Colorado State University (CSU) weather forecasters on Friday slightly lowered their odds that a major hurricane would strike the U.S. coastline this year, but they continued to call for a “very active” Atlantic basin season. Landfall probabilities for the U.S. coastline “remain above their long-period averages.”

WeatherBug, which owns 8,000 weather-monitoring stations in the United States, on Thursday also maintained its forecast for an above-average storm season in the Atlantic Basin because conditions are becoming “more conducive” to hurricanes.

According to the CSU revised forecast, there is a 68% chance of a storm strike somewhere along the U.S. coast; the 100-year average is 52%. In May, CSU forecasters called for a 74% chance of a major hurricane making landfall on the U.S. coastline (see Daily GPI, June 1). The U.S. East Coast, including Florida, has a 43% chance of a strike, compared with the 100-year average of 31%. And the Gulf Coast from the Florida Panhandle westward to Brownsville, TX, has a 44% chance of a strike, compared with a 100-year average of 30%. Above-average “major” hurricane landfall risk exists in the Caribbean.

CSU’s Department of Atmospheric Science forecast team, led by Philip J. Klotzbach and William Gray, with assistance from William Thorson, said the information through July continued to indicate that this year’s Atlantic hurricane season would be more active than the average 1950-2000 season.

With a slight reduction from its forecasts in early April and late May, the CSU team now estimates that 2007 will have about eight hurricanes (average is 5.9); 15 named storms (average is 9.6); 75 named storm days (average is 49.1); 35 hurricane days (average is 24.5), four intense (Category 3-4-5) hurricanes (average is 2.3); and 10 intense hurricane days (average is 5.0).

“The probability of U.S. major hurricane landfall is estimated to be about 130% of the long-period average,” said the CSU team. “We expect Atlantic basin Net Tropical Cyclone (NTC) activity in 2007 to be about 160% of the long-term average.”

The forecasters said there were “slightly less favorable conditions in the tropical Atlantic” for storms. “Sea surface temperature anomalies have cooled across the tropical Atlantic in recent weeks, and there have been several significant dust outbreaks from Africa, signifying a generally stable air mass over the tropical Atlantic.” El Nino/southern oscillation (ENSO) conditions have trended slightly cooler over the past few weeks, and the forecasters said they expect “either cool neutral or weak La Nina conditions to be present during the upcoming hurricane season.”

CSU’s early August forecast is based on a newly devised extended-range statistical forecast procedure that was developed using 40 years of past global reanalysis data that was tested on an additional 15 years of data. CSU said the new procedure “has shown comparable skill during the first half of the 20th century,” and overall, the scheme explains more than 50% of the variance in NTC activity from 1900-2005.

CSU will issue seasonal updates on Sept. 4 and Oct. 2 with separate forecasts for each month.

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