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Severe WX December 10 & 11, 2021 Severe Threat (3 Viewers)

Fred Gossage

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There's an event-duration UH track map to go along with the GFDL plot above. Also, this is that 12z Euro signal I was talking about for the idea of early-on warm sector supercells near or just after sunset over north MS/southwest TN that would track through northwest AL and middle TN in the 7-11pm timeframe, well before the pre-frontal round with the arrival of the main forcing.
 

warneagle

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I haven't had a ton of time for model-watching the last couple of days, so I really appreciate your analysis, Fred. The event that came to mind right away for me was 23/12/15 (the Holly Springs, MS EF4). That was another "wow that's a big warm sector for December", positive-tilt trough event. I could see this being the same type of thing, where it's not a massive outbreak of supercells but instead a few "storms of the day" that were long-lived and produced prolifically.
 

Fred Gossage

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I haven't had a ton of time for model-watching the last couple of days, so I really appreciate your analysis, Fred. The event that came to mind right away for me was 23/12/15 (the Holly Springs, MS EF4). That was another "big warm sector for December", positive-tilt trough event. I could see this being the same type of thing, where it's not a massive outbreak of supercells but instead a few "storms of the day" that were long-lived and produced prolifically.
I've been thinking something similar may be possible. There is one potentially important difference between this one and that one though. There were actually height rises, anticyclonic flow aloft, and warm mid-level temperatures out in the open warm sector that prevented what we thought would be a major threat over north/west Alabama north of I-59 from coming to fruition. This one... there are decent height falls (rising motion), mostly straight flow aloft... but a slight cyclonic curvature from the base of the trough out into the warm sector, and mid-level temperatures are in a cooler and more acceptable range for severe weather. There theoretically should be more large scale ascent and lift with this one, although the lift still isn't overly strong.
 

Fred Gossage

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18z Euro is in, and it is deeper, stronger, a bit more curved with the trough, and it's not as in-your-face positive tilt.

icon_z500_vort_us_28.png
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RGEM and ICON are also more curved with the trough, and these guys were the first to trend toward what we have as the current model evolution. ICON also isn't a positive tilt by Friday late evening either, and for that matter, RGEM isn't as in-your-face with the positive tilt itself.

18z Euro still has the strong signal for renegade stuff in the open warm sector in the evening and overnight Friday well ahead of the predawn prefrontal main round, like the 12z run. 18z Euro brings that prefrontal main round in as cellular stuff though and gets it deep into central MS and north central AL going into Saturday morning...

263727420_622223932242452_3546891699878799974_n.png
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Watching for trends...
 
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Yeah I get what you were saying. Being exposed to sites like YouTube for so long got me used to seeing wannabe mets getting super riled up and acting like there will 100% be another 4/27/11 in December. People see one parameter that is similar to a past event and just go crazy. Trying to steer clear of sites like YouTube now especially given what the comment section has devolved into over the years.

It's OK to watch James Spann's Weather Xtreme Video, or if @Fred Gossage ever decides to start a similar video blog.

This looks like another classic case where the make or break factor will be the balance between the subtle lift/forcing in the open warm sector vs. the capping. It seems to always be a fine line between "you can't get sustained, discrete storms" and "convective mess." This is usually the critical factor and unfortunately seems to be the hardest for the models to pin down even the day of. Same is true on the Plains (5/20/19, 5/26/21 south of the KS/NE line).
 
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500wh.png
500wh.png
500wh.png


18z Euro is in, and it is deeper, stronger, a bit more curved with the trough, and it's not as in-your-face positive tilt.

icon_z500_vort_us_28.png
500wh.png

RGEM and ICON are also more curved with the trough, and these guys were the first to trend toward what we have as the current model evolution. ICON also isn't a positive tilt by Friday late evening either, and for that matter, RGEM isn't as in-your-face with the positive tilt itself.

18z Euro still has the strong signal for renegade stuff in the open warm sector in the evening and overnight Friday well ahead of the predawn prefrontal main round, like the 12z run. 18z Euro brings that prefrontal main round in as cellular stuff though and gets it deep into central MS and north central AL going into Saturday morning...

263727420_622223932242452_3546891699878799974_n.png
263624514_636348524224916_5756463973999412570_n.png


Watching for trends...
Great point Fred … the trough on 18z loosing the positive Tilt look for sure . Trend ? I feel quite confident what I am seeing modeled today spc will put out a rather large day 3 15 percent risk at least
 
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Fred Gossage

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500wh.png
500wh.png
500wh.png


18z Euro is in, and it is deeper, stronger, a bit more curved with the trough, and it's not as in-your-face positive tilt.

icon_z500_vort_us_28.png
500wh.png

RGEM and ICON are also more curved with the trough, and these guys were the first to trend toward what we have as the current model evolution. ICON also isn't a positive tilt by Friday late evening either, and for that matter, RGEM isn't as in-your-face with the positive tilt itself.

18z Euro still has the strong signal for renegade stuff in the open warm sector in the evening and overnight Friday well ahead of the predawn prefrontal main round, like the 12z run. 18z Euro brings that prefrontal main round in as cellular stuff though and gets it deep into central MS and north central AL going into Saturday morning...

263727420_622223932242452_3546891699878799974_n.png
263624514_636348524224916_5756463973999412570_n.png


Watching for trends...
So far, the GFS isn't less favorable than it has been, but it's still a flatter solution than the 18z stuff from above on the other models. The 00z Canadian came in solidly trending toward the 18z Euro:

500wh.conus.png
500wh.conus.png
 

Fred Gossage

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Very similar convective evolution signal to the 18z Euro as well on tonight's Canadian:

700hvv.conus.png
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If the Euro tonight continues its idea from earlier, I'm going to start leaning toward the assumption that the GFS is showing its historically known bias of being too flat and sheared with systems.
 
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chasestormz39

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Very similar convective evolution signal to the 18z Euro as well on tonight's Canadian:

700hvv.conus.png
700hvv.conus.png


If the Euro tonight continues its idea from earlier, I'm going to start leaning toward the assumption that the GFS is showing it's historically known bias of being too flat and sheared with systems.

Do you see storms initiating prior to sunset or is this primarily a nocturnal event?
 
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0 z euro continues the less Positive tilted look to the trough and more of a Deepening surface low as it moves northeast . Key question. Will there be some discrete cells out ahead of the frontal boundary itself . And spc did do what I expected finally placed a fairly large 15 percent risk out . Won’t suprise me to see a 30 percent on tomorrow s 2 day outlook.
 
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Richardjacks

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00z Euro has exact same synoptic evolution as the 18z run... except the shortwave is stronger, and the surface low gets much deeper heading through the early to mid morning of Saturday.
This appears to be leading to higher lapse rates Friday evening over N. Miss. and NW Al....also, the shear values are adequate in this area and something that could increase if current trends continue...I am still questioning how this all evolves late Friday evening into early Saturday. The first area of supercells that develop ahead of the front could eventually become the main squall line, or there could be a break with a seperate line forming along prefrontal trough.
 

KevinH

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0 z euro continues the less Positive tilted look to the trough and more of a Deepening surface low as it moves northeast . Key question. Will there be some discrete cells out ahead of the frontal boundary itself . And spc did do what I expected finally placed a fairly large 15 percent risk out . Won’t suprise me to see a 30 percent on tomorrow s 2 day outlook.
30% over which areas?
 

JPWX

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Here's the 12z GFS and NAM CWASP output.
 

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