Eastern Pacific's Hilary, Irwin Entwined in Weird Fujiwhara Effect Generating High Surf in Southern California

At a Glance

• In the eastern Pacific, Hilary and Irwin performed their daisy-wheel dance called the Fujiwhara effect.
• Hilary and Irwin didn't threaten land directly, but generated high surf for Southern California.
• This Fujiwhara effect also happened thousands of miles away in the western Pacific east of Japan just days prior.
• To see this happen more than once a year is meteorologically impressive.

A pair of Pacific tropical storms participated in a bizarre, circular dance known as the Fujiwhara effect, generating high surf along the Southern California coast just days after a similar interaction involving separate tropical cyclones in the western Pacific Ocean.

(MORE: Hurricane Central)

Tropical Storm Hilary and Tropical Storm Irwin, fortunately, did not threaten Mexico's Pacific coast.

Their centers became sufficiently close – just over 400 miles apart – that a phenomenon meteorologists call the Fujiwhara effect kicked in.

Named after a Japanese researcher who discovered this in experiments with water in the early 1920s, the Fujiwhara effect details how two tropical cyclones less than 900 miles apart rotate counter-clockwise about one another.

Think of the teacup ride at Disney or the Tilt-a-Whirl at your local county fair, but with tropical systems instead. In the teacup ride, adjacent teacups can not only spin, but revolve about each other.

In this case, Irwin, the southern storm of the pair, after being stalled for a day or two, got pulled north and revolved counter-clockwise around the weakening circulation of Hilary.

Each storm rotated about each other, before merging and weakening over cooler water in a more stable air mass.

(If you're interested in a more technical explanation, University of Albany, SUNY Ph.D. candidate Philippe Papin tweeted an excellent explanation.)

While neither named storm impacted land directly, there was one impact. Higher swells were forecast to reach the Southern California coast.

Separate West Pacific Dance

More than 5,000 miles away, Typhoon Noru, the first typhoon of 2017, teamed up with another tropical cyclone named Kulap in a separate Fujiwhara effect.

While Kulap had degenerated to a remnant, one could still pick out its leftover circulation in Himawari-8 visible satellite imagery July 27 south-southwest of Noru.

July 25, thanks in part to the Fujiwhara interaction, Noru crossed its path from the previous week completing an oval-shaped loop.

(MORE: Typhoon Noru Rapidly Intensifies After Fujiwhara Effect)

To see this happen once a season isn't unusual, according to a 2014 study in the western Pacific Ocean, but for this to happen in two separate areas within days is very unusual.

Jonathan Erdman is a senior meteorologist at weather.com and has been an incurable weather geek since a tornado narrowly missed his childhood home in Wisconsin at age 7. Follow him on Facebook and Twitter.

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