Hurricane Erin was downgraded to a Category 3 storm Sunday, but forecasters warn it is expected to intensify and grow in size in coming days, as it lashes Caribbean islands with heavy rains that could cause flash floods and landslides.

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.

As of 5 a.m. on Sunday, Hurricane Erin has weakened back to a Category 3 storm but has grown larger, the National Hurricane Center said.

Erin reached Category 5 status before weakening but has brought significant rain to the Virgin Islands and Puerto Rico.

Hurricane Erin, after strengthening into a Category 5 storm early Saturday, weakened to Category 3 by Sunday morning and has taken a track slightly more south than originally forecast, the National Hurricane Center said.