Severe WX April 11th-13th, 2020 Severe Weather Threat

[J-Rab]

For Easter Sunday, there's still just one burning question to me in which I'd love to hear opinions on. Despite this being a very massive tornado outbreak with more and more tornadoes being reported even still, the one part that did NOT make sense to me was Southern AL having very few tornadoes and a very mitigated threat while being surrounded with strong tornadoes. S AL had a lot of surface heating in the warm sector after the midday mess of storms went through, and if North-Central AL and S GA didn't have these problems with reheating, It doesn't make sense that S AL would've had them.

When the two strong supercells that put down the Bassfield tornado family first came out of the elevated LA convection, it was assumed that these supercells would continue to track deep through AL in a very healthy warm sector. The first cell, however, inexplicably died as it got close to the MS/AL border. The cell, when it died, accelerated and turned VERY hard left, going almost due north up the state line. The second, trail cell that also was producing tornadoes did almost the exact same thing at the exact same location. The third supercell in the line, which never quite produced tornadoes iirc but was a healthy looking supercell that looked to become dominant, also behaved very similarly, and again in almost the exact same location. The HRRR nailed the development of the first two supercells, but did not anticipate their death. The burning question for me is: why did these supercells die in what looked to be an extremely healthy environment? Does the rapid left turn indicate capping problems and them becoming elevated as they got to the border, perhaps because the cap hadn't eroded enough? Or is there something else at play here?

In addition, new elevated development in the LA panhandle, further south than the 2x EF-4 family that started just west of the same location, was fully expected to become supercellular with time as they went more surface based in SE MS. This Cellular development, however, never quite went surface based, and it appeared to form into a messy squall before it could become fully surface based. This squall didn't become organized in a QLCS mode until it reached Georgia, with S AL getting a grungy, messy squall that produced nothing but severe winds. As soon as this squall got past AL, it became more QLCS in nature and produced numerous tornadoes, including eventually leading to the stronger ones that impacted Southern SC at about 11Z (two of which got EF-3 ratings, iirc).

I've heard a few theories about why this has taken place, but I'd like to get more opinions from people who know more about this event than I do. The theories I've heard thus far:
1) Surface heating wasn't as great in AL, and the cap was still in place; as a result only more linear storm modes were sustainable in that area
2) Winds veered unexpectedly early, killing surface convergence in S AL. Even if this was the case, why'd the storms suddenly become more tornadic once they reached GA? The shear profiles in AL were still off the charts during the event.
3) The elevated convection in the LA panhandle was growing upscale before it could become surface based, and by that time needed way more time to organize and mature before being able to produce tornadoes. I would favor this theory being that it most closely resembles what I saw on radar during the event, but it doesn't explain the abrupt death of the first supercell family.
4) The shear in S AL was so great that it tore apart the updrafts of any supercells in the area. This could make sense, but wouldn't deeply established supercells be able to harness that energy anyways?

I'm hoping that some of you have more answers than I do. It's certainly an interesting case study given the ridiculous parameters in place in S AL during the event.

I also would love to know what happened to those two tornadoes just as they neared the AL state line. I expected them to plow right into AL and keep on going.Plus, why that cluster of cells that came out of South LA after the first three didn’t form like the first group. They looked identical to the supercells that dropped the two tornadoes, only they were forming just to the East of where the first ones did. Being in West Hattiesburg, and knowing that the two tornadoes missed me by about 5 miles to the West, I knew that the second group wasn’t going to miss me. I 100% expected one or more of that group of about 5 supercells to form tornadoes. I’m very glad they didn’t because one of them went right over my house. They weren’t even a bad thunderstorm, really though. Completely different from those first two.

I don’t know nearly as much as some of the people here who, frankly, could probably work for the NWS. I‘m hoping that somebody can explain why that first group was so much more potent than that second group that come out of nearly the same place shortly after the first. They looked identical when they formed but acted completely differently. As I said though... very glad they weren’t like the first. That Bassfield storm was terrifying, even from a few miles away.

The great information that can be found on this forum is second to none. I figured this thread would be where I’d find an answer, if I ever get one.

 

[xJownage]

The maximum values in this image are +101 and -106 kts. The maximum gate-to-gate shear in any point is 165.2 kts. Unfortunately, for most of the tornado's most intense portion, the values weren't properly resolved and there was a large area of no return in the tornado's center.

 

[xJownage]

I also would love to know what happened to those two tornadoes just as they neared the AL state line. I expected them to plow right into AL and keep on going.Plus, why that cluster of cells that came out of South LA after the first three didn’t form like the first group. They looked identical to the supercells that dropped the two tornadoes, only they were forming just to the East of where the first ones did. Being in West Hattiesburg, and knowing that the two tornadoes missed me by about 5 miles to the West, I knew that the second group wasn’t going to miss me. I 100% expected one or more of that group of about 5 supercells to form tornadoes. I’m very glad they didn’t because one of them went right over my house. They weren’t even a bad thunderstorm, really though. Completely different from those first two.

