As of early April, the Colorado State University hurricane forecasting team led by William Gray has upped the numbers for 2001 just slightly, suggesting a normal season. For the June 1-Nov. 30, 2001 season, the scientists are calling for 10 named storms, six hurricanes and two intense (Saffir-Simpson category 3 or higher) hurricanes.
That’s one tropical storm and one hurricane more than predicted in the initial 2001 season forecast released last December by Gray. That first forecast was termed by Gray “below the long-term average” (1950-1990) and well below the six-year-period (1995-2000) that set a record for Atlantic Basin tropical storms and hurricane activity.
The long-term average calls for 9.3 named storms, 5.8 hurricanes and 2.2 major hurricanes a year. A professor of atmospheric science at Colorado State, Gray said that despite expected near “normalcy” for the 2001 season, activity will exceed the average storm activity in the quiet period of 1970-94, when population and property grew at a rapid pace along the southeast U.S. coast and the Florida Peninsula.
Gray sounded a warning note when issuing the initial forecast in December 2000, indicating that what he terms climate signals were not as clear as in some years. He said then that the team was not as confident about the 2001 forecast as they had been about the 2000 forecast, in which they predicted 11 of 14 tropical storms, seven of eight hurricanes and three of three intense hurricanes. “There are definitely mixed signals this year, and we’ve been debating the numbers since they became available last week,” he said. “Our statistical calculations are somewhat jumbled. Some years are easier than others, and this is not an easy forecast year.”
What’s confusing the forecasters are strong climate signals, pro and con. An El Niño is forecast that should cause winds in the tropical Atlantic to blow more strongly from a westerly direction and hence be unfavorable to hurricane formation. However, signals from the Atlantic – sea surface temperatures, average barometric pressures at the ocean’s surface and the Azorean high – all seem to indicate a more forceful hurricane season.
“Looking at the factors that both promote and inhibit hurricanes, we feel that our reasoning should hold fairly well,” Gray said of the expectation of an average season. One inhibiting factor is the El Niño, a body of relatively warm, equatorial water in the eastern Pacific. Absent for the past few years, it is expected to bring weak to moderately warm water to the South American west coast.
A characteristic of El Niño is westerly winds in the upper troposphere that act to shear the tops off Atlantic easterly waves coming off the African Coast, preventing them from growing into named storms or hurricanes. “Most of us agree that a weak to moderate El Niño event should be anticipated for this coming summer,” Gray said. “This should be an inhibiting factor in 2001.”
However, he said, the team is “very sure” this year’s El Niño event will fall well short of the one that occurred in 1997, limiting that season’s total named storms to seven. Another inhibiting factor is the Quasi-Biennial Oscillation, an equatorial stratospheric wind pattern that blows east to west for about 14 months, reverses direction for about the same period and then changes yet again.
The winds will be easterly this season, Gray said, causing some further inhibition of Atlantic easterly waves from forming into named storms. On the other hand, Gray said, “the Atlantic sea surface temperatures will be relatively warm and will tend to enhance this season’s prospects for tropical storm formation.” So, too, will the below-average barometric pressures at the Atlantic’s surface, which help hurricanes to form. A ridge of high barometric pressure found over the Azores Islands is very low this year.
That will add to the Atlantic’s hurricane-promoting ability, Gray said.
Gray estimates the probability of one or more major (Saffir-Simpson 3-4-5) hurricanes making landfall along the entire U.S. coastline in 2001 at 65 percent (the past century’s average is 52 percent). For the U.S. East Coast and Florida peninsula, the 2001 landfall probability is 46 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 37 percent compared with 30 percent for the century.
Because of the difficulty of estimating landfall probabilities for 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. Gray and his colleagues have been warning for years about more active hurricane seasons, especially those with higher numbers of intense-storms making landfall on the U.S. East Coast and Florida peninsula.
From 1900-25 and 1970-94, records show, relatively few made landfall. In the period from the late 1920s to the late 1960s, landfall activity picked up. Gray said the United States has encountered good fortune in recent years. Between 1900 and 2000, 218 major storms occurred in the Atlantic Basin, with 73 (one in three) coming ashore in the United States.
Of the 23 major storms of the last six years, only three have made landfall. Meteorologically, Gray knows where the luck comes from. “Upper-level trough frequency over the U.S. East Coast has been above average during the hurricane seasons of the last six years,” Gray said. “This has led to the steering of most major Atlantic Basin hurricanes away from the U.S. mainland.”
On the East Coast and Florida peninsula, the first six decades of the 20th century have averaged 3.4 times more landfalling major hurricanes per year than have the last four decades. “Climatology (averages) will eventually return, and 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.
Gray thinks the expected increase in landfalls will cause unprecedented economic loss, the result of increased development along the U.S. Southeast coastline. 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, 1953, 1957, 1963 and 1965, all of which had weak to moderate El Niños and had average or above average activity.
“What we believe is that 2001 will prove to be less active than the very busy seasons of 1995, 1996, 1998, 1999 and 2000 but definitely more active than the average of the 25-year period of relatively low activity we saw beginning in 1970. U.S. major hurricane landfall probability this year will be above average.”
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