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Hurricane Charley

Shrinking core reduced storm surge

By JAMIE THOMPSON
Published August 17, 2004

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Despite warnings of 15 feet of stormwater in the Tampa Bay area, Hurricane Charley stirred a significantly smaller 6-foot surge as it veered east and slammed into Charlotte Harbor.

What happened, forecasters say, is that Charley shrank to a remarkably small inner core of winds that generate the bulk of surge. It also sped up to almost 25 mph, giving the storm less time to push water ashore.

"It just looked like we were headed for disaster," said Brian Jarvinen, storm surge specialist with the National Hurricane Center in Miami, "but that thing kept getting smaller."

The change minimized flood damage and may have saved lives, forecasters said.

Storm surge is saltwater flooding that rushes over coastal areas, near where the eye comes ashore.

Long before residents evacuated the coast last week, forecasters predicted a large surge based on previously used computer models that account for pressure, size, forward speed, track and winds.

"The surge numbers that you heard - 10, 15, up to 18 feet - were all based on previous runs," Jarvinen said. "You have to do that ahead of time, predict the worst-case scenario and make sure no one is in harm's way."

As the storm neared the coast, the hurricane center began plugging in fresh data gathered by reconnaissance planes. The models still showed the potential for 10 to 15 feet of storm surge.

That was partly because the storm, coming off Cuba, gained strength and appeared to have a 24-mile-wide core - the area of strongest winds.

"It looked like it was going to be a pretty large storm," Jarvinen said.

Since they didn't know precisely where the storm was going to hit, forecasters ran a variety of scenarios, including the worst case, which had the storm's strongest winds pummeling the heavily populated Tampa Bay area and churning up a devastating surge.

But any variation in position changes the potential for surge.

And Charley changed. The storm quickly went from a Category 2 to Category 4, and it shrank from a core of about 24 miles across to one of about 10 miles.

"I'm flabbergasted," Jarvinen said. "I've never seen one this small. When it actually got inside Charlotte Harbor, the whole core fit inside the harbor. It's unusual for a Category 4."

While the core was about 10 miles across, Charley's eye may have been only 5 miles wide.

Precise measurements are not yet available, but Jarvinen and his colleagues believe the core of Charley would have fit inside the eye of Andrew, already considered a small storm, with an eye of about 12 miles.

As it turned out, most of Charley's wrath was dealt in winds, rather than storm surge. But if Charley had been larger, Jarvinen said, it would have been much more devastating.

"If this storm had a 12-to-15-mile radius," he said, "it could have produced our scenarios."

Charley's speed also impacted the surge, as it didn't have time to churn over the gulf.

"Faster moving storms don't have as much time to push that water inland," said Robbie Berg, meteorologist with the hurricane center.

Additionally, Charlotte Harbor was heading toward low tide when the storm hit about 3:45 p.m., which lessened the amount of water around the shoreline.

The shallowness of the gulf makes it particularly vulnerable to storm surge, Jarvinen said.

In Atlantic storms, a lot of the energy produced by a hurricane is absorbed by the ocean. But in a gulf storm, the energy hits bottom, churning water toward the shoreline, Jarvinen said. That creates an even bigger problem if the storm is headed toward Tampa Bay.

"The added impact of the bay creates a funneling effect," Jarvinen said. The churning water surges into the bay, washing over the farthest point.

A computer model Jarvinen created shows flooding up to 17 feet above sea level in parts of Pinellas and Hillsborough counties.

"There's no doubt in our mind that someday we will see a storm like this and create that type of flooding. The question is when, but someday it will occur."

One of the most devastating storm surges of recent years came with Hurricane Opal, which slammed into Pensacola Beach as a Category 3 hurricane in 1995.

Although the storm packed winds up to 144 mph, its main impact came from storm surge and breaking waves that flooded the coast with 10 to 20 feet of water, according to the hurricane center. Most of the $3-billion in damage was attributed to storm surge.

While the weather service released preliminary estimates of surge, the hurricane center says precise measurements will not be available for several weeks.

Jamie Thompson can be reached at 727893-8455. Send e-mail to jthompson@sptimes.com [Last modified August 17, 2004, 00:04:21]

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