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Severe WX 1/24/21-1/26/21 Severe Event (Fultondale, AL) (1 Viewer)

rolltide_130

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The Euro is starting to come around and get pretty gnarly for next week's severe event from the Arklatex eastward into Dixie. I initially had some concerns that the ridge was going to get too pumped up for this system to effectively translate east, but as of the last couple of runs that concern seems to be getting alleviated and your pretty classic relatively low-amplitude trough is coming together.

It's your classic low-CAPE high-shear event you see this time of year, but as usual you need to watch for the thermos to be underdone seeing as there could be some fairly stout WAA out ahead of this, with your classic 990s low near St Louis.

The advertised pattern that has been getting watched closely is showing some early signs of its potential, and an early season MS Valley and eastward event is a hallmark of many of our most notorious La Nina years.. this one will be worth a close watch 1611067757497.png
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Fred Gossage

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With a major +TNI (+1.0 and climbing) now in place, we have to watch every single system that comes rolling by.
 

Fred Gossage

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How fast the shortwave deamplifies will do a lot to determine how significant of a risk this ends up being, but as of now, even the less amplified solutions still look to have a decent potential.
 

Fred Gossage

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Waiting on the 12z Euro, but everything has come in supporting the threat on Monday. If we can keep the slight curvature to the trough as it passes by Dixie, this may be a significant event. However, if it continues to flatten and flatten and flatten, we will lose the height falls and large scale ascent that we have, and the threat won't go away, but it won't be nearly as appreciable. As is, if we can keep the very slight curvature and diffluence aloft with the trough, the low amplitude nature may help to lock in a discrete storm mode.
 

andyhb

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Already seeing signs of a stout EML for the time of year with this wave. Various soundings I've pulled have 700-500 mb lapse rates in the 7-8+ C/km range with a rather pronounced capping inversion at the base overlying mid-upper 60s dewpoints. If this trough deamplifies any less than forecast currently, there is certainly potential for a fairly significant event (might still be the case as the 12z Euro shows with a pretty flat wave).

Example sounding from S/SE TX on the 12z UK at 144 hrs (morning of)
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Fred Gossage

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Having a wave as flat as the 12z Euro might make this evolve more akin to November 5, 2018. That was still a decent severe wind/tornado event, including EF2 tornadoes. But the large scale ascent was focused closer to the boundary than out overtop the warm sector, and that led to a more mixed/linear storm mode. If the trough completely goes flat like the 12z Euro shows, it may evolve more like that. If it still ends up with a bit of curvature and diffluence to it, I think we have enough upward motion for things to go in the open warm sector.

Andy is dead on about the large scale thermodynamic environment. The large scale mechanisms for adequate instability are there, regardless of what models show for CAPE, etc., values right now.
 

andyhb

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00z GFS is a total non-event. Wave is running into too much confluence from the trough over Ontario that is forced south due to the big west-based -NAO block over Baffin Island.

Edit: GDPS/UK don't buy it though.
 
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rolltide_130

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0z Euro came back to a more substantial threat as mentioned above, but one thing sticking out of me is the sheer size of this dry slot modeled starting 12z Monday.. this is the plains/SW drought in action and this may be a regular feature this spring. This is more often than not a hallmark of our biggest Nina years..

Also the Euro has sped up ever so slightly and the core of the threat area from 21z to 0z is squarely over AL. Initially looked like the threat would be more towards the MS Delta during peak heating and we would be getting more of an overnight event, but the Euro is starting to cast some doubt on that idea as well..

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Fred Gossage

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It's going to come down to how the vortex on the west side of the retrograding high associated with our long established but about to break down -NAO block behaves. If that wave is more pronounced and/or slower like the GFS shows, that will cause a fair amount of confluence in the Northeast like Andy previously mentioned, causing the shortwave to shear out faster and more significantly and lead to a lower overall threat. However, at least as of the overnight data, most other models (including the Euro, Canadian, and UKMET) did not have that shortwave there and were leading to much less confluence over the Great Lakes and interior Northeast, allowing the approaching trough to be more pronounced, leading to a more substantial threat. At this stage, until we have a better handle on how that will evolve, either solution is believable.
 

Fred Gossage

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The Canadian and Euro were a tick faster, but generally remain unchanged and stronger/more amplified with the trough. UKMET was as well but was holding the timing. GFS is still off doing GFS things. I'm still not ready to rule out the GFS being right because of what is involved in making it come to its solution, but it's off on its own for the most part. Euro, Canadian, and especially UKMET support a potentially substantial threat, and even the most recent run of the parallel GFS (the 6z run, 12z hasn't come in as of the time of this post) is leaning toward a more curved shortwave with a stronger surface low farther north compared to the operational GFS. The UKMET at face value, the parallel GFS at face value, and hand modifying soundings from the others, show that if we warm to 68-71F in the warm sector and have mid 60 dewpoints, there will be substantial CAPE (by cool season standards). All the guidance shows a 300mb temperature of -40C. That's a Matt Grantham red flag value. That's a sign of a profile that would support tall CAPE and intense updraft acceleration deep through the atmosphere. If the wave is stronger and the op GFS is wrong, this may be significant trouble.

EDIT: 12z parallel GFS is in and does not support the operational GFS solution of a weaker/flatter system. I still want to wait a little longer before completely discounting the operational GFS, but it's just about time to call it on its BS. This is a known and documented bias in the op GFS and GFS ensembles.
 
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andyhb

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Regardless of what happens next Monday, if this general look aloft returns in March/April, there are going to be some serious problems. Classic low amplitude Nina pattern with very strong mid/upper level flow via an enhanced polar/Pacific jet favoring EML advection and discrete convection versus linear.
 

Fred Gossage

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Regardless of what happens next Monday, if this general look aloft returns in March/April, there are going to be some serious problems. Classic low amplitude Nina pattern with very strong mid/upper level flow via an enhanced polar/Pacific jet favoring EML advection and discrete convection versus linear.
Agreed and that echoes Jon's concerns in the thread here too. CFS, GEFS, and Euro take the MJO into the Pacific a bit in February (which, sadly, matches some of those analogs we've talked about in the seasonal thread)… only for the CFS to bring it right back to the maritime continent and classic low amplitude La Nina forcing for the latter part of March and into April.
 
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Regardless of what happens next Monday, if this general look aloft returns in March/April, there are going to be some serious problems. Classic low amplitude Nina pattern with very strong mid/upper level flow via an enhanced polar/Pacific jet favoring EML advection and discrete convection versus linear.

So is it time to start playing "Humans Being" on loop yet, or is that for May Plains chasers (which as you have pointed out on this and other forums, the picture going into spring 2021 is a lot murkier for them)?

 

Fred Gossage

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Parallel GFS has steadily trended stronger with the Monday low for the past several runs and completely abandons the operational GFS/GEFS idea. Op GFS is out on a limb all by itself now.

Parallel GFS has a large warm sector of 500-1500 CAPE, mid 60+ dewpoints, mid-level lapse rates of 7-8 c/km, and 300mb temps of -40C too work with as well. If the op GFS isn't pulling off a miracle, then we have trouble coming...
 

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