12/31/10 FCST TX/LA/MS/AL/AR

[Bob Hartig]

Has anyone been eyeballing Dixie Alley lately? With the GFS and NAM both indicating the right ingredients for a severe event falling into place, I thought I'd open up a forecast thread. In a nutshell, a large mid-level trough moves into the central CONUS Friday as a surface low transitions northward into Iowa, drawing in a surge of moisture through the Gulf states and well northward. This morning's runs of GFS (6Z) and NAM (12Z) indicate 500 J/kg MLCAPE overlaid by ample shear. Check out my latest blog post to compare a few NAM and GFS maps.

ADDENDUM: Mod, would you please add Arkansas to the list. Thanks!

 

[Mike Johnston]

Nice that this thread was started. Actually, severe parameters look pretty decent and prototypical for a winter severe setup in the south; very low LCLs and virtually no capping. If anything, looks like the storms could fire fairly early. I would even add Arkansas to the area covered, as it looks like ample moisture could be pulled at least into the southern part of that state which would place it under the leading edge of the stronger upper level winds and height falls.

Yes, SPC staying fairly cautious on its Day 4 Outlook but from the text definitely leaving the door open.

 

[Sean Miller]

I would say Western Tenn, and even parts of Southern Missouri could see some action as well with this system. Memphis AFD mentioned widespread surface based thunderstorms along the cold front, which at the moment is progged to be around the Mississippi River around 6PM. Obviously timing can change. To me this could be more of a damaging wind threat, but you never know this far out. Should be an interesting one to keep an eye on, and with a 3 day weekend ahead I might even make a trip south. Weather permitting.

 

[Skip Talbot]

12z NAM and GFS differ on the timing with the GFS being several hours faster with the system, like it usually is. The GFS is also a bit stronger with the surface low and moisture advection. I'd like to see those two line up more, but it does look like the jungles from NE TX to the lower MS valley could see some fast moving, low topped severe storms. Keep an eye on Missouri too though. Rather marginal dewpoints in the 50's are forecast to advect in, but with some cold air advection aloft also forecasted, lifted indices are more negative. If this area gets some sun we might see some cold air mini supercell action maybe even as far north as US 36 in MO, kind of like what happened out there a few weeks ago. If forcing from that super strong trough keeps the area socked in, however, I'd expect a rather mediocre line extending down to AR and the MS valley. Best chances for severe weather look like NE TX at this point, with a strong sheared environment and the best instability. Storms could go up early on Friday afternoon with little capping, but I'm not sure how favorable this setup will be for a chase given that storm speeds might be rather fast, terrain unfavorable, and storm mode questionable.

 

[David Reimer]

Good Morning from I-70 in SW PA. I'd like to take this moment to point out whoever built this zig-zagging highway is a fool.

The 09Z SREF once again returned the 30 percent contour to SE LA for the 21Z timeframe on Saturday. I'm returning to Texas a few days early (I'm driving back there now) in order to chase this event, along with a possible local chase in NE TX tomorrow evening. I wouldn't be too concerned with the instability numbers. Back on November 29 many models only introduced 1,000 J/Kg of SBCAPE/MLCAPE in LA. The 18Z LCH sounding had over 2,900 J/Kg of SBCAPE. Models often have issues diagnosing the amount of instability with these winter time events. The same thing happened back on April 24th (This is not a April 24th event, but features are setting up where there are some comparable similarities. Based off what I'm seeing, I wouldn't be surprised to see instability parameters end up over 2,000 J/Kg in parts of the target area by Friday, even though this is December.

For the past few days I've been concerned with the potential for a displaced low level jet (The NAM has been placing the instability axis about 100-200 miles west of the LLJ) but things are appearing to line up better now, so that concern is dwindling. Here is the 12Z NAM's output for surface based instability on Friday at 21Z.

With a zoomed in version...

If these numbers were to come to frutition (which I bet they are even underdone a bit) a similar result from 11/29 could come to fruition I believe. This definitely has the potential to be an interesting event, and I also wouldn't rule tomorrow night out (overnight event) in parts of E/NE TX. The 12Z NAM did show a few cells firing in a favorable environment.

 

[James Gustina]

Its starting to look like the cold front will be nicely draped across northeastern Arkansas back into northeast Texas with the instability axis of around 1000 j/kg of MLCAPE directly ahead of it in Lousiana and Mississippi. The only issue I see with chasing this would be how little capping there is (virtually non-existent). I'd guess storms will initiate within a low topped mess of showers and garden variety thunderstorms and form a messy squall line due to the moisture surging out of the gulf so far north and having such low CAPE values (well, weak instability is all relative I guess ).

 

[Ryan Wichman]

I would agree that the main tornado threat may end be confined to the deep south on New Years Eve. But there will also likely be a secondary area of severe weather in close proximity to the core low. Adam Lucio did a really neat write up today, comparing what models are indicating to past winter set-ups in the Western Great Lakes.

http://convectiveaddiction.com/forecasts-2/new-years-eve-2010-a-midwestern-severe-event-in-the-making/

The models seem to be playing catch up with regards to moisture and overall instability, which is to be expected during these months. Since the storm did come on shore for the 12z runs they are probably catching on to what will likely unfold. Both the GFS and NAM, along with other models have come into excellent agreement with regards to thermal profiles and storm evolution. Still plenty of time to figure out exact details. As always forecasts will need to be updated with nowcasting tools on late Thursday/Friday.

