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

Jon Erdman
Published: July 28,2017

A pair of Pacific tropical are participating 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.
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Tropical Storm Hilary and Tropical Storm Irwin, are, fortunately, no threat to Mexico's Pacific coast.

Current Infrared Satellite Image

Their centers are now sufficiently close - less than 510 miles apart - that a phenomenon meteorologists call the Fujiwhara effect kicks 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, has temporarily stalled, but will soon get pulled north and will revolve counterclockwise around the circulation of Hilary this weekend, according to the latest forecast guidance.
(FORECAST PATHS: Hilary | Irwin)

ECMWF (European) model forecast from July 27, 2017, of the expected Fujiwhara effect of tropical cyclones Irwin and Hilary. Lower pressure is denoted by the deeper orange and red contours. Higher pressure is shown by the brighter teal, aqua colors.

Each storm will rotate about each other, before merging and weakening over cooler water in a more stable air mass next week.
(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 will impact land directly, there will be one impact into the weekend.
Higher swells may reach the Southern California coast as soon as Friday, continuing into early next week. Minor beach erosion and coastal flooding are possible, particularly on south-facing beaches, according to the National Weather Service.
(MORE: How Eastern Pacific Tropical Systems Can Impact the U.S.)

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 on July 27, south-southwest of Noru.

The remnant of former Tropical Storm Kulap and Typhoon Noru are shown in this visible satellite image from the Himawari-8 satellite on July 27, 2017.
(JMA)

Tuesday, thanks, in part, to the Fujiwhara interaction, Noru crossed its previous path from last week completing an oval-shaped loop, and will now migrate westward.
Thanks to weak steering winds over the northwest Pacific, Noru may loaf and lollygag near Iwo To (previously known as Iwo Jima) well into next week.

Noru Path History and Forecast Path

We still can't rule out an eventual land impact in Japan or southeastern Russia, but that may not come until late next week or the following weekend. In the meantime, Noru may intensify, and is likely to at least generate large swells that will reach the Japanese Pacific coast and persist into next week.
Satellite loops of these dual Fujiwhara events should be compelling to meteorologists and weather enthusiasts alike.
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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