2013-12-21 FCST: LA/AR/MS/TN

[Matt Hunt]

I'm surprised nobody has posted about the potential for tomorrow, yet! While some seem to be focused on the snow potential to the north of the front, I've been watching the severe potential over the last several days to the south of the front. Just like in November, we're looking at a negatively tilted trough, which is the biggest reason I think this has outbreak potential. Instability is pretty meager, but that's typical for a cold season setup, and with mid-60s dewpoints and the strong shear, I'm not sure that it will be too much of an issue.

I'm getting ready to leave Indy for Memphis. I'd like to stay in NE Arkansas tomorrow for better terrain and road options, but storms may send me a little further south. Right now I'm looking around the I-40 corridor. Storms will be moving pretty fast once again, so I'll likely just be trying to setup in an area where storms will be mature, and intercept along an east-west route, rather than really chasing them.

 

[Skip Talbot]

My biggest concerns for this setup would be storm mode, interference and cooling of the boundary layer by junkvection, and timing. The models sped this system up quite a bit. Earlier runs had the cold front (and sometimes a dryline) way out in central Texas/eastern Oklahoma. Now it's heading for the MS River by evening. That's a positive thing. The location of this event on earlier runs was in such unfavorable terrain that I wouldn't even consider chasing it. Eastern Arkansas is a different story, and there are several really eye catching plots the models are spitting out for this setup. 500+ effective storm relative helicity? Holy smokes.

Storm mode, interference, and timing though... The NAM has been consistently showing a wide open cap and ample amounts of lift for the duration of the day, leading to a warm sector that is completely socked in with cloud cover, rain, and junkvection from the start. That's why the CAPE is so low. Low level lapse rates are going to suffer tremendously. So much so that the tornado potential is probably going to be mitigated as the low levels are stabilized with rain cooled air.

500-850mb crossovers are unidirectional across Arkansas, and parallel the cold front. It means fast storm motions, and training cells. Unlike November 17, where cells stayed fairly discrete and moved away from the cold front pretty quickly, this event looks to kick up huge bands of NE training junk. I'd be very worried about seeing anything discrete at all. The training motion and storm coverage will lead to a great deal of interference. Storm bases will be very low to the ground, very low contrast, and embedded. Visibility, despite the shift to favorable terrain, may be completely lacking.

And then the timing. That 90-100+ knot streak doesn't eject until after dark across the MS. That's when many of the parameters peak as well. You're really looking at 21z for a daylight intercept on this event, since the sun sets at about 23z down there. At that time, the streak is still way out west by TX and OK. You need that streak over the warm sector to compensate for the lack of instability and provide some dynamic forcing. Without it, updrafts in the warm sector will struggle against each other and the poor lapse rates. The peak of the severe threat might not materialize until after dark on or east of the MS when the cold front kicks through and that streak ejects. Then you're probably looking at a big damaging wind event, with a few embedded tornadoes... at night moving at 60+ mph. Yikes, I'll pass (but still watching for changes).

 

[Zack Cooper]

It's too close to home for me to not try on, so I'm heading towards NE LA/ SW AR tomorrow in the morning from Shreveport. Am concerned about the large amounts of precip that will be present, but the negatively tilted trough and insane wind shear make me think there's definitely potential. Storm modes will be junky and the terrain isn't overly favorable, but I'm still taking the chance and heading out.

 

[Chris DeAngelo]

The SPC just put out an enhanced thunderstorm outlook for tomorrow a few minutes ago. It's been very humid and damp all day long here in NE Arkansas. The local media is saying an early start for the severe weather threat North of I-40 & NE Arkansas. I will more than likely stick close by, but my gut tells me to head further South along the Mississippi River line.

 

[Mike Johnston]

Looks like the area around Memphis, perhaps just to the east and south of there, after sundown on Saturday could be primed for some tornadoes. The feature that looks important to me is the potential for a roaring LLJ. If it gets much over 70, I would look for some spin-ups on the nose of LEWPs in the area. Remember what happened several years back with those winter/nighttime tornadoes around Evansville, IN when the LLJ went crazy.