For the first time in its 29 years of predicting hurricane season activity, the Colorado State University team of Phil Klotzbach and William Gray is dispensing with issuing exact numbers for its December forecast. For its first forecast of the 2012 hurricane season, the team is discussing probabilities of key factors influencing the hurricane season, with each resulting scenario indicating a range of the number of named storms, hurricanes, and major hurricanes that could develop.
“We have suspended issuing quantitative forecasts at this extended-range lead time in December, since they have not proved skillful in real time over the last 20 years,” Klotzbach said in a press release issued Wednesday.
Hurricane season officially runs June 1 through Nov. 30.
In April, the team will return to giving numbers for predictions of named storms, hurricanes, and major hurricanes with winds of 111 mph or greater. Forecasts will be issued April 4, June 1, and Aug. 3.
Klotzbach said that no statistical or dynamical models have shown skill at predicting the El Niño and Southern Oscillation factors that go into hurricane forecasting at nine to 12 months out, which poses significant challenges. Instead they’ll look at analyzing factors that influence the hurricane season rather than actually predicting the number of hurricanes.
"There is significant uncertainty with this earliest outlook, issued six months prior to the start of the hurricane season, so we’re looking more at a range of different potential outcomes which might occur,” Gray said. “We believe we’re still in an active multi-decadal period for hurricanes in the Atlantic basin.”
For the December prediction, Klotzbach and Gray reviewed the strength of two major factors known to influence the Atlantic basin hurricane season: Atlantic Thermohaline Circulation and El Niño/Southern Oscillation.
They evaluated Atlantic Thermohaline Circulation, which is a change in ocean circulation driven by temperature and salinity. When the Atlantic Thermohaline Circulation is stronger than normal, a large variety of physical features in the tropical Atlantic are typically more conducive for hurricane formation and intensification.
Tropical Atlantic water temperatures tend to be warmer; additionally, vertical wind shear, which is the change in wind direction with height, tends to be reduced, sea level pressures tend to be lower, and mid-levels of the atmosphere tend to be moister. These factors combine to create an environment favoring an active hurricane season.
El Niño relates to water temperatures, occurs in the tropical Pacific, and creates an environment less conducive for storm formation in the Atlantic.
Klotzbach said that the Southern Oscillation concerns how the atmospheric pressure responds to water temperature.
Typically, vertical wind shear is stronger and the mid-levels of the atmosphere are drier in the tropical Atlantic in El Niño events, both of which are unfavorable for hurricane formation and intensification, Klotzbach and Gray said.
The two forecasters outlined several scenarios:
- For the 2012 Atlantic basin hurricane season, they predicted a 45 percent chance that the Atlantic Thermohaline Circulation continues in the above-average condition it has been in since 1995 and that no El Niño develops. If this happens, hurricane activity would be approximately 140 percent of the average season. This is typically characterized by 12 to 15 named storms, seven to nine hurricanes, and three to four major hurricanes.
“We weigh this one the highest,” Klotzbach said by phone from his Fort Collins, Colo. office.
- They also think there is a 30 percent chance that the Atlantic Thermohaline Circulation continues in the above-average condition it has been in since 1995 with the development of a significant El Niño in which tropical cyclone activity is reduced to approximately 75 percent of the average hurricane season. This means the season would see roughly eight to 11 named storms, three to five hurricanes, and one to two major hurricanes.
- They give a 5 percent chance that the Atlantic Thermohaline Circulation becomes unusually strong in 2012 and no El Niño event occurs. This would bring about hurricane activity typically associated with approximately 180 percent of the average hurricane season. This means that about 14 to 17 named storms, nine to 11 hurricanes, and four to five major hurricanes would develop.
- They think there’s a 10 percent chance that the Atlantic Thermohaline Circulation becomes weaker and there is the development of a significant El Niño. This would be associated with hurricane activity that is approximately 40 percent of the average season, or approximately five to seven named storms, two to three hurricanes, and zero to one major hurricanes.
The hurricane team’s forecasts are based on the premise that global oceanic and atmospheric precursor signals that preceded active or inactive hurricane seasons in the past will provide meaningful information about future seasons.