Although December is obviously much quiter then May in terms of severe weather, there have only been two tornadoes reported thus far this month. The average number of tornadoes reported in December is around 25 tornadoes, so we’ve obviously below average at this point. That can be explained by the strongly negative NAO (North Atlantic Oscillation). When strongly negative, the pattern generally favors a colder then average pattern. Obviously when that happens, the deep tropical moisture is confined to the Caribbean and severe weather is unlikely.

The above graphic depicts the strongly negative trend quite well. Since Thanksgiving we’ve been in a negative trend, hence the colder then normal temperatures nation wide for the most part. Since the Boxing Day Blizzard a few days ago, the pattern has been moderating in conjunction with the NAO. By New Years, the NAO will head into positive territory. Although severe weather is not necessarily likely with such a NAO shift, it does highlight a  pattern change overall, and anytime you start getting those you have to watch for moisture return ahead of the next dynamic system. Sometimes you get the moisture, more times you don’t. It all depends on the length of time the moisture can return, and the quality of moisture sitting in the Gulf of Mexico. If you just had a arctic cold front push through the Gulf of Mexico and the dewpoints are only in 30′s down into the Yucatan then that’s all you’re going to get. As the moisture graphic above shows, moisture is already returning into the Southern Plains with 50F dewpoints as far north as Dallas. As we’ll discuss here in just a moment, very high moisture amounts for December are combining with a strong storm system to create the possibility of a Dixie Alley Severe Weather Outbreak on New Years Eve.

The first thing I think of when seeing that chart is how pretty it looks with all the bright colors. From a meterlogical stand point, it’s quite obvious from looking at this map that a very strong vortmax will be swinging through the base of the trough. This is a very strong system that has a lot of dynamic forcing with it. When a system like this comes into the picture, you definitely start paying attention.

While placement of the cold front differs slightly between the NAM and GFS, both models agree that a tropical plume of moisture, with dewpoints averaging in the 63 to 65F range. Anytime you get high quality moisture of that magnitude in the winter season you definitely pay attention, as the wind shear is much higher in with these winter systems, thus requiring less instability to produce severe weather. When combined correctly with the time of day and instability axis, these systems can produce a volatile severe weather setup.

This graphic, displaying surface based instability for 3 PM Friday, shows values generally around 1,000 J/kg. When combined with the high shear in place (See 850 MB chart) you have a good recipe for surface based supercells and significant severe weather. While it’s still too far out to know, the general setup would support a possible tornado event across parts of the warm sector should everything come together correctly. At this point, it looks like we may be chasing in western Dixie Alley on Friday.

While there is still uncertainty regarding the extent of the tornado/severe threat, folks in the region need to be prepared for a very active New Years Eve. Based off this morning’s model runs, I would not be surprised at all for there to be the potential for a strong tornado with any cell that can become established in the surface layer and low level jet. When the cold front comes plowing through the region it’s likely a squall line will already have developed. With the high amounts of shear in place, it won’t take much for strong wind to mix down to the surface, increasing the potential for wind damage.

Folks in Arkansas, Louisiana, Mississippi, and perhaps even up to Missouri need to keep an eye on the latest forecasts from their local National Weather Service offices. For now, we’ll leave our chase status at Moderate for Friday, but we may upgrade to a high risk later tonight. Stay tuned to the Twitter and Facebook pages for the latest updates.

- David