‘Dirty side’ or not? With Hurricane Ian, it could make big difference along Florida coast

No one who lives along the Florida coast wants to see a hurricane bearing down on their home and they get even more anxious when they wind up on what’s known as the “dirty side.”

The dirty side — typically a storm’s front right quarter in the direction it is heading — can be the most powerful and destructive section of an already powerful and destructive hurricane.

That’s underlined by the shifting forecasts this week for Hurricane Ian, churning toward a still uncertain landfall along Florida’s Gulf Coast. On Monday, its projected track just north of the Tampa Bay area was expected to bring some of the worst winds and, most critically, flooding into a region especially vulnerable to storm surge.

But Tuesday, the track had shifted south and computer models showed reduced but still serious surge risks for the Tampa Bay-Bradenton area but heightened ones for Sarasota and communities to the south. Those areas on the dirty side could also see higher wind speeds as well.

One measure of the difference. Before the track shift, the National Weather Service was projecting that Bradenton Beach could see steady winds of 100 mph or so and gusts nearing 120 mph. The latest modeling dropped estimates of winds of 64 mph and gusts to 86.

Here’s what explains the difference:

Read More: Florida Gulf Coast faces massive storm surge from Ian. Sea level rise could play a role

The National Oceanic and Atmospheric Administration describes the “dirty side” as the quadrant where “weather is rougher and more dangerous.” Its location can vary a bit but generally it’s the right front section of the storm in the same direction the hurricane is headed. Science explains it.

The vortex of hurricane winds spin in a counterclockwise motion. When the storm’s forward motion is going in the same direction, the combination can created the strongest and most damaging winds. On the back side of a hurricane, that same forward motion subtracts from the hurricane wind speeds. But it’s the proximity to the eye that is the real issue — the farther from it, the better, “dirty side” or not.

Brian McNoldy, a senior research associate at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science who researches hurricanes and tropical cyclones, said that the “dirty side” to any hurricane encompasses the entire right portion from the eye. But the closer to the eye an area within the “dirty side” is, the more likely it will see the worst winds.

As for wind damage, Ian’s track will dictate the impact for the Gulf Coast.

“They definitely will have some strong winds, it depends on how close the eye wall gets to the coast,” McNoldy said. “If it’s far enough out, the winds won’t actually be too bad. They’ll still be strong, but if it’s a Category 3 hurricane when it’s next to them, then the Category 3 winds will only be within the eye wall. If they’re far enough away, which looks to be the case, they won’t get that kind of wind.”

If Ian strikes as a Category 3 storm, considered a major hurricane with winds averaging from 111-129 mph, the damage could be serious from wind alone. Currently, Ian’s maximum sustained winds are 125 mph. That’s not even counting the storm surge flooding that is also predicted.

“Well-built framed homes may incur major damage or removal of roof decking and gable ends,” NOAA writes of possible damage posed by Category 3 storms. “Many trees will be snapped or uprooted, blocking numerous roads. Electricity and water will be unavailable for several days to weeks after the storm passes.”

At 5 p.m. Monday, the National Hurricane Center predicted Ian would approach the coast as a major storm as it passes near the west-central coast of Florida on Wednesday and Thursday but weaken before landfall. But since Ian is also expected to slow near the Tampa Bay region, it could “prolong the storm surge, wind, and rainfall impacts along the affected portions of the west coast of Florida.” It is not yet clear where the worst of those impacts will strike.

And as of Tuesday’s 8 a.m. update, NOAA forecasts “a reduction in forward speed” for Tuesday night and Wednesday. The storm will make its way toward the west coast on Wednesday, bringing with it torrential downpours and winds that will extend out as far as 35 miles from its center.

An area does not have to be within the “dirty side” or directly near the eye wall to suffer disastrous consequences.

“You don’t have to be that close to get the storm surge,” McNoldy said, “and rain will also be a big problem.”