El Nino Major Factor In Slightly Reduce '97 Hurricane Forecast; Colorado State's Gray Says More Storms May Form In Higher Latitudes

August 06, 1997

FORT COLLINS--A very strong El Niño present this year is expected to slightly hamper Atlantic Basin hurricane activity for 1997. But El Niño will also likely push some storms to higher latitudes and perhaps closer to the United States, a team of hurricane forecasters at Colorado State University said today.

Calling the 1997 hurricane season an "extremely unusual and difficult year to forecast," the noted team, led by Professor William Gray, released an updated hurricane forecast today that predicts 11 named storms, six hurricanes and two intense hurricanes. The updated forecast reflects one less hurricane and one less intense hurricane than the team's previous forecasts issued on April 4 and June 6.

If the revised forecast holds true, 1997 would still be an above-average year for named storms and hurricanes but below average for intense hurricanes. If the predicted activity is realized, it would still be enough to produce the most active, three-year hurricane span on record (1995-97). On average, 9.3 tropical storms, 5.8 hurricanes and 2.2 intense, or major, hurricanes form annually.

Gray said the slight decrease in hurricanes and intense hurricanes in this latest forecast is a result of a very strong El Niño--warmer than normal water temperatures in the eastern Pacific Ocean off the coast of Peru and along the equator. Gray said the 1997 El Niño is by far the strongest event to occur since the record El Niño of 1982-83. The current El Niño is much stronger than the hurricane team anticipated when it announced its early April and early June predictions.

"We've got a group of very mixed global climatic signals, which makes it a very difficult year," Gray said. "We're still going for a much stronger hurricane year than should occur during a typical El Niño year because many favorable factors for hurricane activity are present in the Atlantic Basin."

When El Niño is present, water temperatures reach 1 or 2 degrees Celcius more than normal in the eastern tropical Pacific. This rise in ocean temperatures causes strong upper tropospheric winds to blow in a westerly direction from the Pacific Ocean to the tropical Atlantic Ocean. These winds typically act to shear off developing hurricanes.

However, the very strong winds produced by El Niño this year may actually help shift hurricane formation from tropical regions in West Africa to the Atlantic Basin, putting some storms at higher latitudes and perhaps closer to the United States. That's because El Niño produces strong upper-level westerly winds at lower latitudes--which helps block African-origin storms. While doing so, it also creates weaker upper-level westerly winds at higher latitudes. These higher-latitude winds are less able to thwart hurricane development. As a result, Gray believes that hurricanes are more likely to form off the east coast of Florida, the Bahamas and in the northern Gulf of Mexico than in the tropical Atlantic or Caribbean Sea.

This pattern was present with Hurricane Danny, which developed in the Gulf of Mexico and first made landfall on the shores of Louisiana, then progressed to Alabama, North Carolina and Virginia before regenerating in the Atlantic. Hurricane Bill moved westward over the Atlantic Ocean between Bermuda and the Bahamas. Both hurricanes were Category 1 on the Saffir-Simpson Scale, the lowest intensity a storm can blow and still qualify as a hurricane. By comparison, intense or major hurricanes have sustained winds of at least 111 mph and fall into categories 3, 4 and 5.

"There are periods when we've had an El Niño and still have active hurricane activity, but these are not typical," Gray said. "However, we've already seen four named storms and two hurricanes even before the most active part of the hurricane season started on August 1."

Based on Gray's forecast, the team expects seven more named storms, four more hurricanes and two more intense hurricanes after Aug. 6. Most of the hurricane activity in the Atlantic Basin occurs after mid-August and before late October.

Although the current El Niño is strong, Gray says other climatic factors in the Atlantic Basin are favorable for hurricane activity this year. Warmer sea surface temperatures in the north- and tropical Atlantic and colder sea surface temperatures in the South Atlantic, as well as colder-than-normal air temperatures 54,000 feet above Singapore are still present. Another favorable factor includes the equatorial stratospheric winds at 68,000-75,000 feet. These winds--known as the Quasi-Biennial Oscillation--also tend to promote the formation of hurricanes when they blow from a westerly direction as they are this year.

Gray's team hurricane forecasts--issued in early December, April, June and August--do not predict landfall and apply only to the Atlantic Basin, which encompasses the Atlantic Ocean, Caribbean Sea and Gulf of Mexico.

In addition to Gray, the hurricane research team includes John Knaff, Paul Mielke and Kenneth Berry from Colorado State; and Chris Landsea, a Colorado State graduate and presently a researcher at NOAA's Hurricane Research Laboratory in Miami, Fla.

 GRAY RESEARCH TEAM'S HURRICANE REVISED FORECAST FOR '97 SEASON TODAY'S 1997 FORECAST 6/97 4/97 1. Named Storms (9.3) 11 11 11 2. Named Storm Days (46.1) 45 55 55 3. Hurricanes (5.7) 6 7 7 4. Hurricane Days (23) 20 25 25 5. Intense Hurricanes (2.1) 2 3 3 6. Intense Hurricane Days (4.5) 3 5 5 7. Hurricane Destruction Potential (68.1) 60 75 75 8. Net Tropical Cyclone Activity% 100 110 110 9. Maximum Potential Destruction 60 70 70 '96 ACTUAL 8/96 6/96 4/96 11/95 1. Named Storms (9.3) 13 11 10 11 8 2. Named Storm Days (46.1) 78 50 45 55 40 3. Hurricanes (5.7) 9 7 6 7 5 4. Hurricane Days (23) 45 25 20 25 20 5. Intense Hurricanes (2.1) 6 3 2 2 2 6. Intense Hurricane Days (4.5) 13 4 5 5 5 7. Hurricane Destruction Potential (68.1)# 135 70 60 75 50 8. Net Tropical Cyclone Activity (100%) 198 105 95 105 85 () Represents average year totals based on 1950-1990 #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.

Colorado State University

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