Colorado State Hurricane Forecast Team Continues Call for Less Active Season Than that of Recent Years; Activity Still Above Average

Note to Editors: Forecast totals are in the attached chart. The complete hurricane forecast and related research and press releases are available on the World Wide Web at: Taped comments by Professor William Gray will be available at 970/491-1525 beginning at approximately 8:30 a.m. MDT.

Colorado State University’s hurricane forecast team continues to call for a "moderate" season this year with 11 named storms, seven hurricanes and three intense hurricanes predicted.

These numbers, presented in the initial 2000 hurricane season forecast issued in December 1999, are above the long-term average but lower than the number of storms than occurred in four of the past five years. William Gray, professor of atmospheric science, and his colleagues are calling for an above-average probability of U.S. major hurricane landfall on the East Coast and Florida Peninsula and an about-average probability of major storm landfall for the Gulf Coast during the hurricane season, which runs from June 1-Nov. 30.

"We do not anticipate a season as active as those in 1995, 1996, 1998 or 1999," Gray said. "Still, we believe we are entering a new era for increased storm activity and for East Coast landfalls by major storms. There is a strong likelihood that in coming years we’ll see more major storms as we did during the 1940s, 1950s and 1960s."

Long-term averages based on the period 1950-1990 indicate 9.3 named storms, 5.8 hurricanes and 2.2 intense hurricanes per year. The 2000 forecast, while anticipated to be less active than several recent seasons, is still expected to exceed significantly the average season during the relatively quiet period between 1970-94.

Factors at work this year include the continued presence of La Nina.

"As we see it now, things are progressing about as we thought they would in our early December forecast," Gray said. "We do not believe that an El Ni o will occur this year. However, the very cold (La Ni a) water that’s been out in the eastern equatorial tropical Pacific for the last two years we think will modify somewhat and not be quite as cold.

"That is a bit of an enhancing factor for this year’s activity."

Another of what Gray calls "climate signals" is the Quasi-Biennial Oscillation. These are stratospheric, equatorial east-west winds, ranging from 16 to 35 kilometers in altitude, that oscillate. The direction changes about every 26-30 months, typically blowing for 12-16 months from the east, then reversing and blowing 12-16 months from the west, then back to easterly again.

"This year, the Quasi-Biennial Oscillation is expected to blow from the east, a direction not normally favorable to hurricane formation," Gray said. "However, this year the winds have failed to drop as low as we expected, somewhat neutralizing their effect."

Gray said that North Atlantic sea surface temperatures continue to be relatively warm, indicating that the Atlantic Ocean thermohaline circulation system, or Atlantic conveyor belt, remains strong.

A strong Atlantic conveyor belt, Gray and colleagues believe, contributes to the formation of greater numbers of major or intense (Saffir-Simpson category 3-5) storms. This increases the probability of major hurricane landfall on the U.S. East Coast and in the Caribbean. Gray has frequently noted that the Atlantic conveyor belt, as measured by relatively high sea surface temperatures and high salinity in the North Atlantic, was strong during the period from the 1930s through the late 1960s, when major storms lashed the Eastern Seaboard.

Those conditions, which seem to be echoed by those since 1995, contrast with the periods 1900-1925 and 1970-1994, when the Atlantic conveyor belt was weak and relatively few major storms made landfall. The years 1995-99 were the most-active five consecutive years of hurricane activity on record in the Atlantic Basin, yielding 65 named storms, 41 hurricanes and 20 major hurricanes.

"Some contend that human-induced global warming is causing the increase in storms, but if this were true and the effect ‘global,’ I would not expect to see lower number of storms in the Pacific, which is what is occurring," Gray said.

Gray and co-authors Chris Landsea, Paul Mielke and Kenneth Berry, with the assistance of Todd Kimberlain, Eric Blake and Bill Thorson, use a variety of climatic factors in their forecasts. In addition to stratospheric winds, North Atlantic sea surface temperatures and La Niña-El Niño conditions, they look at a ridge of barometric high pressure called the Azores High, warmer sea-surface temperatures throughout the Atlantic, two measures of west African rainfall and mid-latitude oceanic wind patterns in the Atlantic and Pacific oceans.

In terms of the probability of landfall, this year’s forecast continues to call for the U.S. Atlantic Coast, including peninsular Florida, to have a 39 percent chance of being hit by one or more major storms in Saffir-Simpson category 3-5 with winds above 110 mph. The long-term (100-year) mean is 31 percent. The Gulf Coast faces a 34 percent probability of one or more landfalling major hurricanes, while the long-term Gulf Coast average is 30 percent. The chances of one or more intense storms coming ashore somewhere along the entire U.S. coast, from Brownsville to the Canadian border, is 60 percent (the long-term average is 52 percent).

Gray estimates the landfall probability of one or more major storms striking the Caribbean at 10 percent above the long-term mean.

Gray also believes that the 2000 season will see hurricane formation earlier than in the seasons of 1998 and 1999, which started very late.

"Only one tropical storm, Arlene, had occurred during the first two months of the season last year, after which, in late August, four hurricanes formed in quick succession," he said.


December 1999 April 2000
Named Storms (9.3)* 11 11
Named Storm Days (46.9) 55 55
Hurricanes (5.8) 7 7
Hurricane Days (23.7) 25 25
Intense Hurricanes (2.2) 3 3
Intense Hurricane Days (4.7) 6 6
Hurricane Destruction Potential (70.6)** 85 85
Maximum Potential Destruction (61.7) 70 70
Net Tropical Cyclone Activity (100%) 125 125

* 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.