Across the United States, there’s more snow on the ground now than there was a year ago.

In fact, the last time there was more snow across the country was in 2011, according to the National Operational Hydrologic Remote Sensing Center in Chanhassen, Minn., the arm of the National Weather Service that tracks such things.

You couldn’t tell that in any of the large cities along the East Coast.

Through Tuesday, New York’s Central Park has had just a trace of snow, Boston 0.9 inch, and Washington and Philadelphia nothing at all, according to National Weather Service records.

It isn’t surprising, because across the contiguous 48 states, December was the warmest on record. While January readings have chilled the region from Boston to Washington, they are still lingering above normal levels in the large Northeast cities, the weather service said.

To understand the temperatures, just look to the equatorial Pacific Ocean, where a warming of the sea’s surface and a ruffling of the atmosphere above it have upset winter weather patterns over the nation.

‘‘It’s a classic El Niño pattern,’’ National Weather Service director Louis Uccellini said in an interview at the American Meteorological Society’s annual meeting in New Orleans.

Warmer air came in off the Pacific, where it got a boost crossing the mountains of the western states and brought about the spring-like December, Uccellini said. Since then, cold has begun to descend from the Pole and sweep into the central United States, due in part to another weather system known as the Arctic Oscillation, which operates independently of El Niño.

Typically, an El Niño is supposed to mean a mild winter for the Northeast. This is where things get interesting. One of the hallmarks of an El Niño is wet Pacific storms that sweep across the southern states, often picking up more moisture in the Gulf of Mexico before moving up and over the Atlantic coast off Washington, New York, and Boston.

Touch any of them with the wand of cold, and snow can fall by the foot. In 2010, a storm dubbed Snowmageddon clobbered the mid-Atlantic states, along with the cities of Washington and Baltimore.

Uccellini said 2010 was also an El Niño year. So where is all the snow this time?

As it turns out, El Niños seem to be all or nothing when it comes to snow in the East, Uccellini said. The sample size is low — just 11 instances with good data — but in El Niño years, Washington, for instance, has received either its most snow or its least snow.

New York’s snowiest month was February 2010, which recorded 36.9 inches during an El Niño, according to weather service data.

So far this year, the center of the El Niño-spurred storms has been taking a track much further inland. This means the warm sides of the storms, where the rain falls, have been what the East Coast cities have been getting.

On Sunday, New York’s Central Park had 1.8 inches of rain, a record for the date, as another of these storms moved northeast. If that had fallen as snow, according to some estimates it might have been at least 18 inches.