Several hospitals were running on backup generators. Most decided to cancel elective surgeries and out-patient testing.
Dr. John Kelly, chief of staff at Abington Memorial Hospital outside Philadelphia, one of the affected facilities, said critical staff needed for any emergencies would be staying overnight. He said the hospital had plenty of fuel and food.
The American Red Cross opened three shelters in southeastern Pennsylvania and stood ready to open more.
"We've been told to be prepared for four to six days. We are gathering staff and volunteers for up to a week," spokesman Dave Schrader said.
Associated Press writers Marc Levy and Mark Scolforo in Harrisburg, Kevin Begos in Pittsburgh, and Ron Todt, Maryclaire Dale and Matt Moore in Philadelphia contributed to this report.