Forecasters are watching a new tropical system that may form behind Hurricane Erin, which is intensifying again Monday as it tracks off the U.S. coast.

After pummeling the Caribbean as a Category 3 hurricane over the weekend, Hurricane Erin has restrengthened to a Category 4 ...

Most of Erin’s intensification occurred during a 12- to 15-hour window overnight, according to Dan Pydynowski, a meteorologist at AccuWeather. By 5 p.m. Friday, Erin’s winds had remained only 75 mph.

Life-threatening surf and rip currents were forecast into midweek for the U.S. East Coast as Erin turns north and then northeast.

Hurricane Erin was downgraded to a Category 3 storm Sunday, but forecasters warn it is expected to intensify and grow in size ...