Severe WX December 10 & 11, 2021 Severe Threat

[MichelleH]

Who is bringing the marshmallows?

S'mores!!

 

[Clancy]

Here's BMX's AFD for the Friday into Saturday period. They don't seem too enthusiastic but at least they are mentioning the potential for severe storms during the time period.

 

[Richardjacks]

Yes, the surface low is pushing northeast, but still there are good pressure falls and the GFS is trying to 2ndry...would be a game changer for low level shear.

 

[Richardjacks]

9pm Thursday night dewpoints...a full 24+ hours before the event. How many times in December have we seen these same numbers 1 hour before the event?

 

[TH2002]

I'm not sure how I feel about this whole upcoming severe threat. There certainly have been some massive outbreaks with a positively tilted trough and I don't think that alone would mitigate a major severe weather event, but at the same time when MLCAPE is 1000 J/KG or less and shear is 50 knts or less, that combined with a positively tilted trough doesn't scream "major tornado outbreak" to me. That being said, there could be higher values west of the BMX forecast area and setups like this are certainly hard to forecast. The GFS model (at least in SCP mode) is still pretty much saying "nothing to see here".

 

[Clancy]

For posterity, I checked CIPS and it shows an area for severe potential for D5 (Saturday).

 

[andyhb]

Machine learning model from CSU is hitting Friday pretty hard.

 

[Fred Gossage]

Alright, so let's talk a minute...

The reasons cited in the outlook mainly revolve around the positive tilt nature of the trough. It is mentioned that the positive tilt is helping promote flow aloft parallel to the front which would undercut thunderstorms along the front. Here's the thing... that front is still WAY west at the time that we're watching. We are concerned with open warm sector development in the WAA regime along the broad low-level jet axis. This is not one of those meridional positive tilt troughs where the deep-layer flow is sharply southerly and the low-level jet is confined to areas immediately along the front. When those situations happen, most of the warm sector is actually east of the low-level jet, where there is low-level divergence. In this case, the low-level jet is well out ahead of the surface front, and the broad zone of WAA helps promote large scale ascent, even if subtle, well ahead of the front they are talking about. In addition to that, there are significant height falls that overspread the warm sector through the afternoon, evening, overnight, and predawn well ahead of the front. This is another source for large scale ascent. We will also have east AR, north MS, west/middle TN/KY, and north AL getting into the right rear quadrant of the 500mb jet streak associated with that lead disturbance. There are some signals there may actually be a double barrel upper jet, similar to Super Tuesday 2008, with a secondary mid-level jet streak over these areas. Those things would promote upper-level divergence, which is yet more subtle large scale ascent. Are those mechanisms strong? No. But in an open warm sector in Dixie Alley where the capping is weak, and the boundary layer moisture is rich and substantially deeper than what you see in the Plains, you wouldn't want strong lift in a situation like this... otherwise you're either going to have a linear storm mode or a messy and grungy mode. Once you add those large scale ascent mechanisms together, and especially account for surface pressure falls and prefrontal confluence bands in the warm sector, there are mechanisms to get robust discrete convective development deep into the warm sector. Last night's Euro and today's Euro actually show a signal for discrete storms in north central MS to west/middle TN as early as 5-6pm, and a strong signal for long-track discrete storms coming out of north Mississippi and southwest Tennessee, across north Alabama and Middle TN as early as 8-11pm with time. That is well ahead of the main round of activity that begins moving into north AL and middle TN after midnight, which may also be supercells or QLCS with embedded supercells, with deep-layer vectors still a bit more veered prior to 12z. Once you get toward the daybreak hours and after, as the main trough axis is stepping out toward the area, the low-level winds are veering with time, the deep-layer vectors are more parallel to the line, and the trough is shearing itself, and dynamics are pulling away with time.

