Severe Event March 25th 2021

[ATDoel]

2 minutes ago, jpeters3 said:

Careful, this is starting to sound like 5-19-19 all over again, with the HRRR showing apocalypse and the NAM nest showing a seemingly unbelievable non-outbreak.  The model isn't violating the laws of physics - it is showing us a dynamically consistently possibility.  In examining some soundings, it looks like there is more warming at ~ 850 hPa in the nam nest that acts as a CAP in the warm sector.  Again, eerily similar to the "bust" mechanism on 5-20-19.  Obviously these two events are quite synoptically different, but we want to learn a lesson from that event and be careful to too hastily throw away CAM solutions that don't show the apocalypse.  

which scenario are you leaning towards?

[jpeters3]

1 minute ago, ATDoel said:

which scenario are you leaning towards?

I honestly don't know.  It's basically a coin toss.  Lets see how CAM outcomes play out throughout the rest of the day.

My gut tells me that prolific tornado outbreaks are always going to be less likely than a "bust" with this kind of spread in CAM solutions, because there are many pathways to a bust and few to a prolific outbreak. 

[olafminesaw]

1 minute ago, jpeters3 said:

I honestly don't know.  It's basically a coin toss.  Lets see how CAM outcomes play out throughout the rest of the day.

My gut tells me that prolific tornado outbreaks are always going to be less likely than a "bust" with this kind of spread in CAM solutions, because there are many pathways to a bust and few to a prolific outbreak. 

I think statistical analysis is an underutilized tool in meteorology. In any case of extreme weather, the distribution is skewed right. I figure in setups like these the distribution drops off to the right side of the curve very quickly because of how fragile they are. Extreme environments have a very narrow envelope of CIN, or they fail spectacularly, one what or the other

[ATDoel]

6 minutes ago, jpeters3 said:

I honestly don't know.  It's basically a coin toss.  Lets see how CAM outcomes play out throughout the rest of the day.

My gut tells me that prolific tornado outbreaks are always going to be less likely than a "bust" with this kind of spread in CAM solutions, because there are many pathways to a bust and few to a prolific outbreak. 

what could lead to a bust tomorrow?  Too much of a cap and discrete cells can't fire?  Too little cap and we get a bunch of garbage convection?  What else?

[Quincy]

I’d say the range of scenarios start at being similar to last Wednesday and that’s hoping either it’s a convective mess or there is some sort of cap that tempers the warm sector... and higher end being a generational event. Hard to envision an evolution without at least a couple of intense, long track tornadic supercells. The wind profiles and lapse rates alone also suggest a much greater large hail threat than we saw last week. 

[jpeters3]

Just now, olafminesaw said:

I think statistical analysis is an underutilized tool in meteorology. In any case of extreme weather, the distribution is skewed right. I figure in setups like these the distribution drops off to the right side of the curve very quickly because of how fragile they are. Extreme environments have a very narrow envelope of CIN, or they fail spectacularly, one what or the other

Agreed.  I suspect that if we did an exhaustive analysis of all the CAM solutions that showed apocalypse like this HRRR run, we'd find that 20% or less of them verified as advertised.  Not saying we won't have tornadoes tomorrow, but the chances of an actual super outbreak (or even a sub-super, but historic outbreak) remain relatively small.

[jpeters3]

Just now, Quincy said:

I’d say the range of scenarios start at being similar to last Wednesday and that’s hoping either it’s a convective mess or there is some sort of cap that tempers the warm sector... and higher end being a generational event. Hard to envision an evolution without at least a couple of intense, long track tornadic supercells. The wind profiles and lapse rates alone also suggest a much greater large hail threat than we saw last week. 

Yeah, this pretty well sums it up.  If I had to bullshit, i'd say:

20% total bust, with fewer tornadoes than last week

60% an outbreak, with at least a few EF3 (+) destructive tornadoes

10% Historic outbreak of the magnitude of 4-12-20, super tuesday, 4-14-12, etc

10% Superoutbreak.

Please keep in mind that these numbers are totally pulled out of my ass.

[Quincy]

You would hope for a cap as well, given the fact that early day convection lifts north and could very well leave down multiple boundaries... that’s one of the key differences between a significant event and a super outbreak. 

[ATDoel]

2 minutes ago, Quincy said:

You would hope for a cap as well, given the fact that early day convection lifts north and could very well leave down multiple boundaries... that’s one of the key differences between a significant event and a super outbreak. 

from my inferior knowledge, it seems that there's a sweet spot in the amount of cap present to lead to a significant outbreak.  You want some to suppress all the garbage convection but not too much to suppress it all.  What exactly are you looking for in the models that give you an idea of what that cap is going to be and where in that "sweet spot" it falls?

