The National Oceanic and Atmospheric Administration's (NOAA) Climate Prediction Center last week joined a growing number of forecasters that have trimmed the number of hurricanes they say will form this year, while still predicting an above-normal number of storms.

According to NOAA's update to the 2007 Atlantic Hurricane Season Outlook, it is likely there will be a total of 13-16 named storms, with seven to nine becoming hurricanes, including three to five becoming major (Category 3 or higher) hurricanes. The majority of tropical storms and hurricanes are expected to form over the tropical Atlantic Ocean, which is typical for above-normal seasons. Those systems generally track westward toward the Caribbean Sea and/or the United States as they strengthen. The number of storms forecast is slightly lower than in May, when NOAA predicted 13-17 storms, with seven to 10 becoming hurricanes and three to five becoming major hurricanes (see NGI, May 28).

The climate patterns responsible for NOAA's above-normal forecast continue to be the ongoing multi-decadal signal (the set of oceanic and atmospheric conditions that have spawned increased Atlantic hurricane activity since 1995), warmer-than-normal sea surface temperatures in key areas of the Atlantic Ocean and Caribbean Sea and the continued La Nina-like pattern of tropical convection (an unusual cooling of the Pacific Ocean's equatorial waters), according to Gerry Bell, lead seasonal hurricane forecaster at the Climate Prediction Center in Camp Springs, MD.

"Most of the atmospheric and oceanic conditions have developed as expected, and are consistent with those predicted in May," said Bell. "The biggest wild card in the May outlook was whether or not La Nina would form, and if so, how strong it would be.

"Today's El Nino/La Nina forecast from the Climate Prediction Center indicates a slightly greater than 50% probability that La Nina will form during the peak of the hurricane season. But more importantly, we are already observing wind patterns similar to those created by La Nina across the tropical Pacific Ocean and Caribbean Sea that encourage tropical cyclone development. The conditions are ripe for an above-normal season."

Hurricane seasons during 1995-2006 averaged 14.4 named storms, 8.2 hurricanes and four major hurricanes. NOAA classifies nine of the last 12 hurricane seasons as above normal, with seven being hyperactive. Only three seasons since 1995 -- the El Nino years of 1997, 2002 and 2006 -- have not been above normal.

The NOAA report follows announcements by other forecasting groups that said they were trimming the number of hurricanes they were predicting for 2007. Colorado State University weather forecasters Aug. 3 slightly lowered their odds that a major hurricane will strike the U.S. coastline this year. But they continue to call for a "very active" Atlantic basin season (see NGI, Aug. 6). In July, WSI Corp. cut the number of storms it forecast, predicting 14 named storms, six hurricanes and three intense hurricanes before the Atlantic hurricane season ends Nov. 1 (see NGI, July 30). WeatherBug, which owns 8,000 weather-monitoring stations in the United States, maintained its forecast for an above-average storm season in the Atlantic Basin because conditions are becoming "more conducive" to hurricanes.

Standing in contrast to most of the other forecasts was UK-based Meteorological Office's (MET) prediction that the 2007 Atlantic tropical storm season will bring "below normal" activity relative to the last 15 years (see NGI, June 25).

Energy traders have expressed some disappointment in the Atlantic hurricane season's inactivity so far -- the official Atlantic hurricane season began June 1 -- compared to some of the loftier forecasts issued earlier this year. While there have only been three named storms so far this year -- Andrea, Barry and Chantal -- the peak activity occurs from August through October. Three named Atlantic storms at this point in the hurricane season is slightly above average, NOAA said.

Last year, seasonal hurricane predictions proved to be too high when an unexpected El Nino rapidly developed and created a hostile environment for Atlantic storms to form and strengthen (see NGI, Dec. 4, 2006). When storms did develop, steering currents kept most of them over the open water in the middle of the Atlantic.

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