tampabay.com

Westerly winds vanquish storms, but for how long?

By PAT FARNAN
Published August 29, 2006


As of Sunday afternoon, forecasters were predicting that Ernesto would enter the Gulf of Mexico this week as a major hurricane and head for the Tampa Bay area.

Then the storm lost its punch, took a dramatic jog and eased anxieties on Florida's west coast.

The cause?

Westerly winds - the nemesis of Atlantic hurricanes - sheared Ernesto and pushed it north into the 8,000-foot-high mountains of Haiti, further destabilizing the storm.

Ernesto was downgraded from a hurricane of at least 74 mph to a tropical storm with winds of about 50 mph. By Monday afternoon the storm had weakened further after slogging through the mountains of Cuba.

So far, westerly winds have been a prominent feature in the 2006 hurricane season, thwarting the formation of some storms and weakening others.

In the case of Ernesto, the winds came from a massive low pressure system in the southwestern gulf. But westerly winds from other systems have helped prevent the formation of storms in the central Atlantic Ocean this summer.

"We've had far more shear this year in the upper levels of the atmosphere, and it's sheared some of these systems apart," said Chris Landsea, science and operations officer for the National Hurricane Center in Miami.

So where were these winds in the last two years, when hurricanes clobbered Florida?

It's unclear.

"We know in active hurricane periods ... we have a couple of factors," Landsea said. "One, average water temperatures are a little high. Two, upper-level winds shift from the west to the east.

"In the last 11 hurricane seasons in the Atlantic, nine of 11 have been above normal with more of these east winds showing up," he said.

Those upper-level winds are in the upper troposphere, roughly 30,000 feet to 45,000 feet. Why they seem to come from the east in active hurricane seasons and the west during less active periods is the subject of much uncertainty.

"We do know that within a season we might have cycles where (westerly winds) are strong, and then we have a few weeks when we have a slackening period" more favorable to hurricane formation, Landsea said.

That's a normal scenario.

"The last two years the hurricane season started so early that it was a little misleading," said Mark Mcinerney, meteorologist with the National Hurricane Center in Miami. "We are in more of a normal pattern."

That means more storms.

"Overall, the expectation is still a very busy season," Landsea said, "and we've had just one hurricane so far."