Just as the US northeast was easing from a mild winter into a historically warm spring, a storm left thousands without power and prematurely leafy trees sagging under snow.

The spring storm that started Sunday and moved slowly through on Monday left tens of thousands of people without electricity, including nearly 25,000 in New York state, Governor Andrew Cuomo said.

"Outage numbers are likely to continue to climb," his office said.

The hard-hit areas ranged from upstate New York and Pennsylvania to West Virginia and Maryland.

Flood watches were in place in New Hampshire and Massachusetts, the national weather service said, while on higher ground, the rain turned to heavy snow.

An inch (2.5 centimeter) an hour of snow was expected, with 14 inches (35 centimeter) already recorded in Somerset County, Pennsylvania, weather.com reported.

"Winds will combine with heavy snow to reduce visibilities, leading to hazardous driving conditions through Monday night from western New York to northern West Virginia," the weather site said.

Trees could collapse under the weight of early leaves and wet snow.

This will "likely lead to downed trees and powerlines, leading to power outages, possibly for several days," weather.com said. "These downed trees may make roads impassable in some areas."

The region saw record warm spring temperatures after an unusually mild winter.