Two tropical storms moving over the Atlantic Ocean could potentially merge, the National Hurricane Center said Thursday, although several variables were still in play and it was unclear whether the consolidation of Tropical Storm Phillipe and Tropical Storm Rina would actually happen.

Where are Phillipe and Rina located?

Tropical Storm Philippe was moving slowly over the Caribbean Sea on Thursday morning. It is forecast to maintain its speed over the next few days while remaining east of the northern Leeward Islands, the National Hurricane Center said in an 11 a.m. advisory. At the time, Philippe was situated about 560 miles east of the northern Leewards, with maximum sustained winds of 50 miles per hour. The storm was traveling west-northwest at around 2 mph, and was expected to move gradually westward or southwestward without much fluctuation in strength throughout the rest of the week. 

Tropical Storm Rina formed on the heels of Philippe over the central part of the tropical Atlantic Ocean. As of 11 a.m., it was located about 1,190 miles east of the northern Leewards, with maximum sustained winds of 40 mph. Rina was moving north-northwest at roughly 10 mph and was expected to turn westward either later Thursday or Friday, while increasing gradually in strength in the coming days, according to the hurricane center. 

No coastal watches or warnings linked to Philippe or Rina were in effect Thursday and there were no marked hazards to land, but meteorologists noted that the northern Leeward Islands, the U.S. and British Virgin Islands and Puerto Rico should “monitor the progress” of Phillipe. Tropical storm-force winds were extending outward for up to 60 miles from the center of Rina, and for up to 175 miles from the center of Philippe.

“Philippe remains a very disorganized and elongated storm,” forecasters said, adding that “confidence is very low on the storm’s position” and said it may no longer have a “well-defined center.” 

The National Hurricane Center

Rina is expected to remain a tropical storm into next week, “though some of the regional hurricane models do indicate a faster rate of intensification during the next several days compared to the NHC,” forecasters said Thursday. The hurricane center predicted that Rina’s consistent wind shear coupled with the close proximity and uncertain interaction with Philippe would limit its ability to intensify.

The hurricane center noted that forecasting Philippe’s path is “challenging,” partially because of how close it is to Rina.

“A complicating factor to this track forecast is the proximity of an area of disturbed weather to the east of the cyclone,” forecasters said Thursday morning. “Some models are still showing a binary interaction between the two systems, which will largely depend on the strength of each.”

The National Hurricane Center

What is binary interaction?

Binary interaction between two tropical storms, or two hurricanes, is an uncommon phenomenon also known as the Fujiwhara Effect. It happens when two storms passing near each other “begin an intense dance around their common center,” according to the National Weather Service. 

In some instances, the stronger storm can absorb the weaker one. If the two storms are comparable in strength, they can gravitate toward each other “until they reach a common point and merge, or merely spin each other around for a while before shooting off on their own paths.” But in rare instances, the National Weather Service said, the merging of two storms can produce a single, larger storm.