2021-03-25 EVENT: AR/LA/MS/AL/TN/KY

Patrick K

EF0
May 2, 2019
16
21
1
Southeast USA
On Thursday, a positively-tilted trough is forecast to move from SW Texas into the MS valley and deepen as it develops into a closed low. Being Dixie Alley, moisture appears in place on morning of, with dew points climbing into the upper 60s / lower 70s as early convection lifts northward on the heels of a rather diffuse-looking warm front moving into TN. Behind this convection, some degree of air mass recovery looks likely, with favorable parameters spreading northward across much of central MS by about 18z. Certain parameters appear to align in this timeframe with good 0-1km SRH, STPs in the 4-5 range over a wide area, and widespread 1500-2000 j/kg MLCAPE. NAM3k forecast soundings from today's 12z model are of the PDS tornado variety in this timeframe.

The potential mitigating factors for a major tornado outbreak are similar to the 3/17 event, with a more spatially confined warm sector for storm development, also relatively uncapped and so instability, cloud cover, and early convection timing could be issues. Additionally, storm motion is close to parallel to the forcing gradient, with storms firing closer to said gradient in this event, which could be a potential storm mode issue. Despite early indications of PDS tornado soundings, hodographs are a bit messy above 2-3km, which could present another storm mode issue. And, yes, positively tilt situation as well. Still, the setup seems too favorable to ignore.

The usual Dixie Alley chasing considerations will be of interest, of course, and despite small-scale details needing resolved closer to the event I would target near Philadelphia, MS.
 

Jason N

EF1
Mar 2, 2021
60
54
6
South Carolina
I agree with your assessment overall. The marginal mid Lapse rates, total cloud cover and mixed mode convection, the limited spatial region as you mentioned being the largest potential detractors Vs. the expected dynamic forcing and CAPE/SHEAR instability. A challenge that I think will become more clear as we close in timing wise. I kind of eyed up a box from Batesville to clarksville, south to indianola, east to Winona, mainly for the better chase terrain, and potential stronger pre-frontal forcing, though the STP/Shear parameters drop over there a bit. whatever Discrete does form seems to quickly evolve into QLCS over that area.
 
Oct 10, 2004
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292
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35
Madison, WI
Is it really a positively tilted, closed low (like last Wednesday), though? On this forecast 500mb chart from today's 12Z GFS, it looks more like a big open trough with a subtle negative tilt (look at the 549 to 558 dam lines from NC KS to NE TX). What am I missing? Screenshot_2021-03-23 Models GFS — Pivotal Weather.png
 

Jason N

EF1
Mar 2, 2021
60
54
6
South Carolina
Well if you follow the 500Mb forecast out to beyond 72hrs, that Trof/Open wave closes off again over Quebec. That may be what he was referring to. When I was agreeing with him on his analysis, I was more referencing the sub synoptic stuff.
 

Jeff House

Supporter
Jun 1, 2008
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Chattanooga, TN
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Upper level pattern is characterized by a broad mean trough in the south-central US with a vigorous short-wave ejecting across Dixie Alley on Thursday. Regardless of how one labels it, the set-up favors tornadoes. Note forecast 500/200 mb winds are WSW. No sloppy SSW. Even SW gets it done in the South. WSW 500/200 mb as forecast has accompanied regional tornado outbreaks in the past.

Then the LLJ is straight S or SSW, which is correct for Dixie Alley. So far no strange 700 mb kinks are forecast in the Hodo. In fact the forecast hodograph is pretty clean in the undisturbed warm sector. EML is forecast less sloppy than that last week. Day 3 is just a bit early for soundings, but they show the same gradual turning and strengthening wind with height found on forecast constant level charts. Just had to cross-check the temp profile too.

