While hurricane forecaster William Gray and his colleagues at Colorado State University are satisfied with their April forecast update, any possible changes in the June 7 forecast may slightly increase the number of storms.
In a mid-May interview, Gray held out a slight chance that April’s numbers – 10 named storms, six hurricanes and two major hurricanes – might go up.
"There’s perhaps a little greater chance of raising than lowering those forecast numbers, but there’s also a good chance we’ll just keep it the same," he said.
The causes of uncertainty are "mixed climate signals," according to Gray, professor of atmospheric science at Colorado State.
Rainfall is moving into the Sahel region of West Africa in typical seasonal patterns at this point, he said. In the Atlantic, sea-surface temperatures are warm and a high-pressure formation called the Azores Ridge is relatively weak. Both are positive indicators of hurricane formation.
On a larger scale, stratospheric winds near the equator called the Quasi-Biennial Oscillation, or QBO, are blowing from an easterly direction, inhibiting hurricane formation. But the big question remains just how warm the eastern equatorial Pacific water temperatures are going to become.
"This year, we’re very confident we’re not going to get an El Niño like the one that occurred in 1997," said Gray, referring to events that led to one of the Colorado State team’s worst forecasts. When an east Pacific warm equatorial water mass (El Niño) is set up, tropical Atlantic hurricane activity almost always is reduced.
But the team’s not confident what degree of warming will occur, and Gray said the degree of eastern Pacific warming is probably the biggest single variable in the forecast.
Interestingly, Gray admitted that the initial forecast issued in early December 2000, which called for nine named storms, five hurricanes and two major (Saffir-Simpson 3-5) hurricanes, and the April update, which called for 10 named storms, six hurricanes and two major hurricanes, involved debate about the number of major storms. Some team members thought the season would bring three. Gray said they will carefully re-examine this question for the early June forecast.
The Colorado State prediction team has advanced a theory that an ocean current called the Atlantic Ocean thermohaline circulation system, which includes the Gulf Stream, carries salty, relatively warm water to the North Atlantic. A stronger thermohaline circulation doesn’t necessarily spawn more named storms overall but does produce more major, landfalling hurricanes along the East Coast and peninsular Florida.
A more active, stronger thermohaline system has been observed between 1995 and 2000, during which 23 major storms have occurred in the Atlantic Basin. Only three, or about one in seven, have made landfall. However, in the last century, 73 major storms have come ashore, approximately one in three. Gray points out that, in the last six years, the East Coast has been largely protected by upper-level troughs that produce southwesterly winds that "recurve" most major storms, forcing them into a clockwise curve that leads out into mid-ocean.
"In the next decade or two, we think we’re going to get landfalling hurricane activity more typical of the 1930s through the 1960s," he said. "We think we’re in an era of major, landfalling storms (on the East Coast of the United States.)"
Another area whose good fortune may be ending is peninsular Florida – essentially the state minus the western panhandle. Gray finds it "amazing" that in the past 35 years only one major hurricane (Andrew in 1992) made landfall. In the previous 35-year period (1931-1966), he points out, 11 major hurricanes made landfall.
Similarly, in the past 50 years, the Florida peninsula encountered only three major landfalling hurricanes. During the previous half-century (1900-1949), 15 came ashore in that state. "There were a variety of climate factors reducing peninsular Florida storms the last 35 years, but some of it was due to just plain luck," Gray said. Because he believes more storms will strike the state, he estimates that about 10 times as much hurricane-caused damage will occur in the next 35 years as what has taken place in the past 35 years.
Gray’s team will issue the next official forecast update on June 7.
"I think there’s a possibility we might raise (storm numbers in) our early June forecast somewhat, but we reserve the right to look at all the data that comes in and discuss it at length before making any decisions," he said.