What to know about December's deadly tornado outbreak

At least 39 tornadoes touched down in more than six states Friday into Saturday, as a powerful storm system moved across the Central and southern U.S., the National Weather Service has concluded. Those numbers will fluctuate, and are likely to increase, in coming days, the agency cautioned.

The big picture: The outbreak left at least 88 dead, dozens injured and caused billions in damage. NWS survey teams continue to assess damage to assign intensity ratings to individual tornadoes, and several have turned out to be on the high end of the Enhanced Fujita Scale.

By the numbers: The 39 tornadoes that touched down during the outbreak include the tornado that began in northeastern Arkansas, and tracked for at least 163.5 miles into Kentucky, though damage surveys on this twister will take several more days.

In Kentucky:

• The NWS office in Paducah gave the tornado that devastated Mayfield and nearby towns a preliminary damage rating of EF-4, with estimated peak winds of at least 190 mph.
• There were two tornadoes that so far have been rated as "at least EF-3" intensity, including one that tracked from Todd County to Marion County, and another that moved from Fulton County to Breckinridge County. Indications of estimated winds of "at least" 155 mph were seen in Bowling Green.
• There was another tornado in Warren County that resulted in EF-2 damage.

In Missouri:

• There was an EF-3 tornado confirmed near Defiance that had peak winds of 165 mph, which is equivalent to a Category 5 hurricane.

In Illinois:

• One EF-3 tornado has been confirmed near Edwardsville, with peak winds of 150 mph.

In Tennessee:

• An EF-3 tornado has been confirmed from Newbern to Paris Landing, with estimated peak winds of 160 mph. In addition, two EF-2 tornadoes have been confirmed in the state as well.
• The Newbern to Paris Landing tornado has a confirmed path length so far of 71 miles, but it may actually be longer, the NWS told Axios, given ongoing damage surveys that could show a continuation of its track.

Where it stands: While it's not yet completely clear how long an individual tornado stayed on the ground between northeastern Arkansas and southwestern Kentucky, the NWS office in Paducah has confirmed that at least 163.5-mile stretch was a continuous tornado there. That includes the tornado that devastated Mayfield, Ky.

• If the path exceeds 150 miles, the tornado would be just one of 13 in U.S. history to have remained on the ground for that long. And a track length of more than 200 miles would place it among the rarest of the rare, with only four recorded instances of similar twisters, the Washington Post reported.
• In total, the NWS issued 150 tornado warnings Friday night into Saturday, and eight of these were "tornado emergencies," which are the most dire type of warning issued for an ongoing significant tornado in a heavily populated area.

Context: The record warm and unseasonably humid air mass was partially the result of the unusually warm waters in the Gulf of Mexico, as winds blowing from the southwest pulled moisture northward from the Gulf Coast all the way into Indiana.

• The overall environment in which this storm took place is being viewed by many climate scientists and meteorologists with suspicion, since climate change is increasing the odds of heat extremes, as well as raising ocean temperatures.

• Winter is also the fastest warming season in most states, which may make December tornado outbreaks more common.

Between the lines: Going into this outbreak, just 14 people had perished from tornadoes in the U.S. this year, an unusually low number, according to NOAA.

• Last weekend's outbreak was the deadliest on record to occur in December, and if the Arkansas to Kentucky long track tornado or family of tornadoes is rated as an EF-5, it would be only the third such twister to occur in the U.S. during the month.
• The parent thunderstorm that spawned that tornado traveled for well over 250 miles, showing signs of rotation on Doppler radar for an astonishing estimate of about 600 miles.

State of play: According to Steve Bowen, head of catastrophe insight for Aon, "It can be said with near certainty that the financial cost from this outbreak is going to reach into the single-digit billions."

• He told Axios it will "certainly be one of the more expensive thunderstorm outbreaks on record," but may not surpass the Midwest derecho of 2020 or the April 2011 Super Outbreak of tornadoes across the South and Southeast. "The fact that those outbreaks included larger metro areas is a main reason why the losses were higher," Bowen said via Twitter message.
• "This does not in any way minimize the devastation that so many communities have recently suffered. The recovery period will likely take years."

Go deeper:

• How to help tornado victims in Kentucky and other states
• Tornado outbreak offers a grim climate warning

Editor's note: This story has been updated to include new details about the number of tornadoes and how far one traveled.