LOUISVILLE, Kentucky—Call this the winter that wasn't. Or the new reality.
Here and across much of the Eastern United States, dozens of cities experienced a "meteorological winter"—the three months from December through February—that ranked among their top 10 warmest on record.
In Louisville, the average mean temperature was 5.1 degrees warmer than normal for those three months. There was hardly any snow. Winter bike riding was more pleasant. Home and business heating demand was less.
Daffodils and tulips were peeking out of the ground a month early in Washington, D.C., and clouds of pollen emerged from pine trees in North Carolina two months early.
In addition to inflamed allergies, public health officials and outdoor enthusiasts in eastern states can now worry about more disease-carrying bugs come spring.
"While I might like the increased opportunity to get outside without battling low temperatures, I know there will be issues to contend with in the spring," said Mike Bucayu, a Louisville resident who enjoyed kayaking the Floyds Fork waterway in January. "Pesky insect and tick numbers also boom in the spring, which is why I like a good winter freeze."
Winter failed to live up to its historical norms in big ways in dozens of cities east of the Mississippi River, from Atlanta to to Boston, which had its second warmest winter in 146 years of record keeping.
Nashville's average mean temperature was 6.3 degrees above normal for the three months, including 8 degrees above normal in December and 7 degrees in January.
The difference wasn't quite as great in Cincinnati, which nonetheless had its 12th warmest winter on record, 5.1 degrees above the norm. In Indianapolis, the three-month stretch was 4.8 degrees above normal, making it the 15th warmest winter.
Scientists have an explanation.
A natural weather phenomenon kept cold air bottled up in the Arctic. And global warming also played a role.
January was the hottest January globally on record.
The last five years rank as the top five hottest, globally.
This year, it is already nearly certain, will be among the five hottest years on record globally, and it's almost a coin toss that 2020 will be the warmest year on record, according to the National Centers for Environmental Information, part of the National Oceanic and Atmospheric Administration (NOAA).
"There is year to year variability," said Karin L. Gleason, a meteorologist with the centers. "We expect that all the time. But the overall trend is warming."
Global warming, she said, is like being on an "escalator that's always going up."
The Arctic Oscillation Has Been Especially Strong
The natural weather pattern that's kept winter at bay in much of the United States is called the Arctic oscillation. It comes in both positive and negative phases, depending on differences in air pressure between the Arctic region and the North Atlantic and Pacific oceans.
It affects the polar vortex, an area of low pressure and cold air over the north pole, and also the storm- and temperature-steering jet stream, experts said.
Deepti Singh, a climate scientist in the School of the Environment at Washington State University in Vancouver, Washington, said the Arctic oscillation has been in an unusually strong positive phase this winter, which has resulted in a strong polar vortex that's kept the cold Arctic air trapped up north.
That's a contrast to some recent winters, when a weaker polar vortex has allowed frigid air to descend from the north, resulting in extremely cold days and strong snowstorms across portions of the United States, she said.
But some scientists also suspect that climate change is playing a role in the weakening of the polar vortex in some winters, adding to the weather extremes that scientists have linked to human activities like burning fossil fuels.
Even when there are cold air outbreaks associated with a weak polar vortex, the cold air coming from the Arctic regions over the eastern U.S. is warmer than what it used to be a couple of decades ago, Singh said.
Overall, with climate change, she said, spring is generally coming sooner, colder seasons are warming faster, "and we are getting fewer extreme cold events."
Up the Eastern Seaboard, There's Been Hardly Any Snow
In an era of climate change, snow can be unpredictable. Warmer temperatures can produce more winter-season rain, but NOAA has observed that, with more moisture in the air, an increase in the frequency of extreme snow storms during cold spells is also possible.
This year so far Louisville has had only 3.3 inches of snow, compared to a typical year of 10 inches through the end of February. Philadelphia, New York and Washington, D.C. also have had hardly any snowfall.
But to illustrate the weather extremes related to climate change, upstate New York received one to four feet of snow just last week, in a surprising "lake effect" snowstorm.
Lake effect snow occurs when cold air moves across the open waters of the Great Lakes. As the cold air passes over the unfrozen and relatively warm waters, it picks up moisture, forms clouds and then can dump snow at two to three inches per hour.
