FORT LAUDERDALE, Fla. — Neighboring Tropical Storm Rina and Tropical Storm Philippe are continuing to interact this weekend, with Philippe expected to become a Category 1 hurricane and steer a weakening Rina to the northwest.

The phenomenon of two tropical systems interacting is not a common one in the Atlantic.

Tropical Storm Rina was several hundred miles to Tropical Storm Philippe’s east-northeast, and the “more pronounced binary interaction” with Rina had moved Philippe slightly farther to the south than forecasters expected, the National Hurricane Center said early Saturday.

The National Oceanic and Atmospheric Administration says 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.

The bigger of the two sometimes absorbs the smaller storm, or the smaller storm “will satellite around” the bigger, Craig Setzer, a hurricane preparedness specialist, said.

The National Hurricane Center describes the Fujiwhara Effect as “an intense dance around their common center.” The smaller storm, after orbiting the bigger, can “eventually come crashing into its vortex” and be absorbed.

“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 says. 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 Tuesday while Rina will gradually weekend this weekend and dissipate by early next week.

No coastal watches or warnings have been issued for either storm as they spin in the open Atlantic Ocean far from land, but swells from Philippe will affect parts of the northern Leeward Islands, the U.S Virgin Islands and Puerto Rico through the early parts of next week and could cause dangerous rip currents and surf, according to the NHC.

Rina was located about 810 miles east-northeast of the northern Leeward Islands in the far eastern Caribbean, moving northwest at 14 mph, as of 5 p.m. Saturday. Its maximum sustained winds were 45 mph, slightly weaker from 50 mph earlier in the day. Rina’s tropical-storm-force winds extended out to 80 miles.

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