Tropical Storm Erin has formed on a path toward the Caribbean and is expected to strengthen to a hurricane later in the week as it passes by Puerto Rico. Most models forecast the storm turning upward into the Atlantic Ocean, but a few see the storm continuing onward to the U.S. mainland.

The chances that the storm will make landfall in the U.S. have increased slightly after the path shifted south.

Erin is expected to gradually strengthen over the coming days, with forecasters predicting it could reach hurricane status by late in the week.