Hurricane Iota was declared a Category 5 monster Monday morning, the buzz saw of atmospheric fury inching toward unavoidable disaster in Nicaragua. Winds of 160 mph swirl around the atmospheric sink drain’s center, with a devastating storm surge and widespread flash flooding expected in Central America.

The storm is set to hit an area that endured a direct strike from Category 4 Eta less than two weeks ago and is the strongest Atlantic hurricane ever to develop this late in the season.

The sun sets on a catastrophic storm

The NOAA GOES East weather satellite captured this chilling loop of Hurricane Iota’s eye as the system prepared to make a catastrophic landfall in Nicaragua on Monday evening. Shadows could be seen lengthening as the satellite peered straight down into Iota’s eye, the low sun angle revealing roiling “overshooting tops” of tall thunderstorms in the eyewall.

It’s the seventh Category 5 eyewall the sun has set on over the Atlantic in the past five years, and probably the last visible glimpse we’ll have of Iota before inevitable disaster ensues in hard-hit Nicaragua.

Inside the eye

The Air Force Reserve Hurricane Hunters captured a striking photo inside the eye of Iota on Monday, revealing a small swirl of low-level cloudiness wrapped up inside the 12-mile-wide eye. The blue sky reached above the ominous void, belying the fury that lurked nearby.

An eyewall shrouded in electricity

Iota’s furious eyewall was crackling with electricity Monday afternoon, a rarity in tropical storms and hurricanes. Usually, the updrafts and sideways spiral of air is insufficient to induce charge separation, which gives rise to lightning. But not with Iota. When a hurricane produces frequent lightning in its eyewall, you know it’s a top-tier storm.

Iota’s lightning formed a continuous wall that surrounded its eye, the “enveloped eyewall lightning” signature that is a staple of a rapidly intensifying storm. A Hurricane Hunters mission even reported hail at flight level in the eyewall Sunday night, virtually unheard of in hurricane eyewalls.

A slice through the monster

This is a slice through a simulation of Hurricane Iota using the HWRF model, which specializes in tracking tropical systems. Air pressure, analogous to height in the atmosphere, is plotted on the vertical.

The brown strip at the surface represents the air pressure at ground level. Near the middle of Iota, the air pressure is extremely low because of the strength of the storm’s vortex and the air being evacuated up and away from the center. That’s why the brown shape bulges upward, since the air pressure is more characteristic of something you’d ordinarily find a half-mile above the ground.

In the eyewall, the doughnut of extreme winds surrounding Iota’s center, gusts topping 160 mph are likely — especially a few hundred feet above the ground. But in the eye, there’s an oasis of calm.

Sinking in Iota’s eye

The same weather model also revealed the striking inner structure of Iota, a commonality of most high-end hurricanes. In the lowest 10,000 to 15,000 feet of the atmosphere, the air is saturated with extreme humidity, accounting for the exceptional rainfall and moisture field associated with storms like Iota.

But in the eye, a column of dry air is present. Everywhere else in the storm, air rises, but in the eye, it sinks. Dry air from the upper atmosphere descends, eroding cloud cover and clearing out the eye in the most powerful hurricanes. In Eta, that’s no exception.

That sinking air is also very warm. On Monday morning, the Hurricane Hunters encountered a temperatures spike of 25 degrees flying into the eye. Outside the eye, temperatures hovered around 50 degrees, but a column of 75-degree warmth extended nearly 10,000 feet high within it.

Trochoidal wiggles

The most powerful hurricanes occasionally appear to “wobble” as they churn, and Iota is no exception. Early Monday, its eye was organizing, the ragged, misshapen feature becoming carved out into a perfect circle. That’s around the same time Iota’s winds spiked to Category 5 strength.

As it marches west, however, it’s easy to spot a series of slight meanderings north and south of the track’s centerline, as though the eye is periodically sidestepping roadblocks. These are called “trochoidal oscillations,” and they result from intense thunderstorm activity in Iota’s eyewall. If an individual thunderstorm updraft on the south side of the eye becomes excessively vigorous, for example, the whole eye and eyewall may be tugged momentarily south.

Sometimes, these wiggles can be the result of an eyewall replacement cycle, meaning a second ring of violent thunderstorms and winds could be forming and surrounding the first, which the storm may soon shed.