Hurricanes Hilary and Irwin in the Pacific Ocean.

Yep, we’ve been amused that hurricanes Don and Hilary spun up simultaneously. Don never was really able to mature — instead, soon after receiving his “hurricane” title, he fell apart. But the short-lived storm managed to stir up some big-time waves.

Meanwhile, in the Pacific, Hilary was putting on quite a show.

Hurricane Hilary exhibited a rarely seen phenomena that occasionally accompanies particularly active regions of tropical development — the Fujiwara Effect. As Hilary shimmied across the Pacific, she approached tropical storm Irwin, encroaching to within 600 miles of the tiny storm on Thursday.

When the two neared, they began to dance in a style rarely seen by tropical cyclones.

(The Weather Channel)

The Fujiwara Effect is most commonly visible across the Pacific, and occurs when two or more tropical systems draw sufficiently near to interrupt each other’s traditional outflow/inflow pattern.

Eventually, the upper-level outflow bands of one storm descend, are compressed, and warm up in temperature — soon acting as the inflow of the other. When this occurs, they become “pinwheel cyclones,” and begin rotating around each other.

Hurricane scientist David Longshore ascertained that this takes place when two storms of similar strength (within 20 millibars of central barometric pressure) find themselves “between 600 and 900 miles apart.” Prior to interaction, their respective motions must be alike as well.

Generally speaking, the two cannot dance together for more than three days, according to Longshore. Eventually they will merge into one, large convective mass and establish a new center of rotation — or, what is more likely, one storm dies and spirals inward to fuel the dominant cyclone.


The influx of additional heat, moisture, and positive vorticity (atmospheric spin) may even help to intensify the primary circulation.

Amid this seemingly complicated process, the computer models used by forecasters illustrated this quite well in the days leading up to Saturday’s tango. As of early Sunday, Irwin found himself as the stronger system as a 60-mph cyclone, while Hilary, due north, appeared to be losing ground fast.

Though the two were rather close, their motions over the weekend were in very different directions — at 2 a.m. Sunday, Irwin was moving due north at 9 knots, whereas Hilary was heading west-northwest at 12 knots. However, these movements would quickly change as the two continued to do-si-do before their demise into tropical depressions Monday morning.

While not a typical run-of-the-mill scenario, there have been several notable hurricanes subjected to the Fujiwara Effect, including a fleeting relationship between typhoons Doug and Ellie in August of 1994 in the northeast Pacific. Within a few days of first meeting, they split apart and went their separate ways. When it was clear things were over between them, Ellie stormed out — taking the water vapor and upper-level divergence with her before making landfall in northeast China on Aug. 16.

On occasion, tornadoes can exhibit similar behavior when multiple vortices come in close contact. Generally speaking, it is quite difficult for a parent thunderstorm to produce more than one tornado within eyesight of another. But it has happened — the first recorded instance of a tornadic merger was on March 13, 1990, in Hesston, Kan.

As a new tornado was developing, the weaker of the two roped out, eventually orbiting and finally being absorbed into the developing circulation. This gave rise to an enormous, violent F5 tornado.

“Satellite tornadoes” also can spin up along the periphery of large wedge tornadoes in supercell thunderstorms. In addition, interactions of this sort are sometimes seen within the suction vortices of multi-vortex tornadoes, with each individual whirl rotating about a common axis. In most scenarios, every low pressure system, regardless of its size, tends to have smaller lows spinning about inside it, whether it be a large planetary wave or something as minute as a dust devil.

Both storms meandered over cooler water and quickly fell apart early Monday morning, but not before leaving us some gorgeous imagery to take in and appreciate. After all, while hurricanes may be destructive, their striking beauty and awe-inspiring dynamics are a sight to behold so long as they remain away from land.

Matthew Cappucci is studying atmospheric science at Harvard University. Follow him on Twitter at @MatthewCappucci.