Severe WX December 10 & 11, 2021 Severe Threat (10 Viewers)

Fred Gossage

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GFS/GEFS/Canadian are speeding up the evolution of the Friday night threat a little, and height falls start reaching middle TN, northwest/north AL, northeast MS sooner. I'm thinking more and more that these areas get involved Friday overnight, and that's when 0-6 km vectors will be more veered and moderate large ascent extends farther out into the warm sector. We'll see if UKMET/Euro continue support for the idea. Saturday, whatever happens with it, looks more and more like a resurgence of whatever ongoing threat there is from overnight Friday night into Saturday morning. Canadian is fastest with this idea, with north/central Alabama being a Friday evening through Friday overnight threat, and things lining out and sweeping through Alabama going into Saturday daybreak. If we're leaning toward a faster timing, I think the supercell threat would increase because deep-layer shear is more veered, and the trough is less meridional prior to the ejection of any second piece going deeper into Saturday.

EDIT: ICON model heavily supports this evolution as well. UKMET is out through 84 hrs as the of the time of this, and Euro is coming relatively soon.
 

Fred Gossage

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You can throw UKMET's hat into the ring for getting the MS/north AL/middle TN portion of this going during the evening and overnight hours of Friday. It keeps the threat going into Saturday, but I'd have to think we establish a cold pool at some point going deeper into Saturday morning, and that trough goes increasingly meridional as we head deeper into Saturday as well.
 

Richardjacks

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You can throw UKMET's hat into the ring for getting the MS/north AL/middle TN portion of this going during the evening and overnight hours of Friday. It keeps the threat going into Saturday, but I'd have to think we establish a cold pool at some point going deeper into Saturday morning, and that trough goes increasingly meridional as we head deeper into Saturday as well.
I am leaning toward an overnight Friday as well, but I could see it becoming a very long night central Miss into Central Alabama as the main trough and lead energy kicks out enough to push the ongoing storms eastward, then just slowly migrates as the whole system starts to move.
 

Fred Gossage

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This may be the first time I've seen the GFSX (MEX) MOS have 65+ dewpoints in the warm sector of a system like this. Even in events where we have 67-70 dewpoints verify, the MEX guidance out ahead of time, because of the biases and such that it has, often has dewpoints in the 50s to low 60s at most. Really high confidence that, regardless of evolution, we're going to have the low-level thermodynamics well in place for this one.
 

chasestormz39

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This may be the first time I've seen the GFSX (MEX) MOS have 65+ dewpoints in the warm sector of a system like this. Even in events where we have 67-70 dewpoints verify, the MEX guidance out ahead of time, because of the biases and such that it has, often has dewpoints in the 50s to low 60s at most. Really high confidence that, regardless of evolution, we're going to have the low-level thermodynamics well in place for this one.
Thoughts on this expansive warm sector reaching areas as far north as MO/IL/IN/KY?
 

Fred Gossage

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Thoughts on this expansive warm sector reaching areas as far north as MO/IL/IN/KY?
I have high confidence that mid 60 dewpoints spread up through a good bit of Kentucky. I'm thinking more and more that lower 60 dewpoints get into southern IL/IN/OH. However, we will have to look at cloud cover and surface heating (which can be accomplished by WAA if there aren't many sun breaks) to see how much instability can be realized. I do anticipate a few hundred j/kg of CAPE as a bare minimum north of the Ohio River though, which will be enough for a tornado threat, and then southern west/middle Kentucky, CAPE may end up being similar to what it is from Tennessee southward. The southeastern areas of Missouri also get in on the more significant low-level moisture and instability as well.
 

Fred Gossage

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Euro supports the idea of a faster evolution, with maybe north Mississippi into west Tennessee possibly getting involved out in the open warm sector as early as mid/late afternoon. Looks increasingly likely this is a Friday evening to predawn Saturday event for northeast MS, northwest/north AL, middle TN. Euro has the storms over HSV to BHM by 6am Saturday, and moving east from there. Canadian supported a similar timing, and GFS was only about 3 or 6 hours behind, but then it laid the line over on itself and flared it back up over central to northeast AL going deeper into the day Saturday. We are getting the models get into agreement on the overall setup and the main idea for the timing and evolution though.
 

Fred Gossage

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The strongest convective signal on the Euro for the event is from Midnight to 9 am Central Time Saturday morning as the storms are moving into northeast Mississippi and middle Tennessee, and then across north AL, middle TN, and into northwest GA and east TN. There is stuff to the west prior to that from the midday/afternoon into the evening, including a signal we may have to watch for renegade stuff in northeast MS/northwest AL as early as late afternoon and early evening... but the strongest convective signal is MS/AL/TN. It makes me wonder if the southeast trend/bias we've seen several times may come into play again.
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Those storms after midnight are strong enough to be disrupting the 0-6 km shear field, and that continues through 9am Saturday. There is no signal for that prior to then.
 

akt1985

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Overnight events are never fun. However, I wonder how this potential event will be recognized by the general population going into the weekend when more people are likely to sleep in early Saturday morning versus having to get up for work the next day? Since I don’t have to get up for work on Saturday, I can stay up all night if need be tracking the storms.
 

KevinH

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They are mentioning potential addition of probabilities in subsequent outlooks, however. They're still sticking to citing positive tilt as a main limiting factor though.
I saw that too. As previously mentioned before, we can get some pretty bad storms/outbreaks even with positively tilted troughs. It will be interesting to see if/when the SPC “changes their tone” in the coming days lol
 

MichelleH

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I saw that too. As previously mentioned before, we can get some pretty bad storms/outbreaks even with positively tilted troughs. It will be interesting to see if/when the SPC “changes their tone” in the coming days lol

Knowing them lately, probably not until the first tornado has dropped.
 

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