‘average’ Hurricane Season Previously Predicted by Colorado State’s Team is Amended Upward; More Activity Foreseen for 2001

Note to Editors: Forecast totals are in the chart at the end of this release. Complete hurricane forecasts and related research are available at http://tropical.atmos.colostate.edu/forecasts/index.html.

Please Note: The August forecast update has been changed to Aug. 7 and will include a monthly forecast for August alone.

The normal hurricane season that William Gray and his Colorado State University forecast team have been calling for since December 2000 now is giving signs that it will be more active than expected.

For the forecast issued today (June 7), Gray and his team are calling for 12 named storms, seven hurricanes and three intense hurricanes (those registering Category 3 or higher on the Saffir-Simpson scale).

For the 2001 season, which extends from June 1-Nov. 30, the researchers had predicted two intense hurricanes, five hurricanes and nine named storms in their initial December forecast. Those numbers were raised slightly, to 10 named storms and six hurricanes, in the April update. The April numbers approximated rather closely the long-term (1950-1990) yearly average of 9.3 named storms, 5.8 hurricanes and 2.2 intense hurricanes.

What’s raising the numbers to activity levels seen in recent years? Gray has two answers: an Atlantic Basin (North Atlantic, Caribbean Sea and Gulf of Mexico) demonstrating more hurricane-promoting signals in May than in March, and the Pacific Ocean now being judged less likely to produce a significant El Niño event.

Gray, professor of atmospheric science at Colorado State, said the increase in the latest forecast doesn’t surprise him.

"All of the climate signals in the Atlantic Basin that we’ve been monitoring are very positive for above-average storm activity this year," he said. "We tend to view them as ‘local’ signals, affecting only the Atlantic, but they’re strong.

"The sea surface temperatures are above average," he said. "The team projects a low atmospheric surface pressure by the late summer. The Azores high (the normal high pressure area near the Azores Islands) and the Atlantic trade winds are below average. These are conditions that promote hurricane activity. The team also foresees above-average rainfall in the Sahel region of West Africa that tends to promote hurricane activity."

"The El Niño phenomenon isn’t proceeding the way we earlier believed it would," Gray said. "There was, of course, the unprecedented El Niño of 1997, which wrecked our forecast and produced only seven named storms in the entire Atlantic Basin that year. Such cyclone reductions will not occur this year."

El Niño is a warm body of water which extends from the South American west coast to the International Date Line in the Pacific. When present, it can produce strong, upper trophospheric westerlies that move across the Atlantic and literally rip the tops off forming storms.

La Niña is the opposite-relatively cool water near the equator that at higher latitudes generates weak upper trophospheric westerlies, or even weak easterly winds, that promote hurricane formation. Since Gray and his colleagues initially anticipated a stronger warming for this year, the team believes the earlier forecast underestimated the number of Atlantic Basin storms.

"There were mixed signals this year," Gray said of the April 6 forecast update. Now it appears the only primary inhibiting factor is an easterly Quasi-Biennial Oscillation. Equatorial stratospheric winds that change direction every 13-15 months, the Quasi-Biennial Oscillation has a full period of 26-30 months (hence its name). This atmospheric phenomenon is blowing from the east this year, an inhibiting factor for Atlantic hurricane activity.

Gray estimates the probability of one or more major (Saffir-Simpson Categories 3-5) hurricanes making landfall along the entire U.S. coastline in 2001 at 69 percent (the past century’s average is 52 percent). For the U.S. East Coast and Florida peninsula, the 2001 landfall probability is 50 percent (vs. 31 percent for the past century). The Gulf Coast, from the Florida Panhandle westward to Brownsville, has a major hurricane landfall probability of 39 percent compared with 30 percent for the century’s annual average.

Because of the difficulty of estimating landfall probabilities for the small Caribbean land masses, Gray offered no probabilities but said that this year the chances of a major storm coming ashore in this region are about average.

While the season now appears set to be more active, Gray and his colleagues have a stronger motive for thinking that the East Coast and Florida peninsula will experience landfalls by an intense hurricane. They theorize that ocean currents (the Gulf Stream among them) bring warm, relatively salty water to northern Atlantic waters, where the heavier, salt-laden water sinks. This redistributes energy and warms the Atlantic, giving storms a greater potential to intensify to a major hurricane more often, perhaps hitting the eastern United States.

Those periods come and go, but the Colorado State team believes the phenomenon, called the Atlantic Ocean thermohaline circulation system, is increasing in strength (i.e., it’s saltier and warmer) since 1995. Prior to that time, from 1970-94, the system was weak and few major storms made landfall. That, Gray and associates warn, is changing rapidly.

Since 1995, and counting the El Niño-dominated year of 1997, 79 named storms, 49 hurricanes and 23 major hurricanes have been observed in the Atlantic Basin. However, unlike the century-long average in which 73 of 218 major storms came ashore, only three of 23 have made landfall in the past six years. The East Coast has been helped by frequent upper level troughs that have steered storms to sea, Gray said, but he believes this luck won’t hold.

"Climatological averages will eventually return," he said. "When that happens, we can expect a significant increase in the number of major hurricanes making landfall on the East Coast and in Florida," he said. Since these areas have become populated and built up during the quiescent 1970-94 quarter century, Gray thinks the expected increase in landfalls will cause unprecedented economic loss.

Besides statistical analyses and climate data, Gray and co-authors Chris Landsea, Paul Mielke Jr., Kenneth Berry and Eric Blake also apply "analog years" to the forecast. These are years when precursor climate signals statistically resemble those of the current year. In the case of 2001, the best "analog years" are 1951, 1952, 1960, 1963 and 1996, most of which had average or above average activity.

"Even with the increased forecast numbers this won’t be one of the worst seasons of the century, but we are anticipating an active season not unlike several of the most recent years," Gray said.


DEC 2000 APR 2001 JUNE 2001
Named Storms (9.3)* 9 10 12
Named Storm Days (46.9) 45 50 60
Hurricanes (5.8) 5 6 7
Hurricane Days (23.7) 20 25 30
Intense Hurricanes (2.2) 2 2 3
Intense Hurricane Days (4.7) 4 4 5
Hurricane Destruction Potential (70.6)** 65 65 75
Maximum Potential Destruction (61.7) 60 60 70
Net Tropical Cyclone Activity (100%) 90 100 120

* 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-long periods where storms attain wind speeds appropriate to their category on the Saffir-Simpson scale.