Relatively Slow Start to Hurricane Season Does Not Change Colorado State’s Forecast Numbers; Active 1999 Season Still Predicted

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In the final update for the 1999 hurricane season, Colorado State University hurricane forecaster William Gray and his team held to the same numbers they predicted earlier in the year, calling for an active year with 14 named storms, nine hurricanes and four intense hurricanes.

The slow beginning of the season, which runs from June 1 to Nov. 30, has not been a factor to lower the numbers, said Gray, professor of atmospheric science. Those figures have been consistent through the initial December 1998 forecast and updates in April and June.

Gray still anticipates an active year.

"I know there are many people out there looking at conditions during the last month who’ll say, ‘Look, in July there’s been nothing there.’ But we’ve had many years when we’ve had almost nothing going on in the Atlantic Basin until mid- or late-August and still have had active seasons.

"People should not be lulled into thinking that because we’ve had a rather slow start to the season that we’re going to get a slow season."

Only one named storm, Arlene, has formed thus far, and that was in June. On average, Gray said, one storm before the beginning of the intense part of the season (generally considered to extend from mid-August to late October) doesn’t necessarily indicate a lower than average season. Long-term, 1950-1990 averages yield 9.3 tropical storms, 5.8 hurricanes and 2.2 intense hurricanes each year.

The 1998 year brought 14 tropical storms, 10 hurricanes and three intense hurricanes.

"We think this year should be much like last year and 1996, but not as active as the 1995 year was. This is probably not (going to be) one of those eight or 10 blockbuster storm years of the century, but it still should be a very active year," Gray said.

The team believes there is a roughly 54 percent chance that one or more intense storms (with wind speeds of 111 mph or above) will make landfall along the U.S. East Coast, including peninsular 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.

What Gray refers to as "climate signals," measures of the global oceanic and atmospheric circulation system, have remained both consistent throughout the year and in all but one case favorable to hurricane formation. Those 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.

*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 causing less disruption to hurricane formation.

In addition, Gray said, the monsoon is running well in southern Asia, a factor weakly correlated with Atlantic Basin (North Atlantic, Caribbean Ocean and Gulf of Mexico) hurricanes. Another phenomenon, relatively cold water off the west coast of North America, is correlated with more landfalling intense hurricanes off the nation’s Eastern Seaboard.

The one climate signal that has not been favorable to hurricane formation may, in fact, be rapidly changing. Above-average, sea-level barometric pressure in the western Atlantic and Caribbean basin began dropping quickly during the last 10 days.

"That’s the only negative factor we’ve been following, and we think the pressure will continue to fall as it has in recent days," Gray said.

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. The years 1995-98 were the most-active four consecutive years of hurricane activity on record, yielding 53 named storms, 33 hurricanes and 15 major hurricanes.

Increasing North Atlantic sea surface temperatures and salinity suggest that changes observed since 1995 indicate a 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.

Gray has said that while the Atlantic conveyor belt affects the Eastern Seaboard, this year also could see more activity at low latitudes from easterly waves progressing out of Africa.

"The probability for United States or Caribbean intense-storm landfall is distinctly higher this year than in the average year, and particularly in comparison to the relatively quiet period of 1970-1994," Gray said. While he expects low-latitude storms that could affect the Caribbean, they can continue moving west-northwest and affect the Gulf Coast.

Gray said that the cold water along the Pacific, from southern Canada to Baja California, is not associated with La Niña, which is much farther south, but is connected historically with higher-than-average numbers of landfalling storms along the East Coast.

The probability of the formation of an intense storm "ramps up" rather quickly from mid-August and drops with equal speed after mid-September.

"Our forecast for this coming 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.

"Things look favorable for an active season. 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."


December 1998 April 7 June 4 August 6
Named Storms (9.3)* 14 14 14 14
Named Storm Days (46.9) 65 65 75 75
Hurricanes (5.8) 9 9 9 9
Hurricane Days (23.7) 40 40 40 40
Intense Hurricanes (2.2) 4 4 4 4
Intense Hurricane Days (4.7) 10 10 10 10
Hurricane Destruction Potential (70.6) 130 130 130 130
Maximum Potential Destruction (61.7) 130 130 130 130
Net Tropical Cyclone Activity (100%) 160 160 160 160

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