Note to Reporters: The entire forecast report will be available on the web at http://hurricane.atmos.colostate.edu/ or with the news release at http://www.news.colostate.edu/ no later than 8 a.m. MST.
The Tropical Meteorology Project at Colorado State University today released its initial outlook for the 2012 Atlantic basin hurricane season.
The report marks the 29th year for the CSU hurricane forecasting team, which is led by Philip Klotzbach and William Gray, but is the first time in its early December forecast that the team is relying on probabilities of key factors influencing the hurricane season rather than issuing a numerical forecast for the number of storms.
“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,” said Phil Klotzbach, lead author of the forecasts. “One of the significant challenges in the early December prediction is that no statistical or dynamical models have shown skill at predicting El Niño – Southern Oscillation (ENSO) at this extended forecast lead time of 9-12 months, so we’re going to look instead at analyzing factors that influence the hurricane season rather than actually predicting the number of hurricanes.”
With its April forecast, the CSU team will return to identifying an anticipated number of named storms, hurricanes and major hurricanes (Saffir/Simpson category 3-4-5). Seasonal updates of the 2012 Atlantic basin hurricane activity forecast are scheduled for April 4, June 1 and Aug. 3.
For the December prediction, Klotzbach and Gray reviewed the strength of two major factors known to influence the Atlantic basin hurricane season:
1) Atlantic thermohaline circulation (THC): When the THC 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, vertical wind shear (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, all of which combine to create an environment favoring an active hurricane season.
2) El Nino – Southern Oscillation: When El Nino occurs in the tropical Pacific, it creates an environment less conducive for storm formation in the Atlantic. Typically vertical wind shear is stronger and the mid-levels of the atmosphere are drier in the tropical Atlantic in El Nino events, both of which are unfavorable for hurricane formation and intensification.
Specific predictions for the 2012 Atlantic basin hurricane season:
• A 45 percent chance that the THC 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 12-15 named storms, seven to nine hurricanes, and three to four major hurricanes.
• A 30 percent chance that the THC continues in the above-average condition it has been in since 1995 with the development of a significant El Niño in which tropical cyclone (TC) activity is reduced to approximately 75 percent of the average hurricane season (roughly eight to 11 named storms, three to five hurricanes, and one to two major hurricanes).
• A 15 percent chance that the THC becomes unusually strong in 2012 and no El Niño event occurs. This would bring about TC activity typically associated with approximately 180 percent of the average hurricane season (roughly 14-17 named storms, nine to 11 hurricanes, and four to five major hurricanes).
• A 10 percent chance that the THC becomes weaker and there is the development of a significant El Niño. This would be associated with TC 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.
For 2011, the team accurately predicted well above-average hurricane activity for the Atlantic basin. Just as hurricane season began in June, the team called for 16 named storms, 9 hurricanes and five major (Category 3-4-5) hurricanes. Observed were 19 named storms, 7 hurricanes and three major 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.”
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.