Three Reasons Why Irma’s Florida Strike Wasn’t as Bad as Forecast

Hurricane Irma was a powerful, sprawling storm that decimated parts of the Caribbean and as a weaker tropical storm, it continues to inundate northern Florida and the South Carolina coast. But the storm didn’t obliterate Miami, inundate the Gulf Coast with excessive storm surge or destroy thousands of homes on the west coast of Florida as feared.

Meteorologists say there are three main reasons why.

First, Irma ran low on fuel. The storm had grown ferociously for more than a week, forming off the west coast of Africa and fueled by the evaporation of unusually warm waters. It traveled unimpeded by land, building up intensity and forming a massive and symmetrical Category 5 storm.

But after nicking the coast of Cuba Friday, the hurricane winds slowed and Irma was downgraded to Category 4. Even a comparatively brief brush with land can cause speed to diminish rapidly and destabilize a storm’s internal dynamics, said Gary Lackmann, a climatology professor at North Carolina State University.

“Hurricanes don’t survive very long when they’re over land,” he said. They need warm water, he said.

If Irma had taken a track just 10 to 15 miles north, the storm would have likely missed Cuba and continued to intensify, Mr. Lackmann said. Instead, the storm never regained its previous ferocity of winds of 160 miles an hour.

Then, the winds changed.

A hurricane is influenced by thousands of variables, from other storms faraway to changes in water temperature to wind patterns.

The winds had been relatively calm and consistent around the storm until Irma approached the Florida Keys, when it encountered a different weather system. The winds at the upper level of the atmosphere where planes fly were blowing much stronger south to north than the winds at the surface of the Earth, said Brian Tang, a hurricane expert at the State University of New York at Albany. The upper-level winds sheared off part of the top of the storm.

Those factors contributed to a shift in Irma’s path from first aiming at Miami to then turning to Tampa Bay. Ultimately, on Sunday, after making landfall on Marco Island, Irma went right up the center of the state on a new path altogether and lost power.

“Think of the hurricane as a cork in a stream and the river around it moves the cork,” Mr. Tang said.

Predicting Irma’s Storm Surge

As forecasts for Hurricane Irma’s path changed, so did predictions of the threat of storm surges to Florida and other states.

Predicted storm surge

7 ft.

5 ft.

3 ft.

10 ft.

A 1 in 10 chance of exceeding ground level by:

Irma’s

actual

route

Irma’s projected

route

GEORGIA

FLA.

Monday

Thursday

Saturday

Irma was expected to move north on Florida’s east coast and on into southern Georgia and South Carolina, where storm surges were expected.

As the storm dissipated, so did the intensity of predicted storm surges. But its route through central Florida threatened flooding in some areas.

Forecasts for Irma’s route shifted west, showing the storm turning north along Florida’s west coast and making landfall south of Tampa.

Predicted storm surge

3 ft.

5 ft.

10 ft.

7 ft.

A 1 in 10 chance of exceeding ground level by:

Irma’s

actual

route

Irma’s projected

route

GEORGIA

FLA.

Saturday

Thursday

Monday

Forecasts for Irma’s route shifted west, showing the storm turning north along Florida’s west coast and making landfall south of Tampa.

As the storm dissipated, so did the intensity of predicted storm surges. But its route through central Florida threatened flooding in some areas.

Irma was expected to move north on Florida’s east coast and on into southern Georgia and South Carolina, where storm surges were expected.

Predicted storm surge

3 ft.

7 ft.

5 ft.

10 ft.

A 1 in 10 chance of exceeding ground level by:

Irma’s

actual

route

Irma’s projected

route

GEORGIA

FLA.

Saturday

Monday

Thursday

Forecasts for Irma’s route shifted west, showing the storm turning north along Florida’s west coast and making landfall south of Tampa.

Irma was expected to move north on Florida’s east coast and on into southern Georgia and South Carolina, where storm surges were expected.

As the storm dissipated, so did the intensity of predicted storm surges. But its route through central Florida threatened flooding in some areas.

Predicted storm surge

A 1 in 10 chance of exceeding ground level by:

5 ft.

7 ft.

10 ft.

3 ft.

Irma’s projected route

GEORGIA

FLA.

Thursday

Irma was expected to move north on Florida’s east coast and on into southern Georgia and South Carolina, where storm surges were expected.

Saturday

Forecasts for Irma’s route shifted west, showing the storm turning north along Florida’s west coast and making landfall south of Tampa.

Irma’s

actual

route

Monday

As the storm dissipated, so did the intensity of predicted storm surges. But its route through central Florida threatened flooding in some areas.

Source: National Oceanic and Atmospheric Administration

A third reason was the weakening of the eyewall, the whirling vortex of intense wind and heavy rain at the center of a hurricane. The rainstorms encircling the eye moved less quickly and evenly—in part because of the loss of fuel and the storm getting knocked off-kilter by wind shear. By late Sunday, satellite images of Irma looked like a wheel with its bottom broken, meteorologists said.

When the storms are circling the eye symmetrically, the eyewall strengthens and the hurricane moves faster and faster. It is the reason an ice skater draws in her arms, Mr. Tang said, to spin faster and more stably.

A hurricane “is like a perfect engine,” Mr. Tang said. “Even if you mix just a little bit of water in your gasoline, it’s probably going to start knocking, because you’re disturbing the machinery of the hurricane itself.”

Write to Valerie Bauerlein at valerie.bauerlein@wsj.com