Similar to last week, an outflow boundary OFB should be found south of the synoptic warm front WF. OFB should have greater instability. The WF will probably make it into Tennessee this time. NAM has Kentucky, but we'll see. Regardless I believe the OFB will set up from Alabama and arc back to somewhere in Mississippi. Could get up into West Tennessee. Instaiblity will likely be greater along the OFB than WF.

Chasers might consider the OFB intersection with a pre-frontal trough PFT coming out of Mississippi. Last week the Mississippi PFT kind of disappointed relative to Alabama, but that's last week. I like the intersection of any OFB and PFT or convergence zone Thursday.
 

Patrick K

EF0
May 2, 2019
16
21
1
Southeast USA
Is it really a positively tilted, closed low (like last Wednesday), though? On this forecast 500mb chart from today's 12Z GFS, it looks more like a big open trough with a subtle negative tilt (look at the 549 to 558 dam lines from NC KS to NE TX). What am I missing? View attachment 21292
Yes, sorry, I should be more careful with wording and which products I'm looking at when writing this. I got the positive tilt below, and the closed low will develop close to concurrently with the end of the event.
 

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Mar 30, 2008
1,276
1,166
21
Norman, OK
www.benholcomb.com
Dug into this and will be pulling the trigger. I felt Deja Vu looking over this setup and was reminded of the Yazoo City day 2010-04-24. Very simlar setups and even the hodographs are similarly shaped. While that setup generally busted, the one strong-long tracked tornado made it a memorable day. Here's the observed 13Z sounding from Jackson that day

20100424_JAN_13Z.gif

And here is KJAN for 15Z tomorrow

nam_2021032412_027_KJAN.png

Yikes. 60 knot Low-level jet cranking all night and morning direct from the gulf. Very rich gulf moisture will be in place tomorrow. The dynamics of this system are on the high end.

Chasing will be a nightmare with storm motions on that aforementioned sounding of 45knots. Hopefully the terrain improved since Yazoo day down there
 
Oct 10, 2004
1,219
292
11
35
Madison, WI
Dug into this and will be pulling the trigger. I felt Deja Vu looking over this setup and was reminded of the Yazoo City day 2010-04-24. Very simlar setups and even the hodographs are similarly shaped. While that setup generally busted, the one strong-long tracked tornado made it a memorable day. Here's the observed 13Z sounding from Jackson that day

View attachment 21303

And here is KJAN for 15Z tomorrow

View attachment 21304

Yikes. 60 knot Low-level jet cranking all night and morning direct from the gulf. Very rich gulf moisture will be in place tomorrow. The dynamics of this system are on the high end.

Chasing will be a nightmare with storm motions on that aforementioned sounding of 45knots. Hopefully the terrain improved since Yazoo day down there
The Yazoo City EF4 was so exceptional that it almost singlehandedly gives that day a DPI (Destructive Potential Index, a method of ranking tornado events through some formula of EF-rating, path length, and path width) higher than all but the two Super Outbreaks and maybe one or two other days since (1950?, I think is as far as they went back and calculated it). In my mind, the occurrence of a tornado like that (Vilonia, AR 2014 is another example) verifies a high risk "in spirit" more so than a smattering of EF2s and below even if they do technically verify the criteria, such last last Wednesday.
 
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Patrick K

EF0
May 2, 2019
16
21
1
Southeast USA
I see this too, the target area appears a bit larger. The warm front is more northward and both it and the trailing cold front are more sharply defined in the recent runs, perhaps from having the system deepen more quickly. The dewpoints are also a bit higher, into the low 70s, along a corridor from eastern MS into the AL/MS border area on both NAM3k and HRRR 18z. HRRR and NAM are now both bullish on storms in the north-central AL/MS border area.
 

