Forecast expect between 4 and 7 inches of rain to fall over several days in the Outer Banks, with some spots receiving 8 or more inches.
Gov. Tom Corbett declared a statewide disaster emergency ahead of the storm, Corbett's office said.
Flooding, power outages and sustained high winds are anticipated, his office said. Sandy could even bring snow to parts of southwestern Pennsylvania and in higher elevations.
"Essentially, this is a hurricane wrapped in a nor'easter," Corbett said.
Public schools in Philadelphia will be closed on Monday.
Public transportation in the Philadelphia area has been suspended.
The Rhode Island Emergency Management Agency urged all residents to prepare for prolonged power outages, wind damage and water damage by keeping an emergency kit, securing property and taking boats out of the water.
State authorities have taken precautions such as checking and clearing drains in flood-prone areas and relocating state equipment if necessary.
Public schools in Providence, the state capital, will be closed Monday.
Obama has declared a state of emergency for Rhode Island.
Radar showed heavy rains from the fringes of Sandy pelting much of South Carolina's coast, from Charleston to Myrtle Beach.
The Palmetto State is expected to avoid a direct hit from the storm, which is expected to make landfall well to the north.
Virginia was one of several states to declare a state of emergency ahead of the storm. Computer models predict parts of the state could see as much as a foot of rain.
Sandbags piled up inside restaurants in the Old Town section of Alexandria, along the banks of the Potomac River.
The Virginia National Guard was authorized to bring as many as 500 personnel onto active duty.
All public schools in Fairfax County, a large school district in northern Virginia, will be closed Monday and Tuesday. Schools in Arlington, Norfolk, and Newport will be closed on Monday.