We have to get beyond conceptual models and actually have an understanding of how previous events have happened, instead of just assuming every significant event in the past was a negative tilt trough without ever going back to look at what the setups looked like. Most of the "classic" rules like that were born from the Fawbush-Miller days of the 40s and early 50s. There are many events that don't fit that conceptual model. The thing about those rules... they were originally born from the Tinker 1948 tornadoes in March of that year. The problem is, if you pay attention to the descriptions of the conditions leading up to the tornadoes... with reports of a wall of clouds stretching from one end of the horizon to the other... not only are rules based on an area which has stronger capping most times and NEEDs stronger lift, but those descriptions make it sound like those may have been QLCS tornadoes, or at least supercells embedded within a QLCS. The descriptions sound like there was a big horizon-stretching shelf cloud immediately before the tornadoes. Conceptual models and anecdotal ideas can only take us so far in forecasting. Severe weather has to be an ingredients-based forecast approach. Most of the time we see a setup in Dixie that looks like that classic Fawbush-Miller conceptual model, we end up with either a solid line of storms or a grungy, messy warm sector. Let's look at how actual events have happened in the past and adjust our conceptual models accordingly. Those events happened the way they did for a reason.

 

[Fred Gossage]

Machine learning model from CSU is hitting Friday pretty hard.

CWASP from the GFS output also has a significant signal over a large area.

 

[andyhb]

I would like to see more southerly surface winds in the warm sector than some of the newest guidance is suggesting, as the current progs are lacking a bit on hodograph size/curvature especially in the 18-21z window.

 

[Fred Gossage]

I would like to see more southerly surface winds in the warm sector than some of the newest guidance is suggesting, as the current progs are lacking a bit on hodograph size/curvature especially in the 18-21z window.

I noticed that on the Euro in that timeframe, but that was also in the face of decent 3-hour sfc pressure falls, as the low is deepening, and it's not occluded. All those things point toward winds that should be backing, and we know from experience that none of the models ever have as much of an isallobaric response as they should until the last minute, and sometimes not even then.

 

[chasestormz39]

I've noticed that too. Critical angles are a bit shallow in the 40-50 degree range. Obviously that does start to change beyond sunset as the LLJ continues to increase. Speed shear is through the roof.

 

[Clancy]

Regardless of the tilt issues, the whole big warm sector in December thing makes this system something to be reckoned with.

 

[JPWX]

View attachment 10796
Alright, so let's talk a minute...

The reasons cited in the outlook mainly revolve around the positive tilt nature of the trough. It is mentioned that the positive tilt is helping promote flow aloft parallel to the front which would undercut thunderstorms along the front. Here's the thing... that front is still WAY west at the time that we're watching. We are concerned with open warm sector development in the WAA regime along the broad low-level jet axis. This is not one of those meridional positive tilt troughs where the deep-layer flow is sharply southerly and the low-level jet is confined to areas immediately along the front. When those situations happen, most of the warm sector is actually east of the low-level jet, where there is low-level divergence. In this case, the low-level jet is well out ahead of the surface front, and the broad zone of WAA helps promote large scale ascent, even if subtle, well ahead of the front they are talking about. In addition to that, there are significant height falls that overspread the warm sector through the afternoon, evening, overnight, and predawn well ahead of the front. This is another source for large scale ascent. We will also have east AR, north MS, west/middle TN/KY, and north AL getting into the right rear quadrant of the 500mb jet streak associated with that lead disturbance. There are some signals there may actually be a double barrel upper jet, similar to Super Tuesday 2008, with a secondary mid-level jet streak over these areas. Those things would promote upper-level divergence, which is yet more subtle large scale ascent. Are those mechanisms strong? No. But in an open warm sector in Dixie Alley where the capping is weak, and the boundary layer moisture is rich and substantially deeper than what you see in the Plains, you wouldn't want strong lift in a situation like this... otherwise you're either going to have a linear storm mode or a messy and grungy mode. Once you add those large scale ascent mechanisms together, and especially account for surface pressure falls and prefrontal confluence bands in the warm sector, there are mechanisms to get robust discrete convective development deep into the warm sector. Last night's Euro and today's Euro actually show a signal for discrete storms in north central MS to west/middle TN as early as 5-6pm, and a strong signal for long-track discrete storms coming out of north Mississippi and southwest Tennessee, across north Alabama and Middle TN as early as 8-11pm with time. That is well ahead of the main round of activity that begins moving into north AL and middle TN after midnight, which may also be supercells or QLCS with embedded supercells, with deep-layer vectors still a bit more veered prior to 12z. Once you get toward the daybreak hours and after, as the main trough axis is stepping out toward the area, the low-level winds are veering with time, the deep-layer vectors are more parallel to the line, and the trough is shearing itself, and dynamics are pulling away with time.

