A normal hurricane season is being forecast in 2001 although there has been a slight increase in the number of hurricanes predicted to develop during the five-month period June 1-Nov. 30.
Noted forecaster William Gray, professor of atmospheric science at Colorado State University, and his research team released their updated hurricane projection on Friday. It calls for 10 named storms, six hurricanes and two intense storm systems during the months of the traditional hurricane season. The Gray forecast projects one tropical storm and one hurricane more than an initial 2001 forecast released in December.
"There are definitely mixed signals this year, and we've been debating the numbers since they became available last week," Gray said Friday. "Our statistical calculations are somewhat jumbled. Some years are easier than others, and this is not an easy forecast year."
According to Gray, what's confusing the forecasters are strong climate signals, pro and con. An El Nino is forecast which causes winds in the tropical Atlantic to blow more strongly from a westerly direction and be unfavorable for hurricane formation. But there are also signals from Atlantic sea surface temperatures, average barometric pressures at the ocean surface and the Azorean high which all seem to indicate a more forceful hurricane season.
The El Nino is one of several factors that are likely to inhibit hurricane formation, Gray believes. The Quasi-Biennial Oscillation (QBO), 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 QBO winds will be easterly during this hurricane season, Gray said, causing 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 pressure found over the Azores is very low this year, this he said will add to the Atlantic's hurricane-promoting ability.
An ongoing factor favoring hurricane formation is the Atlantic Ocean thermohaline (temperature plus salinity) circulation system. Relatively warm sea surface temperatures in the North Atlantic are a proxy measure of the strength of this ocean current that moves warm, salty water from the tropics to the North Atlantic. Historically, a strong thermohaline circulation system signals more major hurricanes making landfall on the East Coast and Florida peninsula and weaker tropical storms making landfall along the U.S. Gulf Coast.
In his forecast Gray offered no probabilities for hurricane landfall in the Caribbean this year but said the chances of a major storm coming ashore in this region are about average.
In conclusion, the hurricane research team at CSU noted that while 2001 will prove to be less active than the very busy hurricane seasons of 1995, 1996, 1998, 1999 and 2000, it will definitely be more active than the average of the 25-year period of relatively low activity that was witnessed in the 1970s.
To see the complete research findings of Gray and his team, click here.
