Basin hurricane formation and is expected to remain cool through the entire June 1-Nov. 30 hurricane season.

Other factors promoting hurricane formation are westerly stratospheric winds, called the Quasi-Biennial Oscillation, that exist high in the atmosphere over the earth's equatorial regions and reverse themselves about every two years. When these stratospheric winds blow from the west, as they are doing in 1999, an enhancing effect on hurricane activity, especially major hurricanes, occurs.

In addition, a ridge of barometric high pressure called the Azores High is measuring below average. The Azores High has an enhancing influence on hurricane activity, as does below-average Caribbean Basin sea-level pressure for August and September 1999.

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

Gray believes that signals from the Atlantic, coupled with recent strong hurricane activity, indicate a new era of storm formation. Increasing North Atlantic sea surface temperatures and salinity suggest that changes observed since 1995 mean the continuance of a strong Atlantic Ocean conveyor belt circulation, bringing with it the chance for more intense hurricanes along the Atlantic coast and in the Caribbean. This enhanced period could continue for two or more decades, Gray believes.

While the Atlantic conveyor belt affects the Eastern Seaboard, this year could see more activity at low latitudes from easterly waves progressing out of Africa.

"That could mean more low-latitude storms (from the equator to about 25 degrees north) this year and storms with long tracks, which tend to become more intense ones," Gray said. "They can affect the Caribbean, but as they move west they tend to curve to the north and could affect the Gulf Coast.

"With this cold water along the Pacific, historically we tend to have lots of landfalling storms along the entire coast."

Colorado State University

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