Because of a warm winter, the Great Lakes haven't had as much ice this year, fueling the conditions for last week's big snowfall, Gleason said.
"Usually we don't have lake effect snow this late into the season," she said.
Climate Change Has Led to More Heavy Rain
Had temperatures been colder, there would have been ample opportunities for more snow. Instead, there was flooding and other forms of extreme weather.
Across the southeast, the winter was warm and very wet, inundating communities in states like Tennessee, Georgia and North Carolina, said Charles E. Konrad Jr., director of the Southeast Regional Climate Center at the University of North Carolina. Mississippi was soaked, as was eastern Kentucky, with the governors of both states declaring states of emergencies.
With global warming, scientists are seeing an increase in heavy rain storms, Konrad said.
Last month's heavy rains in eastern Kentucky flooded homes and businesses in eight counties—revealed by dramatic drone video—and prompting Kentucky novelist Silas House, in an essay in The Atlantic, to ponder global warming and wonder whether anyone in big cities cared about rural suffering that went largely unnoticed.
On Monday night and early Tuesday morning, tornadoes ripped through Nashville and and middle Tennessee, leaving behind paths of death and destruction, and raising new questions about climate change and tornadoes. The Associated Press counted at least 24 deaths.
Vanderbilt University climate scientist Jonathan Gilligan described any link between climate change and tornadoes as "extraordinarily uncertain," adding that scientists he considers reliable are making progress.
Still, Gilligan said, there are indications that the total energy dissipated by tornadoes—their destructive power—may be increasing. Tornado outbreaks also appear to have shifted from the Great Plains somewhat east, to include the southeast, in what some are now calling Dixie Alley. But Gilligan said the cause of that remains uncertain.
Despite scientific uncertainties surrounding the study of climate change and tornadoes, researchers have been noticing certain trends, said Harold Brooks, senior research scientist at NOAA's National Severe Storms Laboratory.
"Warmer than normal winters are associated with above average tornado counts for the United States," said Brooks, and that may be true especially in the southeast.
The large number of southeastern tornadoes in January is consistent with that trend, Brooks said.
In January, there was a violent outbreak of 90 tornadoes, 82 of which occurred over the two days of Jan. 10 and 11. NOAA said it was the result of a strong, spring-like storm weather system that brought heavy rain, damaging winds and tornadoes to numerous states across the south and southeast, including Louisiana, Mississippi, Alabama, Georgia, Tennessee and Kentucky.
Fewer Days Below Freezing Mean More Bugs Come Spring
With spring breaking out, earlier gardening and more outdoor activities will increase interactions with the bugs emboldened by a mild winter.
Populations of disease-carrying insects were historically knocked back during deep freezes. Louisville is an example of how that's not happening so much anymore.
The number of days below freezing is now less by two weeks, on average, than it was historically, according to Louisville's draft climate vulnerability assessment. In three decades, there could be as many as 46 fewer days below freezing, and 64 fewer by the 2080s
"With warmer and wetter winters, diseases such as mosquito-borne West Nile Virus, dengue, and Zika, as well as tick-borne Lyme disease and Ehrlichiosis, could become more prevalent," according to the report.
In coastal North Carolina, for example, the warm winter brought out invasive, aggressive and venomous fire ants two months early. "They're pretty terrible when you step on them," said Tiffanee Conrad, an agent with the North Carolina Cooperative Extension in Pender County.
In nature, timing is critical.
Scientists worry that wacky weather may cause birds to migrate and produce offspring after critical food sources that have come and gone.
Flowers on North Carolina peach trees are blooming early, before there are many bees around to pollinate them; other trees are releasing their sneeze-inducing pollen two months early, Conrad said. Now there's a risk of a frost in March that could damage the blooms, resulting in economic losses at harvest, she said.
Native trees are pretty good at dealing with whatever weather winter throws at them, said horticulturist Paul Cappiello, executive director of Yew Dell Gardens in Oldham County, Kentucky. The sugar maple and American beech trees' buds remained "tight as a drum," he said.
In the meantime, Cappiello, an avid bicyclist, doesn't have to look at weather service data to know this past winter was mild.
"I got a fair more miles in this winter than last winter," he said.