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

[SilentShadow87]

Well, with 9 EF3 and two EF4 tornadoes, this is now officially the most intense tornado outbreak since March 2, 2012.
I was definitely expecting a pretty robust outbreak, but this is unbelievable. What a horrific day for so many people. Like I said a few days ago about March 3, you never root for this kind of destruction and loss of life.

 

[Equus]

I really do hope contextual evidence gets a better standard in whatever EF scale updates are coming, it would certainly work if there was a standardized guidance for interpreting additional evidence in borderline cases instead of a subjective free-for-all left to individual surveyor opinions when regarding contextual factors. Have heard murmurings of an expansion in DIs to account for more of these subjective things, but have also heard that tree damage might be taken out, which is a double edged sword (way more tree variation than just 'hardwood' and 'softwood' on the current scale, but I also don't want to see tree damage become entirely subjective without some guidance)

 

[TornadoFan]

Up to over 100 tornadoes in this outbreak in total.

 

[Shakespeare2016]

Up to over 100 tornadoes in this outbreak in total.

The main outbreak must have produced 80+ tornadoes over a period of less than 24 hours. It started on the afternoon of April 12th and went into the morning hours of April 13th.

 

[KoD]

The velocity signature from the Bassfield tornado was so extreme that I'd hope the survey team is very thorough in finding DIs that prove its ground level velocity matched it's radar intensity.

Pixels end to being miles wide on the reflectivity, and well above the ground, but g2g shear was 200+mph.



If historical radar data tells us anything there's no doubt this was violent at ground level. I couldn't imagine anything less than EF4 from this monster.

 

[vanni9283]

The velocity signature from the Bassfield tornado was so extreme that I'd hope the survey team is very thorough in finding DIs that prove its ground level velocity matched it's radar intensity.

Pixels end to being miles wide on the reflectivity, and well above the ground, but g2g shear was 200+mph.

View attachment 2925

If historical radar data tells us anything there's no doubt this was violent at ground level. I couldn't imagine anything less than EF4 from this monster.

Can you include the actual velocity values in this image?

 

[vanni9283]

If you look closely at the official EF scale DIs, the absolute maximum that an average frame home can be assigned is 200mph (high EF4) without using the slider to bump it toward upper bound - which requires 'superior' construction above the building code. The problem with that is not only the rarity of 'superior' homes, but that a 'superior' home has to be hit at PEAK intensity, and be located somewhere that the WFO doesn't assume failure mode was due to other non 'superior' homes nearby disintegrating and pelting it with debris. AND, of course, being in a WFO that will even acknowledge it. Given all that, you can see why EF5s are so rare, especially when frame homes are about the only major DIs to go off of.

What is a WFO?

 

[Equus]

Weather forecast office

 

[Shakespeare2016]

The velocity signature from the Bassfield tornado was so extreme that I'd hope the survey team is very thorough in finding DIs that prove its ground level velocity matched it's radar intensity.

Pixels end to being miles wide on the reflectivity, and well above the ground, but g2g shear was 200+mph.

View attachment 2925

If historical radar data tells us anything there's no doubt this was violent at ground level. I couldn't imagine anything less than EF4 from this monster.

The velocity signature from the Bassfield tornado was so extreme that I'd hope the survey team is very thorough in finding DIs that prove its ground level velocity matched it's radar intensity.

Pixels end to being miles wide on the reflectivity, and well above the ground, but g2g shear was 200+mph.

View attachment 2925

If historical radar data tells us anything there's no doubt this was violent at ground level. I couldn't imagine anything less than EF4 from this monster.

That looks so unreal. It looks eerily similar to the Tuscaloosa radar image on April 27, 2011.

 

[andyhb]

Tornado from the second supercell now with a 93 mile path length.

 

[xJownage]

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.

 

[andyhb]

I think it's probably a combination of all the failure modes you mentioned, aside from maybe #4.

 

[warneagle]

Chalk up another significant tornado.

 

[andyhb]

7 fatalities now from the Bassfield-Soso tornado btw per NWS JAN PNS.

 

[Brice]

From what I’ve heard, there is a specific reason it gets left at 200 MPH. This was done intentionally by those who created the EF Scale, so that the only factor differentiating EF4 and EF5 damage, is contextual damage. Actually a great idea, but the problem is, there is no official guide to interpreting contextual evidence. In fact, some WFOs disregard contextual evidence altogether.

In my opinion, there should be a supplementary guide on how to interpret contextual damage in association with established DIs. It would clear up so many inconsistencies.

Rochelle had max winds of 200 mph but, it wasn't an EF5 for that reason?

 

[Equus]

EF5 technically starts at 201, or 205mph if we go by the NWS 5-MPH rating increments, so a 200mph tornado still falls under the EF4 range by the slightest of margins

 

[Brice]

Okay, that makes sense.

 

[Mike S]

This outbreak was definitely not to the extent of the outbreak of 4/27/11 but there are more pages on this thread than that one.

Keep in mind that from the time the event was unfolding until several days after there were widespread power outages. It was Sunday before I personally had power. That likely cost us quite a few pages.

Also, you guys have done a great job of keeping this going even after the storms exited Alabama and continued east.

 

[Equus]

Also the 4/27 thread kept going in the years following (I have 105 pages saved) but this is just right after the event. The massive power outages on 4/27 no doubt accounts for posts slowing down throughout the event on that brutal day.

 

[KoD]

Can you include the actual velocity values in this image?

I wish I had that data but I didn't save it, just recall inspecting it on Radarscope. If somebody had Gibson Ridge software and data they could probably derive it and post it.