Nearly 30 years after Iris absorbed Humberto, forecasters watch Humberto and Imelda for signs of the rare Fujiwhara Effect that can shift storm paths.

Both Hurricanes Humberto and Imelda are now expected to stay out at sea. It's due, in part, thanks to something called the "Fujiwhara effect."

Tropical Storm Imelda and Hurricane Humberto have both strengthened, now lurking a few hundred miles away from one another.

The Fujiwhara effect is rarely seen in the Atlantic Ocean, and though it's still not a sure thing, Tropical Storm Humberto's forecasts are already being impacted by a nearby tropical wave.

Predicting path and intensity of Tropical Storm Humberto and Invest 94L is difficult due to something called the Fujiwhara effect.

The Fujiwhara effect is a semi-rare process that happens when two storms that are relatively close in geography orbit around a shared center point. Sometimes, if the storms are of equal strength, they can spin around each other and then release, going their separate ways. Sometimes they will merge and create a stronger storm.

Storms' intensity, proximity, and size matter, and there could be significant implications if two storms merge.

There is also a danger to those in its path: The Fujiwhara effect has major implications for forecasting, and could alter a storm's track unpredictably, making it harder to warn communities in its path, according to The Weather Channel. This example can be found in the 2017 track of Hurricane Hilary and Hurricane Irwin.

Possible Fujiwhara Effect between Hurricane Humberto and a developing storm in the tropics.

"The Fujiwhara Effect was also observed during past hurricane seasons, such as with Hurricanes Hilary and Irwin in the Pacific (2017), and occasionally in the Atlantic when systems like Hurricane Sandy interacted with nearby disturbances," Mainolfi said.