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Two tropical storms spinning in the eastern Pacific Ocean are entering into a rare dance that will see one consume the other, thanks to something known as the Fujiwhara effect.
EAST PACIFIC | Fujiwhara Effect

Two tropical storms start a rare dance in the Pacific Ocean


Scott Sutherland
Meteorologist/Science Writer

Saturday, July 29, 2017, 12:42 - Two tropical storms spinning in the eastern Pacific Ocean are entering into a rare dance that will see one consume the other, thanks to something known as the Fujiwhara effect.

This story has been updated.

The US National Hurricane Center continues to tracking two storms in the eastern Pacific - Tropical Storm Irwin and Tropical Storm Hilary. Both storms have now weakened from hurricane status, and neither is expected to make landfall.


Credits: earth.nullschool.net & Scott Sutherland

With these two continuing to behave as forecast, they are drawing closer to one another, bringing about a rare meteorological treat.


GOES-West visible satellite imagery from July 29, 2017, showing TS Irwin (lower-left of center) and TS Hilary (upper-right of center). Credit: NOAA

As of Thursday, Tropical Storm Irwin and Tropical Storm Hilary are close enough to each other that they are starting to directly interact.


GOES-West infrared false-color satellite imagery from July 29, 2017, showing cloud-top temperatures (redder = colder = higher). TS Irwin can be seen lower-left of center and TS Hilary upper-right of center. Credit: NOAA

With Tropical Storm Hilary being the stronger of the two, it is expected to pull Irwin into an orbital dance that will result in Hilary slowly consuming Irwin.


A computer model simulation of the interaction between TS Hilary and TS Irwin, from July 28 to Aug 3, 2017. Credit: WeatherBELL Graphics

This binary interaction of two storms, in which they orbit about a common point between them, is known as the Fujiwhara effect.

Although the original paper describing this effect, written in 1921 by Japanese meteorologist Sakuhei Fujiwhara, described vortices in water, the effect works exactly the same way when it comes to storm vortices in the atmosphere.

When two storms of equal size interact in this way, they get locked into a circulation where they both orbit around a fixed point between them, in a counter-clockwise direction (cyclonically). This interaction causes them to pull closer together, speeding up in their mutual orbit until they eventually merge into one larger storm.

For tropical cyclones like Irwin and Hilary, they will also begin to slowly orbit around a fixed point between them. Since Hilary is the stronger of the two, though, it will pull Irwin into a counter-clockwise orbit around it. Then, over the days ahead, Hilary will siphon off Irwin's strength, until the smaller storm is finally drawn into to be absorbed by Hilary's circulation.

This entire interaction is expected to take place over the next week, so come back for updates as these two begin their slow meteorological dance.

Sources: National Hurricane Center | earth.nullschool.net | NOAA GOES Imagery

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