A wind shear environment unfavorable to hurricane development across the tropical Atlantic, which is being driven by a recently developed El Nino event in the tropical Pacific, has prompted Andover, MA-based WSI Corp. to further reduce the number of hurricanes it expects to form during the 2009 Atlantic hurricane season.

The WSI forecast now calls for a total of 10 named storms, including five hurricanes, with two of them intense (Category Three or greater) forming by Nov. 30. The National Weather Service’s National Hurricane Center has reported no tropical cyclones in the Atlantic basin this year.

In its initial hurricane forecast for the 2009 season WSI had predicted 13 named storms, three of them intense (see Daily GPI, Dec. 29, 2008). But a continuation of relatively cool tropical Atlantic Ocean temperatures, combined with unusually cold temperatures in the eastern equatorial Pacific and normal to above-normal wind shear in the tropical Atlantic, prompted the forecaster in April to reduce those numbers to 11 named storms, including six hurricanes, with two of them intense (see Daily GPI, April 23). Last month WSI said it was sticking to that forecast (see Daily GPI, June 23).

The impacts of the El Nino event — the warming of surface temperatures in the central and eastern equatorial Pacific Ocean, which can influence the formation of Atlantic hurricanes — have emerged more quickly than originally expected, resulting in reduced expectations for the upcoming season, WSI said.

“Ocean temperatures in the tropical Atlantic remain quite cool relative to the temperatures during the last 15, more tropically active years,” said WSI seasonal forecaster Todd Crawford. “Further, the new El Nino event continues to strengthen, resulting in an unfavorable wind shear environment across the tropical Atlantic. The early development of this enhanced wind shear along with the relatively cool tropical Atlantic temperatures will almost certainly result in a less active season than last year, and could potentially result in an unusually quiet season. We have reduced our forecast numbers slightly to account for the impacts of the new El Nino event, and the fact there have been no early season storms through mid-July.”

The number of tropical storms forecast by WSI 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 National Oceanic and Atmospheric Administration (NOAA).

NOAA has said eastern equatorial Pacific sea surface temperatures were at least one degree Centigrade above average at the end of June, signaling the arrival of a new El Nino event (see Daily GPI, July 10). El Nino events, which occur every two to five years and typically last about 12 months, can help suppress Atlantic hurricane activity.

The consensus forecast has been for a relatively mild hurricane season. Colorado State University (see Daily GPI, June 3), NOAA (see Daily GPI, May 22) and AccuWeather.com Chief Long Range Forecaster Joe Bastardi (see Daily GPI, May 15) have all predicted fewer hurricanes this year.

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