[jpeters3]

2 minutes ago, ATDoel said:

from my inferior knowledge, it seems that there's a sweet spot in the amount of cap present to lead to a significant outbreak.  You want some to suppress all the garbage convection but not too much to suppress it all.  What exactly are you looking for in the models that give you an idea of what that cap is going to be and where in that "sweet spot" it falls?

This is generally true.

And it's fairly simple.  Look at the suite of CAMs, and compare the ones that show development to the ones that don't.

[Quincy]

5 minutes ago, Quincy said:

You would hope for a cap as well, given the fact that early day convection lifts north and could very well leave down multiple boundaries... that’s one of the key differences between a significant event and a super outbreak. 

To clarify, I meant to suggest that you don’t want a weak cap with a myriad of outflow boundaries within that instability/shear environment. You can see “swarms” of supercells when there’s a weakly capped environment, in a case like this. 

[andyhb]

The other 12z CAMs (WRF variants) aren’t quite as high end as the HRRR, but they are certainly more active than the 3 km NAM. ARW and NSSL are probably outbreak scenarios taken verbatim.

[jpeters3]

Here's an example:

NAM nest

HRRR

 

Now the MLCIN differences are basically negligible.  However, CIN isn't the entire story.  Growing updrafts (i.e., congestus) are very susceptible to the negative effects of entrainment.  The dry warm region in the Nam NEST at 850 mb may kill everything that tries to form via dry air entrainment.  In contrast, the HRRR shows a much deeper moister layer.  In the HRRR situation, growing cumulus can make it deeper, and parcels can gain more momentum, before they start to feel the negative effects of entrainment.  This difference is subtle, but these types of subtle differences can make or break an outbreak.

[forkyfork]

there isn't much low level cape even on the hrrr

[Quincy]

6 minutes ago, forkyfork said:

there isn't much low level cape even on the hrrr

[ATDoel]

19 minutes ago, jpeters3 said:

Here's an example:

NAM nest

HRRR

 

Now the MLCIN differences are basically negligible.  However, CIN isn't the entire story.  Growing updrafts (i.e., congestus) are very susceptible to the negative effects of entrainment.  The dry warm region in the Nam NEST at 850 mb may kill everything that tries to form via dry air entrainment.  In contrast, the HRRR shows a much deeper moister layer.  In the HRRR situation, growing cumulus can make it deeper, and parcels can gain more momentum, before they start to feel the negative effects of entrainment.  This difference is subtle, but these types of subtle differences can make or break an outbreak.

I see... so the HRR showing deep moisture all the way up to 800mb is a significant difference than the NAM showing dry air starting at around 900mb.  So is this one of the primary factors you're going to be watching on future runs? It looks like the last two runs of the NAM show this dry air, but the previous ones didn't.  thank you for the info.

[forkyfork]

Just now, Quincy said:

on that second sounding peters posted the juicy cape doesn't start until around 700mb

[jpeters3]

11 minutes ago, forkyfork said:

there isn't much low level cape even on the hrrr

 

2 minutes ago, forkyfork said:

on that second sounding peters posted the juicy cape doesn't start until around 700mb

The tropical tidbits soundings are misleading, because they plot parcel and environmental Temperature, rather than Virtual temperature.  If they plotted virtual temperature, the low-level CAPE would show up better.

For those of you who are unfamiliar with virtual temperature, it is a temperature adjustment to the air that accounts for the affect of moisture on density.  Virtual temperature is typically a few C warmer than actual temperature when there is a lot of moisture.  And sometimes Virtual temperature of a parcel can be larger than that of the environment, despite the parcel's actual temperature being cooler than that of the environment.

[Santa Clause]

Looking like we may see some tornadoes, but too many variables in key places for this to be a prolific day. Probably will stick with MDT in day one overnight if I had to guess, but certainly a lot of questions on prominence of tornadoes over other severe modes tomorrow. As always though, ingredients are there..it takes only one for things to be memorable or historic. Great discussion on here today.

[Quincy]

Dixie events aren’t usually driven by “fat” or large CAPE profiles. Interestingly enough, this event to expected to have both impressive wind profiles and climatologically large CAPE. 

[Cheeznado]

3KM NAM obviously the odd CAM out. What impresses me are the forecast incredibly high 0-1KM SRH values of AOA 500. That is at the level of the 2011 superoutbreak. Not saying this will be as bad, but still impressive.