Jason N

EF1
Mar 2, 2021
60
54
6
South Carolina
Well, I don't think there's any question about the setup as a whole, it's certainly primed for Severe and Tornadoes. The box I was interested in my earlier post from today seems to have shifted a bit to the east. Which in terms of chase terrain, gets more difficult for anyone on the ground. Based on the more recent runs of the CAMS , Probabilistic, and deterministic models, they seem to favor an area along I-55 from Winona North to Memphis , From Winona East to Jasper/Cullman AL and North up into Columbia TN. I am especially concerned for Tupelo to Corinth over to Florence and Haleyville based on most recent NAM/SREF .. recent HRRR however seems to currently favor a further south CI location from Jackson to Meridian MS, Tuscaloosa to Birmingham, perhaps firing on a lingering outflows from AM convection or confluent lines, but the theta-e, wind gust and 10M wind forecasts seem to show these old boundaries mixing out by 16Z .. the better mid to upper dynamics seem to favor a slight more northerly location. Perhaps two area CI will result.

Either Way, the RAP/NAM SKew-T's are pretty intense over most of the MS/AL region but for me, It's going to be a busy day it appears.

Current HRRR/NAM NEST sim ref. seem to be slightly over doing convective coverage over LA/MS but not by a lot.

I'll be curious to see if SPC upgrades to High tomorrow or 30% for TOR. who ever goes out, be safe, Stm Motion Vectors are 210-230 @ 45-50kts.. so lets hope whatever happens it does minimal damage and everyone stays as safe as they can.

if I were out chasing this would be my focus box tomorrow.
 

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Mar 30, 2008
1,276
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Norman, OK
www.benholcomb.com
The 00Z NAM seemed to say hold my beer and watch this. Some absolutely ridiculous skew T's in northern Mississippi and Western Tennessee at 18 and 21z. This seems primed for some serious long-trackers. I would not be surprised to see a 100+ mile long track tomorrow.

Can't disagree with the chase region above. I'm staying on the southeast side of Memphis metro tonight and I'm not sure I'll be going much further than Tupelo tomorrow.

0-1 SRH values are anywhere from 250-350. I think like the 2011-04-27 event, even rainshowers will be producing tornadoes tomorrow.

Of big concern to me are 3km CAPE values of 150-175 consistently throughout the warm sector.
 

Patrick K

EF0
May 2, 2019
16
21
1
Southeast USA
Things have really come together for this setup. I haven't seen this kind of SPC language probably since 4/27/11:

This is an uncommon, upper-echelon parameter space. In such an environment, any relatively discrete supercells will be capable of multiple tornadoes, some long-tracked and strong to violent (EF2-5 possible), with considerable destructive potential. A very moist boundary layer also will reduce potential cold-pool/outflow strength via less subcloud evaporation, so that even closely spaced storms may have substantial tornado threats.
Last night I still had some concerns about storm mode and interference but SPC is even poo-pooing that now.

What are the potential flies in the ointment? Perhaps cap erodes more quickly and/or instability builds more quickly, and storms go earlier into a less favorable parameter space? This only seems like a slight mitigator, although recent NAM3k runs seem to show little storm activity in the warm sector, preferring to delay initiation and having it closer to the front.

On a side note, and apologies if this is inappropriate here, but the new NWS mosaic radar is terrible. Does anyone have a decent replacement? I found the College of Dupage site, which is nice, does anyone know of a better alternative?
 
Mar 30, 2008
1,276
1,166
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Norman, OK
www.benholcomb.com
PDS Tornado watch out with >95/>95 tornado probabilities. I'm going to meander over towards Tupelo here shortly. Clearing per vis sat has been on most of this morning from Starkville to Columbus to Tupelo and West Point area. I feel like might be one of the biggest corridors for violent tornadoes later with HRRR and NAM soundings showing 0-1 SRH of 300.
 

Jesse Risley

Staff member
Apr 12, 2006
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LSI values are starting to cool, showing the erosion of CINH across much of the risk area, and latest RAP analysis shows that said trend will continue to occur ENE as the 500 mb perturbation approaches from the WSW and the pre-frontal trough starts to push east while the main surface pressure gradient tightens to the NNW, which will all help ignite the show relatively soon I suspect. Vis sat continues to show a nice area of clearing across the greatest threat areas, i.e., the golden triangle region.

