Posted by Rich Woolley VP Operations and Michael Ferrari, PhD
WTI’s proprietary hurricane season forecast technology indicates that the total number of named storms in 2009 will once again be fewer than last year. The model indicates at the most 13 named storms, but uncertainty due to the upcoming Fall ENSO status necessitates a forecast range. The official forecast is for 10-13 named storms with 6 of them becoming hurricanes and 3 of them becoming major hurricanes. The total number of named storms will be influenced by a number of factors this year which are all pointing toward a less active season. The AMO, TSA, TNA and ENSO signals were the primary tools used in this year’s forecast. CLICK ON IMAGE FOR LARGER VIEW
The AMO or Atlantic Multidecadal Oscillation which is basically an index depicting North Atlantic sea surface temperatures has for the first time since 1994 trended negative for the first 4 months of the year. This is significant as a negative AMO index value is strongly correlated to lower than average tropical activity.
The TNA and TSA are indexes of sea surface temperatures in the Tropical North Atlantic and Tropical South Atlantic. The TNA has begun to trend negatively over the last few months while the TSA has trended more positive. Both of these factors also correlate to lower tropical activity.
The final factor used to build this year’s forecast was the status of El Nino. Currently last winter’s weak La Nina has faded and there are some indications that a weak El Nino may form later this year as tropical waters off South America have shown some unusual warming in the last few weeks. The uncertain status of El Nino later this year has introduced some doubt into this year’s forecast as a rapid development of El Nino this Fall will dramatically reduce the number of named tropical systems due to upper atmospheric wind shear.
Analog years where the 4 major forecast indexes were in a similar alignment were in 1973, 1984 and 1986. Remarkably in those 3 years there were no major Hurricane strikes in the U.S. as most storms remained in the open Atlantic and the few that were in the Gulf of Mexico tended to be weaker systems. There were 6-13 named storms in the analog years.
Readers need to be cautioned that while the number of storms may be less this year it only takes one storm striking a heavily populated area to cause catastrophic loss of life and property.
Accumulated Cyclone Energy
Though the number of named storms is the most popular Hurricane Season metric to track there is another metric that better represents the actual amount of hurricane activity. This is known as ACE or Accumulated Cyclone Energy. The last few years have seen a dramatic increase in the number of marginal storms being named that haven’t survived more than a day or two which has to some extent inflated the actual number of systems. Officially the naming of many of these systems is justified but the advent of satellite imagery and constant 24/7 monitoring of the tropics has resulted in a greater detection frequency. This increase in detection had resulted in many short-lived named storms, thus increasing the number of named storms metric. The ACE tempers the effect of numerous marginal storms when determining overall hurricane activity. The graphic below depicts the last 39 years of ACE which was at its 3rd lowest level in 39 years and lowest in 31 years. This clearly shows that hurricanes have not been increasing despite a global warming trend from the mid 1970s to 2006. It’s interesting to note that as the Oceans have cooled over the past two years, activity has dramatically decreased on a global scale. Looking ahead to this year one can expect a slight rebound in this index as the potential for a developing El Nino will lead to a greater number of storms in the Pacific and a larger probability that storms in the Atlantic basin last longer than expected due in part to them remaining out at sea. CLICK ON IMAGE FOR LARGER VIEW
A Review of Last Year:
There were 16 named storms in 2008 which was one less than the year before and 2 greater than last years WTI forecast of 14. Of the 16 storms, 8 of them strengthened into hurricanes and 5 became major hurricanes (WTIs forecast was for 7 Hurricanes with 4 storms reaching major hurricane status). Overall the number of named storms was still above normal as were the number of strong hurricanes (Cat 3 or higher). The season began in late May with Tropical Storm Arthur and ended in early November with Hurricane Paloma. Notable storms in 2008 were Gustav, Hanna, and Ike. Gustav was a major hurricane before interaction with the Island of Cuba weakened the storm before eventual landfall in Louisiana. Hanna was a short-lived hurricane but a long lived tropical storm which almost single handedly refilled a drought drained Lake Okeechobee in Florida. Some areas in Florida saw over 2 feet of rain from Hanna. The most damaging storm of the year was Hurricane Ike which brought near total devastation to parts of Galveston, TX. Property damage was severe and a number of lives were lost.
