Hurricanes avoiding U.S. mainland

But storm pattern may soon change

September 15, 2010|By Frank D. Roylance, The Baltimore Sun

Forecasters predicted a busy hurricane season, and they've been right so far, with 11 named storms and five hurricanes to date. Four reached dangerous Category 4 strength. And the season is only a bit more than half over.

So far, however, the U.S. mainland has been spared. Alex and Hermine dragged heavy rain and flooding from Mexico into Texas; and Earl stirred up deadly surf while cruising offshore. But the coastal states have been in a kind of protective bubble, with no direct strikes.

That could soon change. Forecasters say the patterns of ocean and atmosphere that have turned this season's storms away from U.S. shores are not likely to hold.

"Those things … have us continuing to think that the balance of the hurricane season will be very active, and potentially more threatening to the U.S.," said Bryan Norcross, a hurricane specialist at the Weather Channel.

The National Hurricane Center is watching two powerful storms in the Atlantic. Igor is threatening Bermuda, while Julia remains a threat only to shipping. Both were spinning at Category 4 on Wednesday, only the second time two storms have grown that strong at the same time.

Tropical Storm Karl was crossing Mexico's Yucatan peninsula Wednesday and could reach hurricane strength in the southwestern Gulf of Mexico before striking Mexico again.

But none of these storms pose a risk to the U.S. mainland. Robbie Berg, a hurricane specialist at the National Hurricane Center, said several factors have kept this season's storms away.

"One of the biggest reasons … is it seems the storms are developing a lot further east," Berg said. They spin up almost as soon as they come off the coast of Africa and move over record-high sea surface temperatures in the far eastern Atlantic.

These storms have become very strong, very quickly, Norcross added. And "when you have a strong storm that far east, the likelihood of it making it all the way across the Atlantic is diminished."

Instead, the stronger storms have been caught by upper-level winds from the southwest, which have turned them north. They're steered into low-pressure paths, or troughs, through the high-pressure "subtropical ridge" that typically dominates the Atlantic in summer. This is why Earl stayed offshore, and why Igor is heading for Bermuda instead of Florida.

Weaker disturbances, like the one that spawned Karl, have slipped beneath the upper-level winds and made it to the Caribbean, where "they seem to have a better chance of redeveloping," Berg said.

But instead of turning north into the U.S. mainland, he said, they find a roadblock. "A ridge [of high pressure] over the Central Plains is driving storms to the west, into Mexico."

Patterns change, Berg said. "Some years we think we're safe, and then things change and we get hit by a storm, maybe in October."

Norcross nevertheless thinks that change is coming.

The far eastern Atlantic is cooling, and the fuel to spawn powerful storms quickly is ebbing. "We would expect that some storms would stay weaker and make it further west and take advantage of generally very favorable conditions in the western Atlantic," he said.

"The weather pattern in the Caribbean and the Gulf [also] looks very conducive for development," he said. All that would be needed for later-season storms to reach the U.S. mainland would be for a door to open in the atmosphere.

What has closed the southeastern U.S. to tropical storms is the same mound of high pressure that's made it so persistently warm in the East this summer, Norcross said.

"As we move into fall, we expect that high will not be stuck in its summertime position. It will tend to move out of the way," he said.

"Whenever cool air filters into the South, that's a sign that the high is pushed out of the way," he said. The door will be open to any storms that come along. "The two things have to happen at the same time," Norcross said.

For now, he said, "there is no reason to think that the hurricane season is in any sense winding down."

frank.roylance@baltsun.com

http://twitter.com/froylance

Maryland weather blog: Frank Roylance on meteorology

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