We have to get beyond conceptual models and actually have an understanding of how previous events have happened, instead of just assuming every significant event in the past was a negative tilt trough without ever going back to look at what the setups looked like. Most of the "classic" rules like that were born from the Fawbush-Miller days of the 40s and early 50s. There are many events that don't fit that conceptual model. The thing about those rules... they were originally born from the Tinker 1948 tornadoes in March of that year. The problem is, if you pay attention to the descriptions of the conditions leading up to the tornadoes... with reports of a wall of clouds stretching from one end of the horizon to the other... not only are rules based on an area which has stronger capping most times and NEEDs stronger lift, but those descriptions make it sound like those may have been QLCS tornadoes, or at least supercells embedded within a QLCS. The descriptions sound like there was a big horizon-stretching shelf cloud immediately before the tornadoes. Conceptual models and anecdotal ideas can only take us so far in forecasting. Severe weather has to be an ingredients-based forecast approach. Most of the time we see a setup in Dixie that looks like that classic Fawbush-Miller conceptual model, we end up with either a solid line of storms or a grungy, messy warm sector. Let's look at how actual events have happened in the past and adjust our conceptual models accordingly. Those events happened the way they did for a reason.

Well said. Always love your analysis.

 

[Clancy]

Small remark but has this thread been pinned yet? If not it probably should be, especially as the event nears and more people may become interested.

 

[TH2002]

Thanks for the analysis, Fred. You're a smart guy and certainly know more about this stuff than me. At the same time though I'm still not sure what to make of this upcoming event, and no my uncertainty is not because of the trough. I probably shouldn't be too quick to dismiss any possibilities and part of that is certainly me just being pessimistic because of how many events this year have failed to materialize. It may very well be another Super Tuesday '08 or if this year's current trend continues it may be another complete bust.

I am still surprised the SPC hasn't introduced 15% probabilities based on what some of the models are showing. I fear we are going back to a "pre-Plainfield" era of stringent forecasting.

 

[Fred Gossage]

Thanks for the analysis, Fred. You're a smart guy and certainly know more about this stuff than me. At the same time though I'm still not sure what to make of this upcoming event, and no my uncertainty is not because of the trough. I probably shouldn't be too quick to dismiss any possibilities and part of that is certainly me just being pessimistic because of how many events this year have failed to materialize. It may very well be another Super Tuesday '08 or if this year's current trend continues it may be another complete bust.

I am still surprised the SPC hasn't introduced 15% probabilities based on what some of the models are showing. I fear we are going back to a "pre-Plainfield" era of stringent forecasting.

To be clear, when I mentioned that date earlier, I was only pointing out that some guidance has a double barrel jet streak similar to what that evening had and was talking about how that was a source of lift. I'm not calling for anything like that or making a comparison for expectations.

 

[TH2002]

To be clear, when I mentioned that date earlier, I was only pointing out that some guidance has a double barrel jet streak similar to what that evening had and was talking about how that was a source of lift. I'm not calling for anything like that or making a comparison for expectations.

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.

 

[bwalk]

This positive/negative tilt issue never goes away. Spann is also riding the positive tilt horse as a reason he is not as concerned about this Friday night's/Saturday's system as he would be if it were negatively tilted.

But - for as many years as I can remember Fred has warned of the danger of downplaying severe systems based on positive tilt. Many of Alabama's deadliest severe weather days (including April 8, 1998; February 5, 2008; May 18, 1995; April 4, 1977; April 3, 1974, etc) were positive tilt troughs.

Trough amplitude is more important than trough tilt (per Fred).

 

[JPWX]

00z GFDL showing a broken line of storms going into Saturday. If this verifies would pose more of supercell risk which would increase tornado potential. Still early but it's concerning.