Hurricane Irma: Best and worst case scenarios for Tampa Bay

Every weather update brings more bad news.

It looks more and more likely that Hurricane Irma will hit Florida, but exactly where is hard to say. Some models show landfall around Tampa Bay while others call for a hit closer to Miami.

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What are the best and worst scenarios for the west coast?

CLICK HERE FOR AN UPDATED VERSION OF THIS STORY: Best and Worst case scenarios for Tampa Bay with Irma nearing Florida

There is a slight chance, maybe about 10 percent, that the hurricane — already a Category 5 storm — curves east and back out to the Atlantic, said Jeff Masters, co-founder Weather Underground. That's the best option.

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There's a roughly 20 percent chance that the storm hits Cuba or Hispaniola, Masters said, weakening before it heads toward the mainland United States.

"Cuba has destroyed a number of intense, major hurricanes," Masters said, including Hurricanes Dennis in 2005 and Ike in 2008. That would also be better for Florida.

But the notorious spaghetti plots show a grimmer picture, the variegated lines essentially blocking out the entire peninsula in forecast maps.

"The spaghetti is centered right over the center of Florida and there's a lot of spaghetti to both sides," Masters said.

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The cone of uncertainty (where a storm could make direct landfall) covers the entire southern third of the state.

The message, then, is familiar: Don't panic, but prepare.

In Tampa Bay, the biggest threat from a hurricane is storm surge. A potentially devastating track for Irma would take it up the state's west coast, just offshore, said 10Weather WTSP meteorologist Bobby Deskins. Some models show Irma blowing over Key West with 135 mph winds before moving north as a Category 3 or 4 storm. That would send water rushing over barrier islands and surging into Tampa Bay.

"That's kind of the worst case scenario for us is a west coast brush," Deskins said.

Irma is unique because it has been a powerful storm for a while, and it's getting bigger. On Tuesday at 2 p.m., the National Hurricane Center announced the storm was already the strongest hurricane ever recorded so far east in the Atlantic Ocean. Winds were up to 185 mph.

"The problem with hurricanes is as they get older, as they're around longer, they grow," said Colorado State University research scientist and hurricane forecaster Phil Klotzbach.

Irma has experienced several eyewall replacement cycles — a common process for major hurricanes that widens the windfield, Klotzbach said.

Satellite imagery suggests Irma has already undergone four such cycles, said Derrick Herndon, an associate researcher at the University of Wisconsin-Madison. Another storm that experienced several eyewall replacements was Hurricane Ivan in 2004, he said.

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If Irma misses Cuba and Hispaniola and heads into the Straits of Florida, it would become even more massive. The storm cold undergo another eyewall replacement as it passes over that area, sucking up warm water and intensifying further, said Herndon, who works at the Cooperative Institute for Meteorological Satellite Studies.

The hurricane will likely be very different than the region's last close call, Hurricane Charley, an extremely tight but powerful cyclone that struck Punta Gorda in 2004.

"Irma could miss Tampa by 100 miles and there could still be significant impact in Tampa because the storm is very large," Herndon said.

Some models call for the hurricane to hit the southern tip of the state and spin straight north, which would bring tropical storm force winds and downpours to both the east and west coasts.

"Heavy rain is going to be an issue," Masters said. "I think regardless of what this storm does Tampa is going to get [at least] 5 to 10 inches of rain."

Based on early models, forecasters are citing Hurricane Donna, which struck Florida in 1960, as a good comparison for what Hurricane Irma could potentially do.

LOOK BACK: Hurricane Irma is being compared to Donna, which slammed Florida back in 1960

Masters, in a blog post Tuesday, wrote of how Donna churned through the Florida Keys as a Category 4 storm before hooking into the area around Naples and Fort Myers. The storm pushed into the center of the state as a Category 2 storm, continuing to wreak havoc before spitting back out into the Atlantic.

Donna showed that even cities inland should not feel safe from an approaching storm, Masters said.

"That storm was no joke," Klotzbach said.

But Floridians should also know Irma will not be a perfect duplicate of Donna, said Rick Knabb, a hurricane expert at The Weather Channel and former director of the National Hurricane Center.

"It's definitely a top-end hurricane, Category 5. It's scary," he said. But just where it hits and how strong it will be at that point remains to be seen.

"Irma will write its own story," Knabb said. "It's not going to behave like any past hurricane."

Contact Zachary T. Sampson at zsampson@tampabay.com or (727) 893-8804. Follow @ZackSampson.