Melting the Arctic in the middle of winter, by @Gaius_Publius

Over the past several days, an alarming string of tornadoes has left dozens dead across the South. At least 68 tornadoes were reported in 15 states from California to the Carolinas from Dec. 21 to Monday, the longest streak on record of December days with a tornado. December tornadoes are twice as common during El Niño years, but this weekend’s atmosphere over the South was something different entirely: By some measures, it was the most moisture-laden ever seen during the winter months. ...

The Texas tornadoes were part of a much larger storm system that at one point encompassed about half the country. The same storm system also brought heavy rains to the Midwest that threaten one of the biggest floods in history on the Mississippi River south of St. Louis, surpassing even the legendary 1993 flood. Road closures due to high water blanketed Missouri, and water levels will continue to rise for several days as record floodwaters from Oklahoma make their way toward the Gulf of Mexico.

On its western and northern fringes, the storm brought snow, the worst of which struck New Mexico. There, Gov. Susana Martinez activated the state’s National Guard and said the historic snowstorm had created a “dire situation.” In fact, at the exact same time that tornadoes were bearing down on Dallas, a record-setting blizzard was burying cars under snowdrifts 10 feet deep on the western side of Texas. Snow fell as far south as northern Mexico. The system also helped bring record-breaking freezing weather to southern California, a fierce ice storm to Chicago and Michigan, and the first significant New England snowfall of the season—just two days after temperatures climbed into the 70s as far north as Vermont. The Wall Street Journal called the juxtaposition of weather extremes “freakish.”

As it departs North America this week, the storm will rapidly intensify over the northern reaches of the Gulf Stream and draw tremendous amounts of warm air northward from Spain and the Mediterranean Sea toward the Arctic. As the storm approaches Iceland, it will have strengthened to the equivalent of some of the strongest hurricanes ever recorded in terms of atmospheric pressure. Intensely high pressure over western Russia, perhaps boosted by melting sea ice, will aid in setting up the tropics-to-pole atmospheric superhighway.

Unlike other recent episodes of extreme weather around the planet, this storm is probably not related to El Niño, which has limited influence in Europe. The storm will be strengthening over the exact spot that North Atlantic temperatures have been cooling over recent years, an effect that scientists have linked to a slowdown of the basin’s circulation triggered in part by melting sea ice ...

The remarkable storm will briefly boost temperatures in the Arctic basin to nearly 10 degrees Fahrenheit warmer than normal—and the North Pole itself will be pushed above the freezing point, with temperatures perhaps as warm as 40 degrees. That’s absolutely terrifying and incredibly rare. Keep in mind: It’s late December and dark 24 hours a day at the North Pole right now. The typical average high temperature this time of year at the North Pole is about minus 15 to minus 20 degrees. To create temperatures warm enough to melt ice to exist in the dead of winter—some 50 or 60 degrees warmer than normal—is unthinkable....