2021-03-17 EVENT: AR/LA/MS/TN/AL/GA

Patrick K

EF0
Joined
May 2, 2019
Messages
34
Location
Southeast USA
Global models are in reasonable agreement of an upper low moving from near the OK panhandle toward the lower MO valley during the day on Wednesday. A warm front will push northward from central MS/AL toward the TN border by 20Z. A broad area of upper-60s dewpoints seems likely with instability building through the afternoon hours, although the warm sector appears to be mostly uncapped, particularly out ahead of the low. Shear, both bulk and directional, appear favorable for supercells across a wide area.

Given the lack of cap and strong instability, it looks like several rounds of initiation will occur, with the initial position of the warm front on Wednesday influenced by Tuesday night's storms across MS/AL/GA. The NAM3k depicts a round of storms moving from eastern AR into northwestern MS and into southwestern TN in the morning hours, crossing the warm front. A second round of initiation appears to follow in the wake of the early morning storms, along/invof the warm front in the early afternoon hours, around the Memphis area and MS/TN border, generally. The 20z-00z timeframe seems to be the real show, with supercells erupting in this timeframe across a broad swath of the warm sector into a highly destabilizing environment. Forecast soundings show long, clockwise veering hodographs with the LLJ ramping up out of the southeast into the evening hours, presumably maximizing the tornado risk around this time.

Question marks seem to be around warm front placement, cloud cover and ongoing precipitation affecting instability and storm timing, plus the lovely area that Dixie is for storm chasing. After chaser convergence on a narrow setup in the TX panhandle this weekend, it seems there will be much less chaser convergence for this broader setup, but also... trees, lot of trees.

I would target the Byhalia, MS area around lunch time and watch those presumed round of storms, with expectations to catch the early evening storms in that same area later in the day, keeping a close eye on the fine-scale details as they develop that afternoon.
 
One thing I like about Wednesday's setup as opposed to most Dixie alley setups is the presence of steep lapse rates on most of the soundings I've pulled up over Northeast AR/West TN/N MS. I feel like though it could be predominantly a nocturnal show though, however if you get any discrete supercells in that type of CAPE+shear combination being modeled, I would imagine a couple intense caliber tornadoes are certainly in the cards. Whether they are chaseable or visible is another question.

If I had to pick a target right now, I'd probably say somewhere in Northeast Arkansas and hope for a daytime initiation in decent terrain. Northern MS/AL/S TN for nocturnal tornadoes along the warm front seems to be a good bet, but that area is rough chasing and I don't think I wanna get into a situation where I'm looking down more fast moving wide tornadoes with limited visibility.

On the fence as to whether I will actually chase this though, it's only a 6 hour drive, but gas prices are soaring over $3 a gallon here...so gotta pick and choose this season wisely.

If it slows down though, there could be something further west on the actual low pressure itself. The overall synoptic setup gives me vibes of May 10, 2008 tornado outbreak in the Deep South.
 
A few things have changed with this setup, notably the timing. CAMs continue to show 3 rounds of initiation, the first lifting northward with the warm front in the morning, the second afternoon round is notable with storms spread across the open warm sector where there is little capping into MS/AL but also no clear forcing mechanism. Presumably, differential heating from broken clouds is driving this, and while Dixie seems to usually produce in similar situations, I am a bit skeptical here for the same reasons in my initial post. Instability could be limited with heavy cloud cover and/or ongoing crapvection. If you're skeptical like me, a triple point play on the 2nd round of storms in NE AR could be in order and besides the MS river issues, this would be a more ideal chase location.

The third round, driven by actual forcing, appears to have slowed down slightly in recent model guidance. So in addition to trees, darkness could become an issue for chasers. The ultimate position of the warm front appears placed more southerly now, so I think this 3rd round may be better chased from further south in south central MS into AL.
 
Just based on the lead in to this system, scouring over the short term CAMS and SREF and medium runs of the NAM, I get this feeling that the general synoptic setup really puts the highest risk box from Jonesboro, East thru Memphis and Jackson TN, and from Greenville MS, East to Tupelo. The southern end of that box seems slightly more conducive once warm frontal surge begins but its a big question as to how much clearing can happen to boost up temps. the northern end seems to get the stronger mid level dynamics. I think the wrinkle will be just how much clearing can occur or will there be a lot of elevated convection around to limit the insolation diurnal recharge. which may be the case closer to the warm front, but further south they may end up getting some clearing which could amplify the threat there later in the period... well because I am just slightly bored at the moment.. I made up a really quick visual. Just for fun, that's all .. Best Chances for EF-2+ in the darkest shade 20-02Z
 

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My target area was, and still is, Memphis TN. It provides a great location to move into NE Arkansas or south into Mississippi for any "round 2" storms that can stay discreet and rotate.

