Philadelphia, PA – A combination of rainfall and melting snow caused rivers across parts of Pennsylvania and New Jersey to rise late this week, though officials say flooding is not expected as colder weather returns.
According to the National Weather Service Middle Atlantic River Forecast Center, widespread rainfall of 0.5 to 1.5 inches from Thursday into Friday, combined with ongoing snowmelt, led to noticeable river rises across the Mid-Atlantic region. The most significant increases were observed in Pennsylvania, New Jersey, and parts of New York.
Forecasters say river levels are now cresting or beginning to recede, and no additional river flooding is anticipated at this time.
Why Rain Causes Snow to Melt Faster
The NWS also shared new graphics explaining a common misconception: rain itself does not melt snow very efficiently. Instead, warm, moist air and wind play the biggest role.
When snowpack temperatures rise to 32°F or higher, the snow becomes “ripe,” allowing rainwater to pass directly through it. As warm, moist air condenses near the snow surface, heat is released, accelerating the melting process. Wind further enhances melting by increasing heat transfer.
If snowpack temperatures are still below freezing, the first rain often freezes on contact, delaying runoff until temperatures rise.
Local Impact
River gauge imagery from Chatham, New Jersey, showed visible loss of shoreline ice and snow between Thursday afternoon and Friday morning as water levels increased. Similar conditions were reported along rivers and streams throughout eastern Pennsylvania and northern New Jersey.
With colder air returning, officials expect river levels to continue stabilizing. Residents near waterways are encouraged to remain aware of changing conditions, especially with additional rain possible later next week.
