While the quiet start to the 2009 Atlantic hurricane season, which has so far produced no tropical cyclones, does not guarantee the next four months will remain calm, the number of tropical storms likely to form this year is lower than originally predicted, scientists at the National Oceanic and Atmospheric Administration (NOAA) said Thursday.
NOAA now expects that the Atlantic hurricane season, which began June 1 and runs through Nov. 30, will see near- to below-normal activity, with the calming effects of El Nino continuing to develop. But despite the calm experienced in June and July, the historical peak months of the hurricane season -- August to October -- could still produce major storms, according to NOAA.
"Early season activity is generally not a good indicator for the activity during the peak months of the season...even in a below normal season with fewer storms, you can certainly get a major hurricane strike which can cause tremendous damage," said Gerry Bell, lead seasonal hurricane forecaster at NOAA's Climate Prediction Center.
In May, on the eve of the 2009 season, NOAA forecasters said there was a 70% chance of having nine to 14 named storms, with four to seven of those becoming hurricanes, including as many as three major hurricanes (Category 3, 4, or 5) (see NGI, May 25). On Thursday NOAA said it is now predicting fewer storms, with a 70% chance of seven to 11 named storms, three to six of them hurricanes, including just one or two major hurricanes. There is a 50% probability of a near-normal hurricane season, a 40% probability of a below-normal season and a 10% probability of an above-normal season, NOAA said. An average season has 11 named storms, including six hurricanes with two qualifying as major hurricanes.
The number of tropical storms forecast by NOAA would be fewer than occurred during the 2008 season, when a total of 16 named storms, including eight hurricanes, five of them intense, formed in the Atlantic. But it would be about the same as an average hurricane season, which has 11 named storms, including two major hurricanes, according to the NOAA.
Last month NOAA scientists said El Nino -- the warming of surface temperatures in the central and eastern equatorial Pacific Ocean -- had arrived (see NGI, July 13). El Nino events, which occur every two to five years and typically last about 12 months, can help suppress Atlantic hurricane activity.
"El Nino continues to develop and is already affecting upper-level atmospheric pressure and winds across the global tropics," Bell said. "El Nino produces stronger upper-level westerly winds over the Caribbean Sea and tropical Atlantic Ocean, which help to reduce hurricane activity by blowing away the tops of growing thunderstorm clouds that would normally lead to tropical storms."
In addition to suppressing Atlantic hurricane formation, El Nino events can increase storminess across the South, produce winter storms in California and the Southwest, and create less wintry weather across the North. El Nino's impacts depend on a variety of factors, including intensity and extent of ocean warming, NOAA said.
The consensus forecast this year has been for a relatively mild hurricane season. Some forecasters, including Andover, MA-based WSI Corp., have said a new El Nino event, combined with cooler Atlantic ocean temperatures, is likely to make the 2009 Atlantic hurricane season "relatively quiet" (see NGI, July 27). Other forecasters calling for a mild hurricane season include Colorado State University (see NGI, June 8) and AccuWeather.com Chief Long Range Forecaster Joe Bastardi (see NGI, May 18).
The first tropical storm in 2009 with sustained winds of at least 39 mph will be named "Ana," NOAA said. Tropical storms become hurricanes when winds reach 74 mph and become major hurricanes when winds increase to 111 mph.
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