Heavy rain and snow wreaked havoc across the northeastern United States Tuesday, disrupting flights and causing power outages, as extreme weather gripped both American coasts.

The "double whammy," as the National Weather Service (NWS) called it, is the latest in an unusual series of weather fronts to have rocked the country simultaneously.

It is hard to establish a direct link between these winter storms, but scientists say human-caused climate change, brought about by the unchecked burning of fossil fuels, is making them wetter and wilder.

New York and New Jersey declared states of emergency to free up additional resources, as a powerful coastal storm called a "nor'easter" barreled through the region and New England.

More than 15 million people across the northern Atlantic seaboard were under winter storm warnings Tuesday, with the NWS warning of widespread minor coastal flooding and tree damage.

Precipitation and strong winds had knocked out power to more than 260,000 homes across New York, Massachusetts, New Hampshire and Vermont as of Tuesday afternoon, according to website PowerOutage.us.

The Berkshires in northwestern Massachusetts had the heaviest snowfall with recordings of 28 inches (71 centimeters) in the town of Windsor.

"We have wires down everywhere. We have trees down everywhere, and it's not going to get any better," the local police department wrote on Facebook, urging residents to stay off roads.

Multiple schools were closed in Massachusetts and in New Hampshire, where dozens of local elections were postponed.

New York Governor Kathy Hochul said the snow was going to "come down like a brick," as she urged residents not to leave their homes on Tuesday.

- 'Very dangerous' -

Monroe County, along the border with Canada, had recorded the highest snowfall in New York state as of Tuesday morning with nine inches falling overnight.

The storm, which arrived late Monday and is expected to cause hazardous conditions until it leaves the region Wednesday, impacted several hundred flights.

Almost 300 flights were canceled Tuesday at Boston's Logan International Airport while more than 250 were scrapped at New York City's LaGuardia airport, according to the website FlightAware.

A plane carrying 61 passengers skidded off a runway in Syracuse airport in upstate New York. No injuries were reported, the airport's operator said.

On the other side of the country, flood watches and warnings were in place over a wide area in California as heavy rains pounded the most populous US state.

The worst affected regions were expected to be the central coast and inland towards the Sierra Nevada mountain range where more snow was forecast to add to an already bumper snowpack.

The NWS warned the storm would cause "considerable flooding impacts below 5,000 feet (1,525 meters) elevation across much of the California Coast and Central Valley and over the southern Sierra Nevada foothills."

Forecasters said conditions in the mountains were "very dangerous" with a high risk of avalanches.

Evacuation orders were in place in parts of Santa Barbara County in the south of the state, including in parts of the swanky coastal enclave of Montecito, home to Britain's Prince Harry and his wife Meghan.

A parade of storms has pummeled California this winter, leading to damage estimated in the billions of dollars as they washed out communities, brought down power lines and caused landslides.