If you've stepped outside lately in most of the lower 48 states, you know it does not usually feel this way in December. Instead of parkas, people are breaking out their lightweight, springtime jackets. Or even -- gasp -- perhaps no jacket at all.
We have a powerful El Niño to thank for the mild November and December so far. During years with a strong El Niño, December tends to be milder than average across the lower 48 states, though this does not mean the winter overall will be mild everywhere.
It also doesn't rule out other factors that also influence the weather, from short-term fluctuations in the jet stream to global warming.
In the contiguous U.S., November was the 13th-warmest such month on record, while the meteorological fall (September, October and November) ranked as the warmest fall on record since 1895. Every state across the contiguous U.S. and Alaska had an above-average autumn temperature, according to the National Oceanic and Atmospheric Administration (NOAA).
Globally, vast swaths of typically freezing places at this time of year -- such as Siberia -- have been unusually mild.
In the U.S., the jet stream pattern right now features a dip, or trough, over the West, with a large ridge in the East that is allowing milder than average air to flood northward from the Gulf of Mexico. So while the Pacific Northwest and parts of the Rockies are getting snow, the East is enjoying record-setting warmth.
Globally, 2015 is going to be the warmest year on record, breaking the previous record, set just last year, by a considerable margin. This is largely due to long-term global warming plus the effects of one of the strongest El Niño events seen since reliable records began in 1950.
Many had similar reactions to the unseasonably warm temperatures.
St. Louis is having a lovely Spring this Winter..— Katie (@Beatrix_Kiddo83) December 12, 2015
Seems we're having a lovely spring this winter. Wth is going on Mother Nature? Your mood swings are giving me whiplash.— AJ Santa's Lazy Elf (@WoahLock) December 12, 2015
It's a lovely spring we're having this winter. 60 degrees in December..? #NewEnglandWeather #lovinit— EvilBeastMonkey (@evlbstmnky) December 12, 2015
What a lovely spring we're having this winter.— Jessika ✌️ (@dont2ndjess) December 14, 2015
Others fretted about the fate of their beloved holiday tradition: ugly sweater parties.
Ugly Sweater Parties bouta be cancelled if it's gon be 80 degrees all month— Kar L. Stine (@karyewest) December 14, 2015
Ugly sweater party! Except it's 65 degrees so I'll wear a t-shirt https://t.co/aoQ07zs44N— Danielle (@smiledani) December 13, 2015
@Semxxj we went to an ugly sweater party tonight. But its hot AF still in south texas so it was an ugly T shirt party instead lmfao— xGS Hogs (@Sway_Hogs) December 13, 2015
This weathers about to turn our ugly sweater party into an ugly t shirt party— Edgar (@that_LIFEGUARD3) December 13, 2015
Ugly Christmas sweater party... But it's 75 degrees out... So ugly Christmas t-shirt and vest!!… https://t.co/lKUHlJuy14— LA (@LA_Kincaid) December 13, 2015
I might just walk into this ugly Christmas sweater party with just a t shirt on lol.— Mel. (@theScruggles_) December 12, 2015
Evening plans: Ugly Christmas Sweater party. Atlanta forecast: 75°F. I may unilaterally declare it an Ugly Christmas T-Shirt party!— Stephen Fleming (@StephenFleming) December 12, 2015
And while some seized the opportunity to have a taste of summer…
Well, if it's gonna be this warm in mid-December pic.twitter.com/Pq09ZRvd6p— Spencer Ackerman (@attackerman) December 14, 2015
...Winter doesn't rest for residents of Madison, Wisconsin.
Please remember that Alternate Side Parking Rules are still in effect, even though it looks like spring out there! https://t.co/AX84M8QQ9t— Madison WI Clerk (@MadisonWIClerk) December 14, 2015