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.’ ”