Hopefully this won’t repeat the Valley Forge winter of 1777, but due to predictions of inclement weather, Segerstrom Center for the Arts is postponing this Saturday’s free “Hamiltunes: An American Sing-along,” which was to take place on the Center’s outdoor plaza. An expected nor’easter in New York City is preventing travel for Hamiltunes host, Broadway veteran Seth Rudetsky, while our own local weather forecast is uncertain. The fun sing-along that celebrates “Hamilton” has been rescheduled to Saturday, March 31 from 4-6 p.m.