Hurricane Erin has been downgraded to a Category 3 hurricane but is gaining in size and raising the risk of life-threatening surf later this week along the U.S.

While Erin won't hit the U.S. directly, the storm's biggest impact on the East Coast will be along the Outer Banks of North Carolina, where coastal flood watches, tropical storm watches and storm surge watches are in effect. The storm surge could be as high as 4 feet.

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.