 

[Bob Hartig]

I would agree that the main tornado threat may end be confined to the deep south on New Years Eve. But there will also likely be a secondary area of severe weather in close proximity to the core low. Adam Lucio did a really neat write up today, comparing what models are indicating to past winter set-ups in the Western Great Lakes.

http://convectiveaddiction.com/fore...2010-a-midwestern-severe-event-in-the-making/

That's a great article by Adam. I hadn't even thought about the possibility of a severe outbreak closer to home, but this time of year 500 J/kg CAPE definitely could get the job done with that kind of shear. Janis Laurens at GRR did a study of tornado climatology in Michigan and found that major outbreaks in our area have tended to occur in high shear/low CAPE setups.

The problem with this kind of setup, though, is that it seems to be so iffy. You could get tornadoes or you could just get a rainy mess ripping along at crazy speeds. Chasewise, if I could teleport to the Missippi River floodplain west of Jackson, MS, that's where I'd presently be thinking about setting up shop. Besides the higher CAPE, storm speeds will be slower while bulk shear should be plenty 'nuff to do the deed.

 

[J Brooks]

My thinking on this one is the area south of I-20 along the Ms. / La. border is where the best ingredients seem to be coming together for this setup. Based on the 12z runs, the highest CAPE of 750-1250 j/kg, combined with 50 kt bulk shear, helicity values of 250-300, and 65+ dewpoints at the time of peak heating appear to be in east and southeast La. and southwest to south central Ms. Discrete supercells will have the best chance of forming during this time. After dark, it looks like the system will transition into a qlcs/squall line as it moves through the area.

At this point I'll more than likely setup in Vicksburg and adjust south from there as needed.

 

[Skip Talbot]

GFS is much better in line with the NAM on this morning's run. Both models are even more bullish with moisture return. Dewpoints approaching 60 are forecast to make it as far north as Quincy, IL on both models. Not bad for the last day of the year, but we'll see if it verifies. As a result of the increased moisture some modest SBCAPE approaching 1000 J/Kg is also popping up on both models. Given the strong speed and directional shear, some honest supercells could be seen if that's realized. We're still pretty marginal on the instability/moisture end of this setup though, so our chances for severe will be conditional on cloud cover and the amount of warm air advection. Right now, I'd target Macon MO to Hannibal MO along US 36 for early afternoon initiation of low topped/minisupercells. I'm not sold on chasing, but this 12z run is teasing me. Points to the south look like they could see some severe weather as well. However, this latest model run actually has higher cape values up in MO, although on a very narrow axis. Winds are better backed and lapse rates should be steeper under that surface low too. You might be fighting the terrain and linear storm modes in the south. I'm biased toward the northern target because of my location too though.

 

[Donovan Gruner]

I'm also liking MO in the US 36/63/24 area. NAM is showing a nice little axis of CAPE around 1250 J/Kg around 3 pm. At this point, the instability is mostly all I'm concerned with, but being so storm deprived and bored, I think this New Years is going to be spent looking at the sky in Missouri. Hope to see some of you out there.

 

[Bob Hartig]

I, too, am becoming a Missouri believer. With my Internet connection finally restored after being offline all day, I've just gotten a chance to glance at some 18Z models. One point of interest is a nice NAM bullseye of 75 J/kg 3km MLCAPE centered over Ashley, MO, northwest of St. Louis. That kind of energy in the lower atmosphere bodes well for low-topped supercells, and it's enough to get me on the road for a backyard chase if I didn't have a gig. After all, how often do you get even half a reason to go storm chasing on New Years Eve! Good luck to all who head out. The fact that I can't go pretty much ensures that at least some of you ought to score.

 

[Jim Bishop]

Glad to see others liking the MO setup too. The shear looks better and the the cold air aloft should support higher CAPE than in central Arkansas. It's gonna be a short window of opportunity, but anything is good in late December. Though storms will be racing so fast it will be very difficult to chase.

Good luck to anyone chasing. A more detailed discussion is on my blog.

Jim

 

[Donovan Gruner]

Despite SPC's thoughts... Still liking MO. 6z model runs still showing a nice little axis of instability in the same area, especially on the GFS. Obviously SPC is going to have to rethink their forecast, being they already have a tornado watch out outside their tornado risk area.

 

[Skip Talbot]

13z Day 1 is about what I expected. I wouldn't expect much of a shot at a tornado north of I 44 in MO now. The cold front is socked in pretty good, and the warm front is being re-enforced by elevated storms. I'm not sure if we'll have any good lift well ahead into the warm sector. That would leave the stuff crossing from AR into MO right now as your play later today (and further development to the south). I'm still going to watch it, but I don't think the northern target is in play anymore. We had to have several things come to together to make this happen like some clearing in the warm sector, cold air advection aloft, and before peak heating is over by early/mid afternoon.