Severe WX Severe Threat 17-18 March 2021

[MattPetrulli]

Something to note here, 00Z CAMS and 06z HRRR have a noticeable lack of UH tracks, At least for most of the warm sector. Definitely something to watch here as albeit far out, still shows the possibilities for failure modes. Right now I think best bet for strong tornadoes is gonna be around the Memphis area atm.

 

[Whyrob1998]

Certainly worth mentioning that there's an enhanced risk for much of Georgie, South and North Caroline on Thursday. SPC outlook states the potential for all hazards of severe weather, including "several tornadoes - a couple of them possibly strong."
View attachment 6953

What does the parameters look like for Central GA on Thursday

 

[MattPetrulli]

Something to note here, 00Z CAMS and 06z HRRR have a noticeable lack of UH tracks, At least for most of the warm sector. Definitely something to watch here as albeit far out, still shows the possibilities for failure modes. Right now I think best bet for strong tornadoes is gonna be around the Memphis area atm.

Looking into it lack of UH tracks across most of the open warm sector is likely due to early initiation in a pretty meager low level environment for tornadoes.

 

[Casuarina Head]

Looking into it lack of UH tracks across most of the open warm sector is likely due to early initiation in a pretty meager low level environment for tornadoes.

Why do these mesoscale models show early initiation? Also, why is the low-level environment so unfavourable then?

 

[Fred Gossage]

Something to note here, 00Z CAMS and 06z HRRR have a noticeable lack of UH tracks, At least for most of the warm sector. Definitely something to watch here as albeit far out, still shows the possibilities for failure modes. Right now I think best bet for strong tornadoes is gonna be around the Memphis area atm.

I'm just trying to figure out from which alternate parallel universe the 6z HRRR you were looking at came from...

 

[Richardjacks]

Why do these mesoscale models show early initiation? Also, why is the low-level environment so unfavourable then?

Notice the change in the 850mb winds tomorrow evening. Earlier there is instability, the low level shear isn't nearly as great as what it will become later.

 

[MattPetrulli]

View attachment 6956

I'm just trying to figure out from which alternate parallel universe the 6z HRRR you were looking at came from...

None of these kidney beans mature and produce long tracked UH tracks, those UH tracks come from messy, semi-discrete super cells embedded in a line later on. That's what I was hinting at in lack of UH tracks

 

[MQATLSNOW]

View attachment 6958
None of these kidney beans mature and produce long tracked UH tracks, those UH tracks come from messy, semi-discrete super cells embedded in a line later on. That's what I was hinting at in lack of UH tracks

Is this good or bad?

 

[Kory]

Something to note here, 00Z CAMS and 06z HRRR have a noticeable lack of UH tracks, At least for most of the warm sector. Definitely something to watch here as albeit far out, still shows the possibilities for failure modes. Right now I think best bet for strong tornadoes is gonna be around the Memphis area atm.

View attachment 6958
None of these kidney beans mature and produce long tracked UH tracks, those UH tracks come from messy, semi-discrete super cells embedded in a line later on. That's what I was hinting at in lack of UH tracks

What you are saying here DIRECTLY conflicts with the guidance you seemingly are referencing. The HRRR shows a strong tornado threat well into Mississippi and Alabama regardless of the discrete convection earlier in the day.

And those cells do mature as the LLJ rapidly increases between 21z and 00z.

 

[Richardjacks]

View attachment 6958
None of these kidney beans mature and produce long tracked UH tracks, those UH tracks come from messy, semi-discrete super cells embedded in a line later on. That's what I was hinting at in lack of UH tracks

I think it is important to remember that you cannot take any one model run for face value. You have to look at the environment as a whole. To me, what I find concerning is I am not seeing a lot of convection/widespread early in the day or even trying to develop in the afternoon. A volatile environment can be expected.

 

[warneagle]

What you are saying here DIRECTLY conflicts with the guidance you seemingly are referencing. The HRRR shows a strong tornado threat well into Mississippi and Alabama regardless of the discrete convection earlier in the day.

And those cells do mature as the LLJ rapidly increases between 21z and 00z.

I was wondering when you'd pop in. I was getting ready to light up the bat signal!

And yeah, I'm already prepared for somebody to come in and say this is a Forecasted Convective Amplification Deficiency when it's still quiet in MS/AL at 3 pm tomorrow.

 

[Richardjacks]

And yeah, I'm already prepared for somebody to come in and say this is a Forecasted Convective Amplification Deficiency when it's still quiet in MS/AL at 3 pm tomorrow.

