With Humberto, an ‘Incredibly Complex’ Hurricane Forecast Is Unfolding

A week ago, during an extended period of subdued hurricane activity in the Atlantic, experts who said this year’s hurricane season would be an above-average one were starting to wonder if they’d been wrong.

But the formation of Hurricane Gabrielle and now Tropical Storm Humberto have kick-started what is shaping up to be the Atlantic’s most active period so far this year.

Humberto formed late in Wednesday afternoon in the Central Atlantic. A third potential storm — currently a disorganized area of rainfall over the northeast Caribbean — was poised to become Tropical Storm Imelda in the next few days.

Hurricane Gabrielle, which at one point had become a powerful Category 4 storm, was moving east through the Azores and away from North America. But where Humberto and the potential Imelda go and how intense they will become remains highly uncertain.

The forecasts for those two storms “are incredibly complex,” said Robbie Berg, a meteorologist with the National Hurricane Center in Miami. “Their future paths could depend on what degree they interact with each other, which itself depends on how strong and big they each get.”

There are several possible scenarios.

A lot depends on which storm becomes the dominant one. If it’s Humberto, and it curves back east over the Atlantic — following a path similar to Hurricane Erin this summer — the other storm, Imelda, would weaken or even be engulfed by the stronger system. This is the more likely scenario, and the one many observers are the most hopeful for, because it means the storm will not make landfall.

Forecasters at the National Hurricane Center said in the first update on the storm that there is quite a bit more uncertainty than typical for the forecast beyond three days.

Forecasters with the National Weather Service in the Outer Banks of North Carolina said that they believed both systems would most likely move to the northwest, then north, potentially interacting with each other as they remain nearby and then eventually move off to the north and east together.

The effect of that close interaction is what is most concerning to forecasters, who are also watching for several outlier scenarios that may unfold.

One of those outliers is if the storm that becomes Imelda moves closer to the United States. Even if it doesn’t move inland, it could stay offshore, creating dangerous rip currents, rough coastal seas and even heavy rains, which is what Hurricane Erin did this summer.

The Fujiwhara effect may come into play.

There is also a possibility that if both storms strengthen, a semi-rare process called the Fujiwhara effect may occur, which could cause one storm to spin toward the East Coast.

The Fujiwhara effect happens when two storms orbit around a shared center point. It was named for a Japanese meteorologist, Sakuhei Fujiwhara, who first described the interaction between whirling masses of fluid or air in 1921.

But it is too early to know with certainty whether that will happen.

How to prepare despite the uncertainty.

With Humberto’s formation on Wednesday, Mr. Berg said meteorologists will soon have “more confidence” in understanding how the next few days will unfold.

He said that people along the entire southeastern coast of the United States should watch closely over the next few days as the situations develop.