(CNN) -- As remnants of Tropical Storm Danny caused high seas Saturday night in the Atlantic Ocean, there were warnings of dangerous rip currents along parts of the East Coast.
The National Weather Service issued a high surf advisory for the coastlines of Rhode Island and Massachusetts and warned that rip currents could form.
A rip current -- different from an undertow -- is a difficult to detect narrow column of water that surges from the beach into the sea. Large waves can sweep a person into the sea from supposedly safe viewing areas, the weather service said.
Storm alerts issued for parts of the North Carolina coast were canceled Saturday evening as Danny was downgraded to a tropical depression, meaning its maximum sustained winds are 38 mph (62 km/hr) or less.
While Danny lost much of its structure, the weather service said the southern Massachusetts and Rhode Island shorelines would be susceptible to swells -- the most significant of which would occur Saturday night into Sunday.
"Stay off rocks or sea walls to avoid being swept into the water," the weather service said.
Forecasters had predicted that the storm would become extratropical Saturday night. Extratropical storms differ from subtropical ones in various technical characteristics relating to temperature and winds.