Hurricane Erin has strengthened into a Category 2 storm as it approaches Puerto Rico and the Virgin Islands, bringing heavy rains that could cause flooding and landslides.

A disturbance in the Atlantic following Hurricane Erin is now expected to track northward, National Hurricane Center data shows.

Hurricane Erin forced tourists to cut their vacations short on North Carolina’s Outer Banks even though the monster storm is expected to stay offshore after lashing part of the Caribbean with rain and wind on Monday.

Storms that ramp up so quickly complicate forecasting and make it harder for government agencies to plan for emergencies. Hurricane Erick, a Pacific storm that made landfall June 19 in Oaxaca, Mexico, also strengthened rapidly, doubling in intensity in less than a day.

Much of North Carolina’s Outer Banks region is under a tropical storm watch with Hurricane Erin expected to skirt the area Wednesday through Thursday, according to the National Hurricane Center.