Capital Weather Gang • Perspective
We still don’t know how to talk about floods
The Washington PostDemocracy Dies in Darkness

Dissecting the parts of a hurricane

Hurricane Irma. (NOAA)

Irma is a big one. Its circulation spans hundreds of miles in all directions, large enough to fully engulf the entire peninsula of Florida at once. Hurricane-force winds extend outward up to 80 miles from the center, with blustery conditions felt from the Caribbean to the Carolinas.

With impacts felt across such a wide area, it’s important to understand the threats of each part of Irma; whether it’s the spiral rainbands or the infamous eyewall, danger extends well beyond Irma’s center.

The most dangerous ‘right-front quadrant’

Irma is like a giant atmospheric buzz saw, scouring the ground wherever she strikes. But if you picture this counterclockwise spinning saw blade moving along the ground at a forward speed of 15 mph, it stands to reason that winds on the right-hand side would be amplified by this motion. If the storm is spinning at 120 mph and trekking along at 15, then gusts to 135 would buffet areas beneath the right-side eyewall.

Likewise, this effect actually makes life a bit easier on the left side; there, surface winds are reduced to “only” 105 mph, lessening the ferocity of the storm. For that reason, you may have heard meteorologists refer to the “right front quadrant” of Irma as the most dangerous sector of the storm.

Some also refer to this part of the storm as its “dirty side.”

This can also sweep ashore a destructive storm surge, since winds on the right most often tend to be oriented onshore when the storm makes landfall. Subsequently, all the seawater that bulges up beneath the low-pressure of the storm is hurled onto land to the right of the eye, inundating shoreline communities with water several feet deep. More than half of all hurricane-related deaths are a direct result of surge, so it comes as no surprise that riding out a storm on the right of the eye can jeopardize your life.

That area is dangerous for another reason, though. Because the surface winds are stronger there than in other part of the cyclone, they enhance the amount of “wind shear” present. In other words, there is a dramatic change in wind speed and direction with height, and this rotational energy can be tapped into by thunderstorms rotating around Irma’s eye.

This can lead to a slurry of brief tornadoes touching down in this front-ride quadrant. They tend to be short-lived, quick-hitting, and tough to predict — they can spin up with little warning due to cloud bases abnormally low to the ground.

As the spiral rainbands of Irma race ashore, monitor for any tornadic activity within them.

The left side of the storm

Meanwhile, the left (west) half of the storm is a little less dangerous in terms of wind and surge, but still poses a formidable threat for inland flooding. Heavy rain falls everywhere in tropical cyclones; there are no two ways about it. But with dry, dense air circulating in on the backside, moist air can be lofted to give an extra boost to generating excessive rainfall.

The eyewall

Impacts increase dramatically on both sides as one nears the center. Within the eyewall, conditions are some of the most furious that will be found anywhere on Earth. It is comparable in intensity to an enormous tornado, sitting and spinning away over locations for upward of an hour.

Whether you’re in the left or right eyewall has little bearing on intensity; with sheets of rain, sustained winds well over 100 mph, pinpoint lightning strikes, and endless projectiles slicing through the air, many have described this regime as “hell on Earth.”

The eye

By some miracle, this atmospheric battering ram is elegantly juxtaposed against the quiet calm of the eye. Within the eye, nothing moves — and an ominous smell of splintered wood and freshly stripped vegetation seems to suffocate anyone courageous enough to venture out from their shelters.

A few peaks of clear blue sky may sneak in overhead, surrounded on all horizons by the dark, boiling clouds ravaging the landscape just a few miles away. It is here that the storm’s lowest pressure is found, enough of a drop to make your ears pop and cause some folks to feel lightheaded.

The transition from eyewall to eye is sharp — normally occurring in just a few moments. During Irma’s rampage in Barbuda, a gust to 155 mph occurred just six minutes before calm conditions were reported; whether this was due the eye passing overhead, or just a result of the wind sensor blowing away, is unknown.

Regardless, everyone in the Sunshine State is in play. Keep your wits about you, your eye to the sky, and stay one step ahead of Hurricane Irma.