CSU Forecast Team Ups Hurricane Prediction for 2000

June 8, 2000

Colorado State University’s hurricane forecast team, led by William Gray, has revised its April forecast, now predicting 12 named storms, eight hurricanes and four major hurricanes for the 2000 season, which began June 1.

Gray, professor of atmospheric science, and his colleagues had forecast 11 named storms, seven hurricanes and three major hurricanes in their initial December 1999 forecast and in their first update, issued April 7. Gray said a series of changes in atmospheric and oceanic phenomena, what he calls “climate signals,” has led the team to postulate the higher numbers.

The long-term, 1950-1990 average calls for 9.3 tropical storms, 5.8 hurricanes and 2.2 major hurricanes (those with wind speeds above 110 mph). The increased numbers suggest an above-average hurricane season roughly similar to or slightly weaker than those of 1995, 1996, 1998 and 1999.

“We’re approaching the numbers we predicted last year,” Gray said. “That was one of our most successful forecast seasons. We predicted 14 named storms, and 12 were observed. We called for nine hurricanes, and eight were observed, and we forecast four major hurricanes. The actual observed number was five.”

Gray pointed out that the increase this year follows a trend begun in 1995, when changes in global circulation systems led to the five most intense consecutive storm seasons on record. The years 1995-99 saw 65 named storms, 41 hurricanes and 20 major hurricanes.

“We’re raising the 2000 season numbers for a number of reasons,” Gray said. “Foremost among them is verification that the current La Niña, or cold water conditions, will continue for the rest of the season. The existing La Niña has weakened but is holding strongly in the Central Pacific. There has never been a case where the La Niña has remained as cold as at present and an El Niña has then developed before the season’s over.”

El Niño, a mass of warm water in the equatorial Pacific Ocean, tends to inhibit hurricane formation; La Niña, by contrast, consists of relatively cold water and does nothing to disrupt the formation of hurricanes in the Atlantic Basin (the North Atlantic Ocean, Caribbean Sea and Gulf of Mexico). However, Gray warned that the impact of these two climate signals, while important, is often overstated.

Other factors leading to the increase in predicted numbers: “The Quasi-Biennial Oscillation is not coming down from an easterly direction as fast as we thought,” Gray said. The Quasi-Biennial Oscillation is a pattern of stratospheric winds that blow in a roughly two-year (actually, 26-30 month) period, from the east and then from the west and then from the east again. Winds from the east tend to inhibit hurricane formation.

In the Atlantic, relatively warm sea-surface temperatures at both high and low latitudes are a favorable indicator of storm formation and are expected to be present later in the season. Also in the Atlantic, lower than normal atmospheric pressures that enhance hurricane development are expected to be present during the height of the hurricane season. In the Pacific Ocean, sea surface temperature patterns associated with higher Atlantic Ocean hurricane activity are present. Easterly upper-level equatorial winds over the Atlantic and South America are favorable for enhanced Atlantic hurricane activity. As indicated by preliminary spring numbers, West African rainfall from June to September, an indicator of hurricanes, is likely to be higher than usual.

Despite the higher numbers, landfall probabilities have increased only slightly from the initial forecast. The team is predicting that the probability of one or more major hurricanes coming ashore somewhere along the U.S. coastline from Brownsville to the Maine-Canada border is 71 percent (the average for the last century is 52 percent). The probability of one or more major hurricanes making landfall on the U.S. East Coast, including the Florida peninsula, is 52 percent, compared with an average the past century of 31 percent. The Gulf Coast faces a 40 percent probability of the onslaught of one or more Saffir-Simpson 3-4-5 storms (the last century’s average is 30 percent). The Caribbean basin faces a possibility of landfall by one or major storms about 15 percent above the past century’s average.

Gray and his colleagues predict number of named storms, hurricanes and major hurricanes, along with measures of duration such as named storm days, plus overall estimates of annual hurricane activity and destructive potential. They do not predict when or where a storm will occur, what category it will achieve or what specific track it will take. However, they are developing landfall probabilities that express the frequencies as they are related to climate changes.

Although the forecasters originally saw the 2000 season as only moderately above average, Gray and his colleagues are not surprised by indications of increased activity. He and others have theorized that an oceanic circulation system called the Atlantic Ocean thermohaline system (Atlantic conveyor belt for short) affects the number of major hurricanes that make landfall along the East Coast. While it doesn’t influence the total number of weaker cyclone systems very much, the number of major hurricanes forming and making landfall is increased.

Major hurricanes, when normalized by coastal population, inflation and wealth per capita, generated about 85 percent of the hurricane-spawned destruction. The Atlantic conveyor belt carries warm, salty from the tropics to the northern Atlantic and then returns southward. This transfer of heat energy in the Atlantic can be traced to circulation patterns in the Pacific and Indian oceans as well, but they add up to one thing: a stronger (i.e., warmer and saltier) conveyor belt flow produces more landfalling major hurricanes along the Eastern Seaboard, as occurred from the 1940s to 1960s.

A weak quarter-century (1900-25) was followed by the active period, and that was followed from 1970-94 by a weak period. Since 1995, Gray believes, evidence points to a decades-long period of increased major storm landfall.

“With the buildup of coastal areas, especially in the southeast United States, and with the shift toward a multi-decadal pattern of a stronger Atlantic thermohaline system and more landfalling major storms, I think we’re going to see more hurricane damage than we’ve ever seen in this country,” he said.

“I would urge people living in coastal areas to heed the warnings and evacuation orders of local emergency management officials,” Gray said. “With Floyd missing Florida last year, there’s apt to be some cynicism, but officials can only predict a couple of days in advance and there will be false calls.

“Heeding an evacuation order that turns out to have been unnecessary is better than not evacuating under order, which could cost human lives,” Gray said. “At the same time, those not directed to evacuate should remain at home and not clog escape routes for those who need them.”

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