The 15Z JAN sounding already shows the environment well-primed then, with ample SRH and profiles supportive of rotating updrafts, and an environment for good vertical parcel acceleration too (see below). I'd expect a 20z special sounding or two as well.

1616692623357.png
 

Dean Baron

Supporter
Sep 25, 2006
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Minneapolis, MN
I'm curious how those veered surface winds in MS will impact things today. Shear values are obviously impressive but a lot of that seems to be coming from the large amount of speed shear. I don't see a ton of directional shear, which is supported by the relatively straight line hodograph Jesse just posted. I could be wrong and the combination of strong instability and speed shear will be enough to get it done today. I just always get wary when surface winds aren't at least out of the due south.
 

Jason N

EF1
Mar 2, 2021
60
54
6
South Carolina
On a side note, and apologies if this is inappropriate here, but the new NWS mosaic radar is terrible. Does anyone have a decent replacement? I found the College of Dupage site, which is nice, does anyone know of a better alternative?
You are completely right, the Mosaic is horrible. As for alternatives, Accu-weather has a national radar Mosaic that's a little better, mainly in the sense that it actually loads and doesn't stop mid way. However the site is advertisement heavy and annoying.
 
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Jesse Risley

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Apr 12, 2006
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I'm curious how those veered surface winds in MS will impact things today. Shear values are obviously impressive but a lot of that seems to be coming from the large amount of speed shear. I don't see a ton of directional shear, which is supported by the relatively straight line hodograph Jesse just posted. I could be wrong and the combination of strong instability and speed shear will be enough to get it done today. I just always get wary when surface winds aren't at least out of the due south.
Just taking a closer look it appears winds are better backed further northeast in the golden triangle region, but it is a valid observation on the special JAN sounding. This looks to be a product of previous convective activity, frontal placement and deepening cyclogenesis relative to the position of the warm front as well, which was just south of JAN at 15z. The CI seems to be co-located with this pre-frontal wind shift axis moving ENE, where winds are more southeasterly from Oxford-Tupelo-Muscle Shoals-Savannah, TN, so that region east of the wind shift and proximal to the WF southeast of the main surface low will be a prime area to watch over the next few hours. You can definitely see the thermal gradient as the WF has peregrinated towards the US 78 corridor.

1616695339202.png
 

Patrick K

EF0
May 2, 2019
16
21
1
Southeast USA
A cyclic supercell has produced damaging tornadoes south of the Birmingham, AL area with major damage in Helena and Pelham.. Terrain, HP nature and low LCLs are making it nearly impossible to see. I heard a chaser utter the "<expletive>, that was close" phrase on stream. Be careful out there!
 
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Jesse Risley

Staff member
Apr 12, 2006
2,190
653
11
41
Macomb, IL
www.tornadoguys.com
A cyclic supercell has produced damaging tornadoes south of the Birmingham, AL area with major damage in Helena and Pelham.. Terrain, HP nature and low LCLs are making it nearly impossible to see. I heard a chaser utter the "<expletive>, that was close" phrase on stream. Be careful out there!
These cells are taking advantage of the more ambient 0-3km MLCAPE and surface vorticity in an environment of rich H85 Theta-e on the periphery of that better instability axis. That axis should continue to shift north and east with the 850 mb jet core over the next 3-4 hours.
 
May 10, 2007
51
5
6
69
North Little Rock, AR
A cyclic supercell has produced damaging tornadoes south of the Birmingham, AL area with major damage in Helena and Pelham.. Terrain, HP nature and low LCLs are making it nearly impossible to see. I heard a chaser utter the "<expletive>, that was close" phrase on stream. Be careful out there!
A little after 2 pm, James Spann reported on-air (ABC 33/40, Birmingham) that there had been "major damage" at his house. His wife was in shelter and was not injured.