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.

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

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.

Tropical Storm Erin making its way across the Atlantic Ocean. What to consider for your Myrtle Beach vacation. August, September and October are the most active months of hurricane season.

Forecasts nudge Erin's likely path to the west, increasing the risks at U.S. beaches. Tropical storm conditions are expected in North Carolina's Outer Banks starting late Wednesday.