Forecasters say the storm is shaping up to become a nor'easter bringing up to 4 inches of rain to parts of eastern Pennsylvania and northern New Jersey on Sunday night through early Monday.
Some local flooding is possible along smaller rivers and creeks, with the highest rainfall totals expected in northern New Jersey, said meteorologist Greg Heavener of the National Weather Service.
In the west, forecasters say the storm could bring 8 to 12 inches of snow to higher elevations in Fayette, Westmoreland, Indiana and Jefferson counties and three to four inches in Pittsburgh and surrounding areas, according to meteorologist John Darnley of the weather service office in Pittsburgh.
"The biggest threat from this storm is probably power outages caused by big limbs being broken off," Darnley said. "Because the foliage is already on the trees, we probably will experience some power outages from the trees falling."
This kind of snowfall late in the spring is unusual, Darnley said, with the record snowfall for the month only 12.7 inches on one early April day in 1901.
"Heavy snow like this in the latter part of April is a true anomaly," he said.
In central Pennsylvania, the storm is expected to bring a mix of rain and snow—heavy, wet snow in the higher elevations and rain in lower areas, said Mike Dangelo, forecaster in the weather service office in State College.
Areas such as Johnstown, Somerset, Bradford, and Warren counties may see a foot or more of snow, especially in the higher elevations, beginning Sunday night and into late Monday, he said.
Lower elevations such as State College and Altoona may see a mix of snow and rain, with rain predominating during the day because temperatures are expected to remain above freezing. Areas such as Williamsport, Harrisburg and Lancaster are expected to see mostly rain, which will be eagerly soaked up by plants in conditions that have been very dry until now, he said.
"The big thing is not so much the amount of snow but how heavy and wet it's going to be, and that there are a lot of leaves on trees," Dangelo said. With that kind of snow settling onto leaves amid northwest winds of 10 to 20 miles per hour, he agreed, "it's probably going to bring down trees and knock down power lines."
In late October, foliage remaining on the trees was also cited as a factor during an unseasonably early snowstorm that resulted in power cut to hundreds of thousands of customers and contributed to eight deaths in the commonwealth.