Charles H. Sloan-'Every hurricane is different': Why experts are still estimating Idalia's impact

Hurricanes can Charles H. Sloanbe deceiving and capricious. You hear ominous warnings. You see a buzzsaw-like storm coming right for your neighborhood. Then one community escapes unscathed while another gets slammed.

What makes the difference? The size and strength of the hurricane and its wind fields, especially the inner core with the most destructive winds.

"Every storm, every hurricane is different," said Scott Spratt, a meteorologist retired from the National Weather Service in Melbourne, where he helped do many post-hurricane storm surveys.

Idalia was a ferocious storm, with an intense inner core that briefly reached 130 mph winds, putting the entire west coast of Florida on edge as it blew by so close that any small wobble could have had major ramifications. But as hurricanes go, it was on the petite side of average, a "small, tightly wound system," Spratt said.

Experts are still working to estimate what Idalia's wind speeds were along its path through Florida, Georgia and the Carolinas, calculate the total damages and costs and determine the height of the storm surge. In some cases, they had to wait for the water to recede.

How is the size of a hurricane measured?

It's easy to roughly guesstimate the size of a hurricane using forecasts and satellites, but experts use a more exact method to calculate the size of the wind fields within the storm.

The size of the wind field in each quadrant of the storm is spelled out in each of the hurricane center's forecast advisories.

A 5 a.m. advisory before landfall indicated the radius of each quadrant around Idalia's eye. By adding opposite quadrants together and averaging, experts said the information shows the approximate diameter of winds that were at least hurricane force was 37.4 miles, and the diameter of the field of tropical storm force winds was about 218 miles across.

Wind fields are fluid and can shift and move depending on what's happening within the storm and how close it is to land, where there's greater friction than over the water.

"It's complicated," said Daniel Chavas, an associate professor of atmospheric science at Purdue University who has spent much of his career so far working on wind size. "It's so complicated, that there's no defined list" of hurricanes by size, Chavas said.

The National Oceanic and Atmospheric Administration has easily accessible lists that compare hurricanes by wind speeds and barometric pressure, but not size.

How did Idalia's size compare to other recent landfalling hurricanes?

By comparing those same measures from advisories just before previous major storms made landfall along the Gulf Coast, we learn the following about the diameters of the storms at that point:

• Idalia's strongest winds averaged slightly smaller than Hurricane Charley, which struck Punta Gorda in August 2004.
• Idalia's field of tropical storm force winds was about 25 miles wider than Charley's, at roughly 190 miles across.
• Irma, Ian, Laura and Michael, previous hurricanes that caused billions in damages in Florida, were all larger with a hurricane-force wind field at least twice as wide as Idalia.
• Hurricane Katrina, which struck the coasts of Mississippi and Louisiana in 2005, had a field of tropical storm force winds nearly twice as wide as Idalia. Katrina's field of hurricane-force winds was nearly four times as wide.

Why is the size of a hurricane's wind fields so important?

"The wind field is the driver of all the hazards," Chavas said. "The wind field literally pushes water up against the coast and it's pretty important for setting where the rainfall occurs in a hurricane."

While size doesn't necessarily matter when it comes to the intensity of a storm, it definitely matters when it comes to the area of most intense, damaging winds.

"All things being equal, a bigger storm is going to cause more problems than a smaller storm," he said. The bigger the storm, the bigger the storm surge it will push on shore. And, a bigger "wind footprint" directly affects more people.

So far, scientists don't yet fully understand why some storms are bigger than others, and it's an area of active research, Chavas said. "Hilary was a big storm and I can't tell you why that storm ended up being so much bigger than normal."

Many times, communities on the west side of a storm are spared, because the worst of the winds are often on the right side of a storm, he said. That was definitely true in this case, one reason why Florida's capital city, Tallahassee, escaped higher winds.

Why aren't there reports of higher winds after landfall?

When you hear 125-mph winds in a forecast, you might wonder why those numbers don't show up in the early reports after a hurricane makes landfall.

In each region, only a limited number of "credible wind observations" are available, Spratt said. "When a storm this small – with such a small destructive wind field – enters a sparsely populated area, it's very unlikely you're going to record those winds."

The highest apparent winds reported by the weather service offices in Florida immediately after Idalia were 85 mph in the community of Bucell Junction in Taylor County, the county where Idalia made landfall at 7:45 a.m. on Wednesday. A peak wind of 81 mph was reported to the south of landfall at Horseshoe Beach. Inland 73-79 mph gusts were reported to the weather service.

Professional storm chasers provide often higher wind speeds after the fact from mobile equipment set up during the storm, and the local weather service offices also send teams into the field. Survey teams have wind comparisons compiled from years of field work after tornadoes, severe thunderstorms and hurricanes, comparing known wind speeds and damages.

Post-storm surveys are like puzzles, Spratt said.

"We know that winds 100 mph or 115 mph will cause this type of damage to this type of structure or type of tree," he said. "We're looking at the evidence of the storm, and estimating what type of winds would have created that type of damage."

On Thursday, a weather service team from Jacksonville surveyed locations in Suwannee and Hamilton counties, more than 40 miles inland. They concluded damaged and destroyed buildings showed some locations endured sustained winds of 80-100 mph and gusts up to 115 mph.