Me too!!

 

[Fred Gossage]

This has the classic earmarks of one of those classic Dixie events where, even if there are a couple of strong tornadoes, the afternoon hours underperform expectation, and then a switch flips at dusk as the low-level jet ramps up. Even the final "main line" is looking increasingly cellular. Both the Baron and WSI broadcast models move it deep across Alabama as a band of spaced, full-on long-track supercells. Even those would be capable of long-track EF3+ tornadoes.

 

[Kory]

I was wondering when you'd pop in. I was getting ready to light up the bat signal!

And yeah, I'm already prepared for somebody to come in and say this is a Forecasted Convective Amplification Deficiency when it's still quiet in MS/AL at 3 pm tomorrow.

Haha I’ve been super busy lately. Glad to be back.

I have to say...this is one of the best thermodynamic systems I’ve seen in a while. How many systems the last few years have we said, “that could’ve been a lot worse had there been more instability.” Well, here we are and we’re about to find out tomorrow.

I could see some of the very early cells struggling prior to 21z in eastern MS and AL, but after that, we see a rapid increase in low level shear and I don’t wanna take my chances with any of the cells in the warm sector or back on the boundary regardless of what UH streaks show. I can see tomorrow evening and overnight will be a volatile set up.

 

[MattPetrulli]

I think it is important to remember that you cannot take any one model run for face value. You have to look at the environment as a whole. To me, what I find concerning is I am not seeing a lot of convection/widespread early in the day or even trying to develop in the afternoon. A volatile environment can be expected.

Oh yeah I understand not taking it at face value, however 00z HRRR and 00z CAMs were hinting at this too.

What you are saying here DIRECTLY conflicts with the guidance you seemingly are referencing. The HRRR shows a strong tornado threat well into Mississippi and Alabama regardless of the discrete convection earlier in the day.

And those cells do mature as the LLJ rapidly increases between 21z and 00z.

I was just noting the lack of UH tracks on CAMs and HRRR. I never said there isn't a strong tornado threat well into MS and AL, but as we've seen time and time again over the past few years those parameters don't mean much if there's messy convective mode like some of those models were referencing.

 

[Kory]

Also, UH products were mostly quiet until the morning of the Fultondale tornado and then they lit up like a Christmas tree (not at all saying UH products are quiet for tomorrow). Some of the mesoscale stuff still needs to be worked out. There appears to be some forecasted scattered morning convection lifting north...how all of that behaves will determine a lot. If I’m remembering correctly, Fultondale also happened with only a 40-45kt LLJ which is about what is forecasted tomorrow prior to 21z over most of the warm sector ahead of the boundary. Small scale stuff determines a lot...

 

[Fred Gossage]

6z run of the 3k Baron is even more volatile than the last one. It is dead quiet on the warm front during the overnight and morning, until that warm front is near Columbia and Hohenwald TN by 10am! Then, a pristine warm sector until mid/late afternoon before multiple discrete, violent long-track supercells develop widely spaced over the western 3/4 of AL back into MS... with semi-discrete storms closer to the front. Several hours of fully discrete long-trackers before the "main line" (in semi-discrete form with violent UH tracks) comes across AL in the late overnight. The UH streaks on this run are even longer in path length than the previous run, and several of them are violent intensity.

 

[Clancy]

12Z HRRR showing this activity across AL/GA at 12Z tomorrow. I know this was expected, but to what degree might this impact convection later on, if at all?

 

[Kory]

12Z HRRR showing this activity across AL/GA at 12Z tomorrow. I know this was expected, but to what degree might this impact convection later on, if at all?
View attachment 6960

Precisely what I’m saying. We have a ways to go to figure out this morning stuff. Along and south of whatever thermal boundary it lays down will be a powder keg ready to go.

 

[Richardjacks]

6z run of the 3k Baron is even more volatile than the last one. It is dead quiet on the warm front during the overnight and morning, until that warm front is near Columbia and Hohenwald TN by 10am! Then, a pristine warm sector until mid/late afternoon before multiple discrete, violent long-track supercells develop widely spaced over the western 3/4 of AL back into MS... with semi-discrete storms closer to the front. Several hours of fully discrete long-trackers before the "main line" (in semi-discrete form with violent UH tracks) comes across AL in the late overnight. The UH streaks on this run are even longer in path length than the previous run, and several of them are violent intensity.

Fred, as you know, this is the kind of setup that could cause this to reach max potential.