A La Nina event — an unusual cooling of ocean surface temperatures off the western coast of South America — will help produce relatively warm temperatures through November and, despite a slow start to the 2010 Atlantic hurricane season, will be one of the drivers behind the creation of an unusually large batch of tropical storms this year, according to Andover, MA-based WSI Corp.

But WSI for the second time this summer lowered the number of hurricanes in its 2010 Atlantic hurricane forecast.

WSI said it expects 18 named storms, including 10 hurricanes, five of them intense (Category 3 or stronger), to form before the season ends Nov. 30. In its previous tropical storm forecast WSI had called for 20 named storms, including 11 hurricanes and five intense hurricanes (see NGI, July 26). WSI warned that the new 2010 forecast numbers are still well above the long-term (1950-2009) averages of 10 named storms, six hurricanes, and two intense hurricanes and slightly above the averages from the more active recent 15-year period (1995-2009) of 14/eight/four.

“Although the season hasn’t had a record-breaking start, historically warm tropical Atlantic ocean temperatures and an enabling wind shear environment should mean that the upcoming three months will be very active,” said WSI Chief Meteorologist Todd Crawford. The current La Nina event is still expected to create tropical storms in the Atlantic, he said.

“Slow starts during emerging La Nina events are par for the course,” Crawford said. “Over the last seven tropical seasons where a transition to La Nina occurred, 85% of all named storms occurred after Aug. 16. This means we are still very, very early in the season.

“More importantly, however, eastern and central tropical Atlantic sea surface temperatures remain at record warm levels, even warmer than the freakishly active season of 2005. While the early part of the season has been hampered by small destructive pockets of wind shear and widespread dry air, likely parting gifts from the recent strong El Nino event, the atmosphere is now quickly becoming more favorable for tropical development. We have reduced our numbers a bit to account for the lagged start, but are still just as bullish as ever on the remainder of the season.”

The East Coast from North Carolina’s Outer Banks to Maine has “an enhanced risk of hurricane landfall this season,” according to WSI.

“Our model suggests that the threat to the Northeast coast this season is on par with that in Florida and the Gulf coastal states,” Crawford said.

The 2010 hurricane season has gotten off to a quiet start. The first three named storms of the season, Hurricane Alex and tropical storms Bonnie and Colin, created little threat to Gulf of Mexico (GOM) oil and natural gas production. Hurricane Danielle, which formed in the central Atlantic last week, was located about 480 miles southeast of Bermuda Friday afternoon and was expected to pass to the east of Bermuda over the weekend and before turning to the north-northeast early in the week, the National Weather Service (NWS). Tropical Storm Earl, the fifth named storm of the season, on Friday was located about 1,300 miles east of the northern Leeward Islands, moving west at 17 mph with maximum sustained winds of 45 mph. Earl was expected to approach Puerto Rico over the weekend and will likely turn to the west-northwest and away from the GOM, NWS said.

Forecasters at the National Oceanic and Atmospheric Association have said they also expect the La Nina event to bring “significant” hurricane activity this year, despite the hurricane season’s slow start (see NGI, Aug. 9). Colorado State University forecasters have predicted an active hurricane season and said they see a “well above-average probability” of a major hurricane making landfall in the United States and the Caribbean. AccuWeather.com Chief Long Range Forecaster Joe Bastardi has said 2010 could be one of the most active seasons on record (see NGI, May 24).

La Nina, a relatively cold north Pacific and a record warm North Atlantic will also help produce relatively warm temperatures through November, WSI forecasters said. WSI’s seasonal forecast calls for warmer-than-normal temperatures to dominate the country, with the exception of the Pacific Coast states..

“For the September-November period as a whole, we are forecasting 901 gas-weighted heating degree days, 3% less than last year and 9% less than the 1971-2000 mean,” according to Crawford.

But the historically persistent negative phase of the North Atlantic Oscillation, which often acts to keep temperatures down in much of the East, is still a source of forecast uncertainty, Crawford said.

WSI predicts temperatures to average warmer than normal across all of the United States in September, except coast California and the Northwest, which is expected to be cooler than normal.

“Warmer-than-normal temperatures in September will extend the summer season dynamics well into the month,” Energy Security Analysis Inc. (ESAI) Director of Power and Gas Paul Flemming said in a statement issued in conjunction with WSI’s outlook. “Any load effects on price due to a seasonal decline in late summer temperatures will be offset somewhat by the start of the planned generator maintenance season. Northeast power prices should remain firm and gas demand above-normal through the end of summer.

“Natural gas prices should be supported with stronger-than-average demand in September, but we do not expect much upside without a hurricane disruption.”

WSI’s temperature forecast map will stay much the same into October, with the only changes coming in the Northeast and North Central regions, which are expected to see much-warmer-than-normal temperatures. Cooler-than-normal temperatures will move into the Southeast in November, but all of the rest of the country, with the exception of coastal California, will average warmer-than-normal through the month, WSI said.

WSI is scheduled to issue its next seasonal outlook on Sept. 21.

In separate forecast also issued last week, the Commodity Weather Group (CWG) said it expects the core heating season (Dec.-Feb.) to be 29% warmer than the same period last year and the warmest since 2006-07. Energy demand should run lower than normal for key population centers on the East Coast, through the Southern production region, and even into many of the Midwest consuming areas, due primarily to the developing La Nina event, CWG said.

“While the Midwest and Northeast have seen cold winters during strong La Nina events, the confluence of many factors appears to argue against this,” said CWG meteorologist Matt Rogers. La Nina is an unusual cooling of ocean surface temperatures off the western coast of South America.

Bastardi has also said he expects La Nina to influence weather across the United States this winter, though he expects it to produce significantly colder-than-normal temperatures in portions of the western United States.

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