Ever heard of the Fujiwhara effect?

In previous articles, I have mentioned many different types and names of meteorological phenomena. There is another one that began to come up in the news media called the Fujiwhara effect. This occurs when two storm systems, such as tropical cyclones, hurricanes or even mid-latitude low-pressure systems, interact with one another. When they get close enough, these storms can engage in a kind of “atmospheric dance,” orbiting around a common center, altering each other’s paths and even merging into one bigger storm.

Depending on the storm’s relative sizes, distances and intensities, several outcomes are possible. The two storms may simply orbit each other and then drift apart, they may merge into a single, larger system or one storm may dominate, absorbing the other and perhaps creating a much stronger single system known as a “binary cyclone.”

As of the weekend, forecasters were indicating that this scenario had the potential for development as Hurricane Humberto coulc combine with Tropical Storm Imelda in the northern Caribbean and affect the southeastern coastline of the U.S. this week.

The Fujiwhara effect is named after Japanese meteorologist Sakuhei Fujiwhara (1884-1950), who many consider to be a pioneering Japanese meteorologist. In the early 1920s, Fujiwhara conducted experiments in fluid dynamics using water tanks to study how vortices behaved when placed in close proximity to one another. In 1921, he published his findings in Japanese, followed by an English paper in 1923. His results demonstrated that two vortices of similar size would orbit cyclonically around a midpoint between them, while a stronger vortex would dominate, potentially absorbing the weaker one. These ideas, originally confined to laboratory experiments, later proved to be invaluable for understanding large-scale atmospheric behavior, especially with the addition of satellite technology.

One of the early documented cases occurred in 1964, when Typhoon Marie and Typhoon Kathy interacted in the western Pacific. Their mutual orbiting confirmed Fujiwhara’s early predictions. Subsequent decades provided numerous additional examples, particularly in the Pacific, where tropical cyclones are abundant. In 1974, Typhoon Irma and Typhoon Carmen demonstrated the effect with partial merging.

The Fujiwhara effect is rare but significant in the Atlantic Ocean. While the Pacific remains the most common stage for these vortex “dances,” the Atlantic has seen some important historical examples. In 1995, Hurricanes Iris and Humberto did orbit around a shared center of circulation, with Iris eventually absorbing Humberto, which led to a stronger system. More recently, in 2017, Hurricanes Hilary and Irwin in the eastern Pacific engaged in a striking Fujiwhara interaction, with Hilary dominating and tugging Irwin along and eventually altering its track.

The Fujiwhara effect can make weather forecasting more difficult, especially in the Atlantic Ocean, where the phenomenon is a rare event. When two storms come close enough to interact, their paths can change dramatically, sometimes unpredictably. This scenario would likely have major implications for regions at risk of landfall, since a storm expected to follow one trajectory may suddenly veer off in another direction due to Fujiwhara interaction.

The Fujiwhara effect can also apply to other atmospheric phenomena, including polar lows and extratropical cyclones, which are large low-pressure systems that form outside of tropical regions. In the case of Superstorm Sandy in 2012, the massive storm was not an instance of the Fujiwhara effect. Instead, Superstorm Sandy resulted from the interaction between a hurricane and a large extratropical storm system, where the extratropical system's upper-level trough steered and influenced the hurricane, causing it to curve into the East Coast of the United States, which resulted in nearly $69 billion worth of damage.

 In terms of the 2025 tropical storm and hurricane season, we’ve passed the peak, but it doesn’t officially end until Nov. 30. As of the weekend, there have been eight named storms and three hurricanes and both were “major.” Hurricane Humberto is the latest to become a major hurricane as it intensified into a Category 5 on Sept. 27.

For September, the Spokane and Coeur d’Alene region will end up with precipitation amounts below normal for the seventh month in a row. Cliff’s station only picked up 0.09 inches. Thanks to the many microclimates in the local area, some of the outlying regions received more rainfall. Early last week, for example, the Athol area received about an inch of rain while other locations barely reported any rainfall. The Spokane International Airport did a little better than parts of Coeur d’Alene, as its moisture total for September was 0.42 inches.

There is a decent chance that conditions will start reversing toward the wetter side, especially in the coming months. The long-range forecast models indicate a series of storms that are expected to bring some much-needed rainfall to the region in the early-to-mid October period. With an expected cooler La Niña, forecasters are pointing to an increase in rain and snow in the coming months across the Pacific Northwest. However, this new La Niña is expected to be relatively weak and brief. New forecast models are indicating that sea-surface temperatures are expected to return to “neutral," or as we call it, a “La Nada” in early 2026. Stay tuned.

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Contact Randy Mann at [email protected].