[Calderon]

14 minutes ago, Cheeznado said:

3KM NAM obviously the odd CAM out. What impresses me are the forecast incredibly high 0-1KM SRH values of AOA 500. That is at the level of the 2011 superoutbreak. Not saying this will be as bad, but still impressive.

Oof, that is certainly a very high ceiling scenario, to say the least.

[Calderon]

2 minutes ago, Cyclone-68 said:

Does anyone anticipate an upgrade to high risk at the next update? 

I personally do not and think very few do as well. If, big if, they do, it'll be during Day 1 issuances but still unlikely for 06z given potential for warm sector contamination. 

[Cyclone-68]

Just now, Calderon said:

I personally do not and think very few do as well. If, big if, they do, it'll be during Day 1 issuances but still unlikely for 06z given potential for warm sector contamination. 

Thank you for the answer. I deleted my original comment because I didn’t want to disturb the great back and forth in here 

[Bob's Burgers]

Expanding the moderate and enhanced northeast would make sense. There isn't quite good enough model agreement for a high risk D2 yet due to storm mode. Some of the HREF members last night had some early convection too which can be a bit of a fly in the ointment. That said, a high risk at 6z would make sense if the 0z HREF shows a strong signal.

[DanLarsen34]

From a probabilistic perspective, I get the hesitancy because there are potential failure modes as many of you have pointed out today. However, I’m not sure you’ll ever see complete consensus in the models ahead of time for any outbreak. 

What has to be considered here is that the ceiling for tomorrow is about as high you’re ever going to see, and even middling scenarios would depict something approaching a high risk across MS and AL. It might be worth it to pull the trigger here now to warn the public because tomorrow has serious potential to be extremely dangerous and life threatening. 

[jojo762]

16 minutes ago, Bob's Burgers said:

Expanding the moderate and enhanced northeast would make sense. There isn't quite good enough model agreement for a high risk D2 yet due to storm mode. Some of the HREF members last night had some early convection too which can be a bit of a fly in the ointment. That said, a high risk at 6z would make sense if the 0z HREF shows a strong signal.

I dunno. Sure seems like there’s plenty of model agreement. 
 

SREF pops a large area of 75% or higher on the significant tornado ingredients combo prob.

The 12z HREF also pops 70% or higher probabilities of UH >75 m2/s2 over parts of Mississippi and Alabama tomorrow. 
 

I get everyone is gun shy because of *gasp* junkvection in Dixie, but it’s really just a reality of setups down there. Albeit, I certainly agree that the lack of CINH (and resulting potential for many more storms) is at least a slight concern for tomorrow reaching its max potential. 
 

Quincy, as always, outlined potential scenarios for tomorrow quite well. If last Wednesday was a *verified high risk*, and a copy of last week is probably the lower-end of possibilities with tomorrow then... you get the rest.

[jpeters3]

1 minute ago, jojo762 said:

I dunno. Sure seems like there’s plenty of model agreement. 
 

SREF pops a large area of 75% or higher on the significant tornado ingredients combo prob.

The 12z HREF also pops 70% or higher probabilities of UH >75 m2/s2 over parts of Mississippi and Alabama tomorrow. 
 

I get everyone is gun shy because of *gasp* junkvection in Dixie, but it’s really just a reality of setups down there. Albeit, I certainly agree that the lack of CINH (and resulting potential for many more storms) is at least a slight concern for tomorrow reaching its max potential. 
 

Quincy, as always, outlined potential scenarios for tomorrow quite well. If last Wednesday was a *verified high risk*, and a copy of last week is probably the lower-end of possibilities with tomorrow then... you get the rest.

I generally agree that a High risk for tornadoes will probably verify tomorrow.  It's almost guaranteed, barring some crazy guidance shift, that tomorrow will have a day 1 high.  So the question is, how much worse does it get.  As last week showed us, the bar, in terms of tornado outbreaks, is pretty low for a high risk to verify.  It can get MUCH MUCH worse.

[yoda]

MOD remains on new Day 2... but hardcore talk for upgrade to high tomorrow 

[jojo762]

2 minutes ago, jpeters3 said:

I generally agree that a High risk for tornadoes will probably verify tomorrow.  It's almost guaranteed, barring some crazy guidance shift, that tomorrow will have a day 1 high.  So the question is, how much worse does it get.  As last week showed us, the bar, in terms of tornado outbreaks, is pretty low for a high risk to verify.  It can get MUCH MUCH worse.

If I’m being honest I’m not sure that even last Wednesday is a good “low bar” comparison given the upper-tier forecast environment. We didn’t see many (if any?) intense, long-tracking tornadoes last week. Tomorrow will *likely feature at least one, if not multiple — unfortunately.