Hurricane forecasting - answers blow in the wind
WILLIAM M. GRAY is pleased with the Atlantic hurricane season that ended officially Nov. 30. For the fourth year in succession, he forecast the season's activity with amazing accuracy. The Colorado State University meteorologist seems to have correctly identified key atmospheric factors that influence Atlantic hurricanes. He can't say anything about the Pacific and Indian Oceans. But even to pin down such factors for the Atlantic is a notable scientific achievement.
The factors include:
El Nino - a period of unusually warm sea surface temperatures in the eastern equatorial Pacific. Its presence tends to suppress Atlantic hurricanes. Stratospheric global winds about 20 kilometers (13 miles) high over the equator. There tends to be twice as much hurricane action in seasons when the winds blow from the west than when they are easterly.
Winds 12 kilometers (7.5 miles) above the lower Caribbean Basin. The stronger they blow from the west, the more they suppress hurricane activity.
Departure from average sea level air pressure over the Caribbean Basin-Gulf of Mexico area. The higher the pressure anomaly is in spring and early summer, the weaker the seasonal hurricane action.
Gray puts these factors into his forecast formula in late May and predicts the action of the Atlantic hurricane season that opens June 1. He updates the prediction in late July as the season enters its most intensive phase.
For the season just ended, he predicted four hurricanes and three tropical cyclones. There were actually three hurricanes and four tropical storms. He forecast 35 days during which a hurricane or tropical storm would be active, and 36 days were observed. That's a stunningly accurate forecast.
The activity was well below the climatological average of six hurricanes plus four tropical storms and 45 hurricane/tropical storm days. Although it's too early for a formal forecast, Gray expects a more active season in 1988. The El Nino present this year should have dissipated, and the statistical odds are for above-average hurricane activity, he says.
He also calculates an index of the total destructive potential of a season's hurricanes (whether or not they actually come ashore and cause damage). By that measure, this was the second-mildest hurricane season in the past four decades. Only 1983 was quieter.
In fact, by this index, the intensity of seasonal hurricane activity has dropped sharply since 1970. The average seasonal destructive potential was twice as great from 1947 to 1969 as it has been during the past 18 years.
Herein lies a great danger. Atmospheric factors, such as those Gray identifies, have weakened Atlantic hurricane activity for nearly two decades. United States and Caribbean coastal development has burgeoned during this period. As Gray notes, it's impossible to tell how long this downturn in hurricane action will continue.
Thus, the Colorado forecaster has done more than help us anticipate a hurricane season coming up. His insight into critical and changeable atmospheric factors adds point to the repeated warnings of hurricane experts that unwise coastal development tempts people to expose themselves to a risk of future hurricane disaster they would be better advised to avoid.
A Tuesday column. Robert C. Cowen is the Monitor's natural science editor.