I considered a chase tomorrow, but my concern about "round 1" convection polluting the area, cell speed, coupled with lack of desire to chase the wide area nocturnal show in the jungles of MS/AL, kept me at home.

Watch the Jonesboro to Pine Bluff, AR area for the afternoon show and try to get them before the nocturnal show lights off farther SE into MS and AL.
 
Likelihood of supercells initiating in the flat clear Delta makes Wednesday somewhat interesting. However the Delta is not that wide and storms will move quickly; so, the easy show may be brief.

Also appears two surface troughs will be in between the cold front CF and warm front WF, which sometimes happens in Dixie Alley. A quasi-dry-line like feature in central Arkansas (ahead of the CF) may go late afternoon, moving toward the Delta evening. This line could have some supercells.

The second pre-frontal trough will be east of that, a convergence zone still west of the warm front / southern Apps mess. Said pre-frontal trough should explode in the Delta late afternoon and through West Tennessee and Mississippi evening. I would probably chase this line, unless it's too sloppy. Line just to its west could be a backup plan.

I don't like anything too much farther east because of terrain. North Central Alabama is decent terrain again (relative terms here) but it'll be after dark then. However Alabama faces the real threat of a strengthening LLJ increasing low-level shear at night.

I also don't like the afternoon warm sector stuff near the warm front because it'll probably be sloppy. Delta seems like the only play, if one can keep up with the storms.
 
Jeff House QUOTE
I don't like anything too much farther east because of terrain. North Central Alabama is decent terrain again (relative terms here) but it'll be after dark then. However Alabama faces the real threat of a strengthening LLJ increasing low-level shear at night.

I also don't like the afternoon warm sector stuff near the warm front because it'll probably be sloppy. Delta seems like the only play, if one can keep up with the storms.
[/QUOTE]
I agree with your southern Alabama nocturnal LLJ increase past 00Z, could be a pretty intense QLCS - BKN line with embedded LEWP tornadoes over that area supported by deeper moisture field and real strong 60kt LLJ Theta-e region form the gulf coast up through I-85. With so Much PW , it's gonna be tough to chase anyway with any pre-frontal discrete cells being HP for sure, and the worst is the QLCS / frontal nocturnal embedded circulations. All moving at 50mph and faster? over rolling hills and heavy foliage areas? yeah no thanks, chasing in the south is definitely more complicated in some ways.

The best play IMO is the River Delta east of Pine Bluff , Tupelo west , Memphis region with passing Vort lobes, best 500mb diffluence - pre frontal trough interplaying the warm front. The complications over this are are outflows cold pool interactions from early morning convection, limited clearing, and not the healthiest Lapse Rates across the area. So I think one of the wrinkles is overall clearing for total instability, but I think even with a BKN skycon, the dynamics and Theta advection strength may offset a chunk of it but, it also may limit the discrete formation further south until it gets closer to the cold frontal passage, tough forecast there for sure.

After that, and as the front becomes more of a solid BKN to solid line with embedded structures, it's selected areas of better chase visibility over Cntrl MS to AL. but by then its way past dark and a chase nightmare.
 
Central Mississippi is experiencing solid insolation to start the day which will only add to the instability. Morning soundings show an EML advecting over the warm sector with steep lapse rates over a very moist boundary layer. Morning models show discrete/semi-discrete supercells across the risk area this afternoon and evening. All indications suggest a major tornado outbreak is on tap today.
 
This is shaping up to be a partly nocturnal, classic Dixie Alley tornado outbreak. Not good for those areas in Mississippi and Alabama, where people will be out celebrating St. Patrick's Day. Also notable: today is the first SPC high risk day in March since 2012, per Wikipedia.
 
I chose to sit today out on purpose, I didn't like the terrain, storm mode, and the fact that things could linger well after dark. However I have a couple chase partners that have ventured south for the day so I'm wishing them the best of luck and hoping for some good visibility for those guys. I think if I had to pick a target I'd be sitting in Columbus, Mississippi this afternoon waiting on open warm sector development. I've been watching this Columbus to Jackson, MS area for a while and as of 10am, I'm still liking what I'm seeing.