I don’t know nearly as much as some of the people here who, frankly, could probably work for the NWS. I‘m hoping that somebody can explain why that first group was so much more potent than that second group that come out of nearly the same place shortly after the first. They looked identical when they formed but acted completely differently. As I said though... very glad they weren’t like the first. That Bassfield storm was terrifying, even from a few miles away.

The great information that can be found on this forum is second to none. I figured this thread would be where I’d find an answer, if I ever get one.

At the time on radar, it appeared that the second group of cells never quite went surface based until they became linear. There was a very strong cap present in the LA panhandle, and the first two supercells that dropped the violent tornadoes were elevated clusters until they got around the McComb, MS area.

 

[J-Rab]

At the time on radar, it appeared that the second group of cells never quite went surface based until they became linear. There was a very strong cap present in the LA panhandle, and the first two supercells that dropped the violent tornadoes were elevated clusters until they got around the McComb, MS area.

Thanks man. Yeah, I kind of get that but why didn’t the second group do the same thing? The cap? The first group had the same thing.

Like I said, you guys know a LOT more than I do, so if that’s a dumb question, I apologize.

 

[xJownage]

Thanks man. Yeah, I kind of get that but why didn’t the second group do the same thing? The cap? The first group had the same thing.

Like I said, you guys know a LOT more than I do, so if that’s a dumb question, I apologize.

Well that's half of the question I posed a few hours ago. Why did S AL avoid the worst of things, which boils down to the two supercells that produced the violent tornadoes dying, and the new convection in the LA panhandle not behaving similarly to those. Truthfully, I don't know.

 

[Equus]

I saved a handful of VWPs from central and south AL radar sites through the event though not sure if the wind profile alone would allude to enough clues to come to a definitive answer. I really wish we had some special soundings throughout the day across S AL to be honest. I thought throughout the day that the cap was slightly stronger than forecast; maybe the cap was just strong enough and low level winds just veered enough to ensure messy storm mode out this way. But that's just a thought. Clearly the Alabama line was an atmospheric brick wall to those massive Mississippi cells for some reason.

 

[Timhsv]

For Easter Sunday, there's still just one burning question to me in which I'd love to hear opinions on. Despite this being a very massive tornado outbreak with more and more tornadoes being reported even still, the one part that did NOT make sense to me was Southern AL having very few tornadoes and a very mitigated threat while being surrounded with strong tornadoes. S AL had a lot of surface heating in the warm sector after the midday mess of storms went through, and if North-Central AL and S GA didn't have these problems with reheating, It doesn't make sense that S AL would've had them.

When the two strong supercells that put down the Bassfield tornado family first came out of the elevated LA convection, it was assumed that these supercells would continue to track deep through AL in a very healthy warm sector. The first cell, however, inexplicably died as it got close to the MS/AL border. The cell, when it died, accelerated and turned VERY hard left, going almost due north up the state line. The second, trail cell that also was producing tornadoes did almost the exact same thing at the exact same location. The third supercell in the line, which never quite produced tornadoes iirc but was a healthy looking supercell that looked to become dominant, also behaved very similarly, and again in almost the exact same location. The HRRR nailed the development of the first two supercells, but did not anticipate their death. The burning question for me is: why did these supercells die in what looked to be an extremely healthy environment? Does the rapid left turn indicate capping problems and them becoming elevated as they got to the border, perhaps because the cap hadn't eroded enough? Or is there something else at play here?

In addition, new elevated development in the LA panhandle, further south than the 2x EF-4 family that started just west of the same location, was fully expected to become supercellular with time as they went more surface based in SE MS. This Cellular development, however, never quite went surface based, and it appeared to form into a messy squall before it could become fully surface based. This squall didn't become organized in a QLCS mode until it reached Georgia, with S AL getting a grungy, messy squall that produced nothing but severe winds. As soon as this squall got past AL, it became more QLCS in nature and produced numerous tornadoes, including eventually leading to the stronger ones that impacted Southern SC at about 11Z (two of which got EF-3 ratings, iirc).

I've heard a few theories about why this has taken place, but I'd like to get more opinions from people who know more about this event than I do. The theories I've heard thus far:
1) Surface heating wasn't as great in AL, and the cap was still in place; as a result only more linear storm modes were sustainable in that area
2) Winds veered unexpectedly early, killing surface convergence in S AL. Even if this was the case, why'd the storms suddenly become more tornadic once they reached GA? The shear profiles in AL were still off the charts during the event.
3) The elevated convection in the LA panhandle was growing upscale before it could become surface based, and by that time needed way more time to organize and mature before being able to produce tornadoes. I would favor this theory being that it most closely resembles what I saw on radar during the event, but it doesn't explain the abrupt death of the first supercell family.
4) The shear in S AL was so great that it tore apart the updrafts of any supercells in the area. This could make sense, but wouldn't deeply established supercells be able to harness that energy anyways?

I'm hoping that some of you have more answers than I do. It's certainly an interesting case study given the ridiculous parameters in place in S AL during the event.

Excellent post and very intriguing as well..

 

[J-Rab]

Well that's half of the question I posed a few hours ago. Why did S AL avoid the worst of things, which boils down to the two supercells that produced the violent tornadoes dying, and the new convection in the LA panhandle not behaving similarly to those. Truthfully, I don't know.

