At the National Arboretum, the white petals of snowdrops — normally an early spring flower — have unfurled. In Maine’s Acadia National Park, lakes still have patches of open water instead of being frozen solid. And in Donna Izlar’s back yard in downtown Atlanta, the apricot tree has started blooming.

It’s not in your imagination. The unusually mild temperatures across several regions of the country in the past few months are disrupting the natural cycles that define the winter landscape.

What began as elevated temperatures at the start of fall in parts of the United States have become “dramatically” warmer around the Great Lakes and New England, according to Deke Arndt, chief of the Climate Monitoring Branch at the National Oceanic and Atmospheric Administration’s National Climatic Data Center. And the Washington area is on track for its fourth-warmest year on record, along with its seventh-warmest December.

That, in turn, has created conditions in which plants are blooming out of season and some birds are lingering before moving south.

“It’s a weird kind of fall blending right into spring,” said Scott Aker, head of horticulture at the National Arboretum.

Arndt said the pattern is most pronounced in eastern Montana, northeastern Minnesota and parts of North Dakota, where December temperatures so far have averaged 10 degrees above normal. But the mild weather extends through the Great Lakes region, along with New England and the mid-Atlantic, with temperatures this month averaging between six and eight degrees above normal.

Just 19.6 percent of the continental United States is covered with snow, according to the latest snow analysis by NOAA, compared with 50.3 percent this time last year.

Scientists — as well as those who question dire global warming predictions — emphasize that one warm season should not be interpreted as a broader sign of climate change. But several researchers said the decline in cold snaps in the United States fits with a pattern of warming driven in part by human activity.

“It’s about long-term trends, and one year does not make a trend,” said Doug Inkley, a senior scientist at the National Wildlife Federation. But he added, “We already, in the lower 48, have long-term warming that has had a large impact on us.”

Temperature anomalies happen for many reasons, and Arndt said some of the mild weather stems from a persistent ridge of high pressure that has settled over the eastern third of the country, bringing southern winds in many areas. But he added that the shifts in seasonality now on display are in line with the warming the United States has experienced in recent years.

“We’ve seen consistently, in the last couple of decades, more consistent warm episodes for the season than cold episodes,” Arndt said, adding that the “climate wrapper” that affects local weather is akin to the connection between parenting and how children behave. “There’s always local factors to a kid’s behavior. Maybe he was stressed out or didn’t get enough sleep or was hanging out with other kids who don’t behave. But when you see a pattern to that, you think, ‘Maybe it is parenting.’ ”

The decreasing frequency of cold snaps should not lead anyone to conclude that there is dramatic warming across the globe, said Patrick J. Michaels, a senior fellow in environmental studies at the libertarian Cato Institute. Climate change is happening, he said, but not at the “magnitude” that some suggest.

Some researchers have detected warming trends in key habitats, but incomplete historic records make it difficult to measure these changes with precision. Abe Miller-Rushing, science coordinator for Acadia National Park, said it’s clear that the time the lakes freeze no longer comports with conventional wisdom. “They typically freeze by January 1st,” he said. “They’re not close to freezing up.”

Miller-Rushing collaborated with a Boston University biology professor, Richard Primack, to examine how the seasonality of plants and animals in Concord, Mass., has shifted since the 1850s, when naturalist Henry David Thoreau recorded their spring patterns with precision. They found that plants, including the highbush blueberry, are blooming an average of 10 days earlier because of warmer temperatures.

Primack said it’s easier to detect these changes in the spring than in the fall, when a combination of temperature, precipitation and day length governs plant behavior. “There’s a climate change signal, but it’s much more complicated,” he said.

Still, Primack added, New England lost almost its entire fall foliage season this year because there were few freezing temperatures in September and October and more rain than usual. Rather than having the leaves turn color at the start of October, he said, they stayed green until late October, and “in a matter of days, all the trees went from having green leaves to having no leaves.”

While many have welcomed the balmy temperatures, they pose potential threats to habitat and humans. Western bark beetles, whose reproductive cycles have sped up and numbers have increased because they don’t face the same cold winter temperatures as in the past, have already ravaged pine trees out West. University of Maine researcher William Livingston recently published a study showing that Cryptococcus fagisuga, a different bark beetle species, boomed during warm winters between 1999 and 2002 and has feasted on Maine’s beech trees ever since.

And in the Plains region, changes in vegetation pose a fire threat and can intensify allergies. The Minnesota Department of Natural Resources is requiring a permit to burn vegetation or trash in light of the state’s dry conditions; a recent wildfire burned 750 acres of forest close to the Hangaard State Wildlife Management Area. Scientists have found that a roughly one-month delay in the onset of the first frost in the Central Plains allows more time for ragweed pollen production, which has extended the region’s allergy season.

“We have been showing these very clear changes in the biota, the living world,” said Jake Weltzin, an ecologist with the U.S. Geological Survey who tracks shifts in timing among plants and animals.

Weltzin, who teams up scientists with citizens to chronicle observational changes about plants and animals’ seasonal rhythms as executive director of the USA National Phenology Network, just started collecting data on plants in 2009 and animals last year.

“We just don’t have observations of these patterns on a national scale,” he said.

The recent mild weather may be affecting some birds’ migration patterns, according to experts, but not to a significant extent. Michael Parr, vice president of the American Bird Conservancy, said that although some waterfowl species appear to be migrating more slowly, many birds have moved south for the winter.

“They wouldn’t have known there was a nice, warm snap in D.C. because they would have already gone,” Parr said.

The blooming of bushes and trees has been far more noticeable, whether it’s flowering dogwoods on 16th Street, the roses and irises that Miller-Rushing spotted in Boston last week, or Izlar’s apricot tree in Atlanta.

“I just noticed two days ago that it’s in full bloom,” she said Thursday.

A cold front is expected to hit the East Coast on Monday. But in the meantime, even Arndt has been surprised by what the warmer temperatures have brought to his surroundings in Asheville, N.C., where the National Climatic Data Center is based.

“We have daffodils blooming in Asheville, which is rare,” he said. “It’s natural botanical evidence that the instruments are getting it right.”

Staff writer Jason Samenow contributed to this report.