Musings: A Milder 2012 Hurricane Season Forecast
The tropical storm forecasting team of Phil Klotzbach and Bill Gray at Colorado State University (CSU) issued their first detailed forecast for the 2012 Atlantic Basin storm season. Given their long-standing efforts to perfect a forecasting model for this meteorological phenomenon, the team introduced a new extended-range, early April statistical prediction scheme. Their new model employs 29 years of past data and also utilizes analog predictors. The bottom line is that these forecasters believe 2012 will be a below-average hurricane activity season due to the combination of a cool tropical Atlantic Ocean and the potential development of El Niño in the Pacific Ocean.
The first CSU forecast of the year calls for 10 named storms to form with only four developing into hurricanes and just two of them becoming major hurricanes. Not only are the forecasters expecting this hurricane season to be below-average, they believe the probability of at least one major hurricane (Category 3-4-5) making landfall on the U.S. coastline is below average, too. At the present time, the model suggests the probability of a major hurricane landfall on the entire U.S. coastline is only 42 percent, which compares to the average for the past century of 52 percent. If the target is just the entire East Coast, including the peninsula of Florida, then the probability is 24 percent versus a 31 percent historical average. The probability of landfall on the Gulf Coast is also 24 percent compared to the historical 30 record.
As a presentation by a NOAA scientist about the 2011 hurricane season demonstrated, there have not been a lot of hurricane landfalls on the U.S. coastline since 1995, but as they, and all forecasters, point out, it only takes one storm hitting the shore to cause extensive damage and loss of life. Notwithstanding that warning, however, the lower-than-historical probabilities for a severe hurricane landing on the U.S. coastline has to be considered good news – not only for residents but also for oilmen and all others who earn their livings from the coastal waters.
The CSU forecasters issued a report early last December in which they announced they were abandoning their normal early 2012 forecast regime in lieu of providing a qualitative assessment of those conditions they believed would impact their traditionally more accurate spring forecast. This change was in recognition that their forecasting record of the coming hurricane season made in December had not proven meaningful. What they did do in their qualitative assessment report was to set forth four possible scenarios for the forces that influence the formation and intensity of tropical storms. For each scenario, they assigned a projection of the number of resulting storms. One scenario involved a continuation of the strong Atlantic Basin Thermohaline Circulation pattern that has been in place since 1995 along with the development of a significant El Niño. They rated that scenario as having a 30 percent probability, the second highest of their four scenarios. That scenario was estimated to produce 8-11 named storms, 3-5 hurricanes and 1-2 major hurricanes. The April CSU forecast fits this December scenario.
The team analyzed the factors they are employing in their forecasting model and reached the following conclusion: "…based on the above information, our best estimate is that we will likely transition to neutral conditions in the next few weeks with a possible transition to El Niño conditions during the early part of the hurricane season." The forecasters warn that this transition needs to be monitored closely as the early spring is the time when the factors influencing this transition are most at risk of altering their trend, which might force them to have to adjust their model. Although 2/3rds of forecasters who input to the European forecasting model, which the CSU team is using, expect a strong El Niño to develop this summer, it certainly is not a given and any change could impact the conditions that influence the formation and strength of hurricanes.
Another step the CSU team took in preparation of its forecast was to utilize analog years to modify their final forecast. They found four years in recent history where the early spring meteorological conditions and the outlook for their development during the hurricane season were similar to current conditions and expectations for changes in the coming months. Those years were: 1957, 1965, 2001 and 2009. The details about the years are displayed in Exhibit 15. Most of the information in that table is straight forward having to do with storm types and number of days. However, most people probably are not familiar with the terms – ACE and NTC. ACE (Accumulated Cyclone Energy) is a measure of a named storm's potential for destruction by wind and storm surge. It is a mathematical calculation of wind speed measured during intervals of the storm's existence. For 1950-2000, the average ACE reading is 96.
NTC (Net Tropical Cyclone Activity) is the seasonal average of the various activity measures: Named Storms, Named Storm Days, Hurricanes, Hurricane Days, Major Hurricanes, and Major Hurricane Days. The 1950-2000 value of this parameter is 100. It is interesting that in three of the analog years, both ACE and NTC measures are below their historical averages as well as the number of storms, etc. However, 2001 stands out because its record is well above the historical measures. The average of all four years falls below the average measures. Based on these analog years, the CSU forecasters have adjusted downward their model's statistically-generated results.
When we consider the CSU professors' April forecast, it is interesting to note how closely the estimates are to the actual storm results experienced in 2009, which just happens to be one of the four analog years. What also stands out is how different the 2009 hurricane season was compared to the two years before and the two years after. It is equally interesting to contemplate just how different 2012 will be compared to recent history if the upcoming storm season unfolds as forecasted.
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