I appreciate the answers man. I’ll just keep checking back and maybe the community here as a whole can answer the very interesting questions you posed. This place knows what it’s talking about, that’s for sure. Everywhere I’ve ever gone, I have been the weather geek that wanted to talk about storms. Here though? I feel like a babe in woods when you guys start talking weather. I love the information here.

Thanks again man.

 

[xJownage]

I guess my point about shear profiles and surface convergence in AL (#2 in the post) during the event was moreso about the lack of QLCS tornadoes. Even without the surface convergence, there was still a very high amount of shear; wouldn't that still facilitate tornadic activity along the QLCS? Instead, we saw none until it got to GA when it put down multiple tornadoes.

 

[pohnpei]

absolutely insane tree damage from bassfield

 

[Jacob]

I guess my point about shear profiles and surface convergence in AL (#2 in the post) during the event was moreso about the lack of QLCS tornadoes. Even without the surface convergence, there was still a very high amount of shear; wouldn't that still facilitate tornadic activity along the QLCS? Instead, we saw none until it got to GA when it put down multiple tornadoes.

I'm a bit confused about your point here. That QLCS was dropping tornadoes left and right all the way across Alabama. BMX has surveyed 17 tornadoes (so far), and HUN surveyed an additional 6.

 

[Kory]

I guess my point about shear profiles and surface convergence in AL (#2 in the post) during the event was moreso about the lack of QLCS tornadoes. Even without the surface convergence, there was still a very high amount of shear; wouldn't that still facilitate tornadic activity along the QLCS? Instead, we saw none until it got to GA when it put down multiple tornadoes.

Huh? We just had the fifth largest outbreak of tornadoes in Alabama from the QLCS and embedded super cell structures. Several of them strong...

0-3KM CAPE was lacking a bit on the 00z BMX sounding but the shear profile was textbook in both direction and speed shear. The ungodly amount of shear showed because that QLCS was spitting out tornadoes left and right (Alabama is actually #1 in tornado count from this outbreak at 23). The tornadoes were just unable to sustain themselves for long periods and I think the 0-3KM CAPE had a lot of due with it in my opinion.

 

[vanni9283]

The maximum values in this image are +101 and -106 kts. The maximum gate-to-gate shear in any point is 165.2 kts. Unfortunately, for most of the tornado's most intense portion, the values weren't properly resolved and there was a large area of no return in the tornado's center.

Wait a minute....I thought the total gate-to-gate shear was the sum of the inbound and outbound wind values???

 

[David in SW Blount]

Feels like we dodged a bullet in Alabama for the most part. Not to belittle the damaged experienced in some areas, but with our state having been squarely in the bullseye of the moderate risk and considering the devastation from the stronger tornadoes, we were very lucky.

 

[Fred Gossage]

We may not have had a swarm of EF4-EF5 tornadoes in Alabama, or a two-mile wide record setting tornado, but Easter Sunday is currently tied with April 28, 2014 for being the fourth largest tornado outbreak in Alabama history (with surveys still ongoing)... and five of those tornadoes were rated as strong. If anyone is saying that the event busted in Alabama or it significantly under-performed, they need to find something else to do with their time other than weather.

 

[Equus]

I think the point being made is that south of Central Alabama - where clusters were dropping swarms of tornadoes - the line in south Alabama was surprisingly tame with just some wind reports and a couple of brief weak tornadoes; the area where the warm sector was supposed to make things really volatile

 

[Fred Gossage]

I think the point being made is that south of Central Alabama - where clusters were dropping swarms of tornadoes - the line in south Alabama was surprisingly tame with just some wind reports and a couple of brief weak tornadoes; the area where the warm sector was supposed to make things really volatile

And had people looked at more than just the placement of higher instability, they would've expected that ahead of time. Ask Kory about the discussion I had with my team. We talked about forcing and the height field not actually supporting major activity in far south Alabama 72+ hours ahead of time. Everybody was letting CAPE drive their placement when the highest threat area is always where forcing intersects where the best shear and instability combo is located. The 576dm height contour at 500mb is very very often a good southern bound for the greatest threat. There are sometimes exceptions to that, but not often at all. Even when the SPC was drawing their higher risk area near the coast, the 576dm height contour was never expected to get south of around Montgomery.

To the people that looked at everything, this always looked like a central/north type of threat. The only real part that busted about the thing is that north/northwest AL stayed locked in the cool air.

 

[Equus]

I guess I've gotten to expect the worst to happen on the northern edge of risk areas for that reason, still was interesting to see those Mississippi cells hit a brick wall and turn left though lol

 

[WesL]

Spotted this on Twitter. A cool product that helps track forest loss makes it easy to track tornados through wooded areas.

 

[andyhb]

I guess my point about shear profiles and surface convergence in AL (#2 in the post) during the event was moreso about the lack of QLCS tornadoes. Even without the surface convergence, there was still a very high amount of shear; wouldn't that still facilitate tornadic activity along the QLCS? Instead, we saw none until it got to GA when it put down multiple tornadoes.

Didn't you drop a "this is busting" post on another forum right before everything went haywire on Sunday afternoon?