Tropical Storm Philippe is expected to become a Category 1 hurricane later this week with top winds of 85 mph, while a downgraded Rina is expected to dissipate Monday.

The two neighboring tropical systems had interacted last week in an uncommon phenomenon in the Atlantic. The Fujiwhara Effect is a binary interaction where two tropical cyclones within 345 miles to about 860 miles of each other start to spin around a common point, according to the National Oceanic and Atmospheric Administration.

“Two storms closer in strength can gravitate towards 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,” the National Hurricane Center said. Rarely, it can create one larger storm rather than two smaller ones.

Philippe is expected to continue strengthening in the next few days and reach Category 1 strength by Thursday, while Rina is forecast to dissipate on Monday.

Tropical storm watches were issued for Antigua and Barbuda, which could get 4 to 6 inches of rain. The northern Leeward Islands were forecast to get 2 to 4 inches of rain. There is potential for isolated flooding, according to the latest advisory.

Swells from Philippe will affect parts of the northern Leeward Islands, the U.S. Virgin Islands and Puerto Rico and could cause dangerous rip currents and surf, according to the NHC.

As of 5 a.m. Monday, Philippe was located about 125 miles east-southeast of Guadeloupe, moving northwest at 7 mph. Its maximum sustained winds were holding steady at 50 mph, with tropical-storm-force winds extended out 175 miles.

Philippe is forecast to pass near or just northeast of the northern Leeward Islands Monday into Monday night.

So far this season in the Atlantic, there have been 16 named storms, six of which were hurricanes. Of those, three were major hurricanes, meaning Category 3 or above.

Those were Hurricane Lee, a rare Category 5; Hurricane Franklin, a Category 4; and Hurricane Idalia, which made landfall on Florida’s Big Bend region at Category 3 strength on Aug. 30.

The next named storm will be Sean.

Hurricane season officially runs through Nov. 30.