If I want a guarantee of getting on severe storms in good terrain, I'd probably hop northwest toward Batesville, MS. I've chased there a few times and its not awful. Although I have concerns about areas further west being more quasi-linear with convection (although this doesn't mean there won't be strong tornadoes, just very hard to spot and fast moving). I think the eastern target over Eastern MS/Western AL is a better bet for a sustained long tracked violent tornado (perhaps similar to what we saw last year on Easter). As with everything weather though, it just takes one or two ingredients to be slightly out of place to make for a total mess.

Right now though I'd stand at Columbus, MS around 20-21z, adjust from there. Long day ahead! Good luck to all chasing!
 
Based on the latest trends, I'd probably adjust my target southeast to Meridian/Hattiesburg, MS over toward Montgomery, AL. A lot of convection has gone up in the warm sector well ahead of the main front further to the west. The BMX 18z sounding showed an awesome wind profile with great hodograph, however limited instability/lapse rates. Jackson 18z sounding shows a bit of veer back veer around 700mb, although looking back to the S/SW near New Orleans, shows better soundings with less issues with regards to VBV and lapse rates.

I'd probably hope something comes out of that cluster that is moving from Louisiana/Arkansas into Mississippi and see if anything can remain discrete. Environment over much of Mississippi and Louisiana is still not quite overturned yet, although surface winds are veering a bit to the SW ahead of the front, this could back more toward 00z as the low pressure approaches. Certainly a messy chasing scenario as it stands now! Patience is key. There have already been several tornadoes reported in that corridor I mentioned including ones near Laurel, MS, Tuscaloosa, AL, and southeast of Birmingham as well. We will see what the rest of the afternoon holds.
 
After Watching today. There was so much going on with juvenile or crap-vection, that I think it some ways it was a blessing that more cells didn't produce large tornadoes. I think it was too moist, with too much in the way of total convection and not enough dry air clearing in the mid levels despite the major amount of shear instability. Where there were pockets of clean undisturbed air from near by convection, there were some small pockets with short duration breakouts, alot of the velocities I saw with the exception of a few cells were pretty low end and that's a good thing, the whole area got pretty lucky I think.

As this thing evolves over my area tomorrow, I kinda think the same, yes the area is warranted on general principle , yes the dynamics are there, but if its all tied to the main line of convection, it will be isolated and embedded mainly in local LEWP formations or embedded discrete.. The only place I see as of right now that seems plausible for more isolated discrete is Charlotte to Raleigh in association with a lee-side surface low that the models are picking up on; will certainly wait to see if that presents itself tomorrow with timing and location of the warm front interaction, but I've seen more than my fair share of tornado genesis from Greenville to Charlotte near and along I-85 due to this.

My second area is over the 95 Corridor from Charlotte to Florence, mainly from Raleigh/Fayetteville east to the near the coast and south from New Bern into Wilmington NC, with more backed winds coming off the water there and a bit more in the way of CAPE.

So again, just for my personal fun, I made a graphic.. and hopefully I will be out chasing to see how this pans out.
 

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I did not do all that much analysis for this event, beyond reading SPC products and looking at a little model output, and while it unfolded I was at work so did little more than check radar every now and then, and read a few MSDs. From what little I had seen leading up to the event, it looked like conditions were most favorable in MS, but reading the storm reports this morning shows almost all of the tornado reports were in AL. Not surprised by the number of TOR reports in AL, but surprised there weren’t more in MS. I assume this is mostly attributable to the factors @Ethan Schisler noted above? Any additional post-mortem thoughts?
 
I kind of want to rescind my overnight post. with conditions as they are based on this mornings Obs , Low Cig/Vis all over the state and cooler SST's funneling moist but cooler conditions, the theta WAA transport while great, is not the best scenario today. the boundary layer is cool still and the satellite and Obs confirm that it will probably remain cloudy most of the day aloft, which will in turn keep temps tempered and the Vis slow to recover until winds and instability rapidly increase just prior to boundary passage. There is still plenty of kinematics, but I think our shot of seeing anything significant really ends up being where and if the sun can beat down on the ground in advance of the boundary which doesn't look all too likely until right now anyway. I am feeling pretty disappointed I didn't see this yesterday, but should have given the synoptic setup and timing. Mucho skeptical now.
 
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