While all has been quiet on the storm front in the Gulf of Mexico over the past few weeks, recent revised forecasts are calling for more tropical storms and hurricanes than previously expected for the remainder of the 2005 Atlantic storm season.
Coming as the second revised forecast in a week, Colorado State University's (CSU) Tropical Meteorology Project said it now foresees one of the most active hurricane seasons on record. The National Oceanic and Atmospheric Administration (NOAA) revised its storm forecast higher a few days early, warning that up to 14 more tropical storms and five more major hurricanes could still be out there, which does not account for Tropical Storm Irene (see NGI, Aug. 8).
After studying recent weather patterns, NOAA said it now expects a seasonal total of 18-21 tropical storms (mean is 10), with nine to 11 becoming hurricanes (mean is six), and five to seven of these becoming major hurricanes (mean is two to three).
Basing their update on new research along with current meteorological information through July 2005, CSU's William M. Gray and Philip J. Klotzbach said they are revising their outlook.
"We estimate that 2005 will have about 20 named storms (average is 9.6), 10 hurricanes (average is 5.9), 95 named storm days (average is 49.1), 55 hurricane days (average is 24.5), six intense (category 3-4-5) hurricanes (average is 2.3) and 18 intense hurricane days (average is 5.0)," the CSU forecasters said. "We expect Atlantic basin net tropical cyclone (NTC) activity in 2005 to be about 235% of the long-term average. The probability of U.S. major hurricane landfall is estimated to be well above the long-period average."
In late May, the CSU report predicted there would be 15 named storms this season in the Atlantic, eight hurricanes, 45 hurricane days, four intense hurricanes and 11 intense hurricane days. (see NGI, June 6). At that time, the researchers also expected Atlantic basin NTC activity in 2005 to be about 170% of the long-term average.
Gray and Klotzbach added that they also believe there is an above-average probability of U.S. major hurricane landfall. The CSU researchers published probabilities of major hurricane (category three, four or five) landfalls on various coastal areas.
They believe the entire U.S. coastline has a 77% chance of seeing a landfall (the average for the last century is 52%). They said there is a 58% chance for a major hurricane landfall on the U.S. East Coast, including the Florida peninsula (average for last century is 31%). They believe that there is a 44% chance that there will be a landfall on the Gulf Coast from the Florida Panhandle westward to Brownsville, TX (average for the last century is 30%).
Looking at the seasonal analogs, Gray and Klotzbach said the best analog years for 2005 appear to be 1886, 1933, 1966, 1995, 1996, 2003 and 2004. As a comparison, actual activity in 2004 consisted of 14 named storms, 90.25 named storm days, nine hurricanes, 45.5 hurricane days, six intense hurricanes and 22.25 intense hurricane days. Atlantic basin NTC activity was 229% of the long-term average.
Commenting on whether the unprecedented landfall of four destructive hurricanes in a seven-week period during August-September 2004 was related in any way to human-induced climate changes, Gray and Klotzbach said there is no evidence that this is the case. "If global warming were the cause of the increase in United States hurricane landfalls in 2004 and the overall increase in Atlantic basin major hurricane activity of the past 10 years (1995-2004), one would expect to see an increase in tropical cyclone activity in the other storm basins as well (i.e., West Pacific, East Pacific, Indian Ocean, etc.)," they said. "This has not occurred.
"When tropical cyclones worldwide are summed, there has actually been a slight decrease since 1995," they said. "In addition, it has been well documented that the measured global warming during the 25-year period of 1970-1994 was accompanied by a downturn in Atlantic basin hurricane activity over what was experienced during the 1930s through the 1960s."
Instead, the researchers said they attribute the heightened Atlantic major hurricane activity between 1995 and 2004 to multidecadal fluctuations in the Atlantic Ocean thermohaline circulation (THC), as they have been discussing in their Atlantic basin seasonal hurricane forecasts for several years. "Major hurricane activity in the Atlantic has been shown to undergo marked multidecadal fluctuations that are directly related to North Atlantic sea surface temperature anomalies," they said. "When the Atlantic Ocean thermohaline circulation is running strong, the central Atlantic equatorial trough (ITCZ) becomes stronger. The stronger the Atlantic equatorial trough becomes, the more favorable are conditions for the development of major hurricanes in the central Atlantic. Since 1995, the THC has been flowing more strongly, and there has been a concomitant increase in major hurricanes in the tropical Atlantic."
Gray and Klotzbach will be issuing seasonal updates of their 2005 Atlantic basin hurricane activity forecast on Sept. 2 and Oct. 3. These forecasts will include separate forecasts of September-only and October-only Atlantic basin tropical cyclone activity. A verification and discussion of all 2005 forecasts will be issued in late November 2005.
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