April 12 Severe Event

[crossbowftw3]

This is no 4/27 but in every way it will be devastating to someone by the time the sun rises Monday morning.

[Bob's Burgers]

Big-time UH swaths coming off the NAMNEST

[jpeters3]

the 12 UTC NAM Nest is showing a substantial warm layer below 700 mb, and almost now buoyancy for a lifted parcel below this level.  Relatedly, there is almost no discrete convection that breaks out in the warm sector.  Rather, everything seems confined to a cold frontal QLCS and a wave of convection proceeding the warm sector.  The latest NAM proper run is a bit cooler between 900 and 700 mb and indicates an outbreak.

This seems like at least one potential "bust" mode. 

[jpeters3]

9 minutes ago, Bob's Burgers said:

Big-time UH swaths coming off the NAMNEST

These UH swaths are all occurring on a progged QLCS along the cold front, and are not related to discrete supercells.

Edit: It also seems like a preponderance of cold front parallel storm motions could be an issue here.
 

[jpeters3]

IMO, there are still a lot of question marks here.  I think MOD day 3 was a good call by SPC given the potential upper envelope, but if other CAMS show similar trends to the NAM NEST, we won't see an upgrade to HIGH tomorrow.

[StormySquares]

3 minutes ago, jpeters3 said:

IMO, there are still a lot of question marks here.  I think MOD day 3 was a good call by SPC given the potential upper envelope, but if other CAMS show similar trends to the NAM NEST, we won't see an upgrade to HIGH tomorrow.

Isn't linear solutions a bias the 3KMNAM has? Should it be taken with a grain of salt?

[jpeters3]

1 minute ago, StormySquares said:

Isn't linear solutions a bias the 3KMNAM has? Should it be taken with a grain of salt?

There isn't any science that supports this alleged bias.  

However, you are right that CAM solutions at this point should be taken with a grain of salt.  I think this run showed us a potential bust mode, but that doesn't mean that it will actually pan out.

The dynamics in this run also support a linear solution, with storm motions that are largely parallel to the front and shitty 0-3 km lapse rates in the warm sector.  So I wouldn't think of this as a bias.

[Santa Clause]

9 minutes ago, jpeters3 said:

There isn't any science that supports this alleged bias.  

However, you are right that CAM solutions at this point should be taken with a grain of salt.  I think this run showed us a potential bust mode, but that doesn't mean that it will actually pan out.

The dynamics in this run also support a linear solution, with storm motions that are largely parallel to the front and shitty 0-3 km lapse rates in the warm sector.  So I wouldn't think of this as a bias.

Good news...even if only a hint of good news.

[Upper Level LOL]

32 minutes ago, jpeters3 said:

the 12 UTC NAM Nest is showing a substantial warm layer below 700 mb, and almost now buoyancy for a lifted parcel below this level.  Relatedly, there is almost no discrete convection that breaks out in the warm sector.  Rather, everything seems confined to a cold frontal QLCS and a wave of convection proceeding the warm sector.  The latest NAM proper run is a bit cooler between 900 and 700 mb and indicates an outbreak.

This seems like at least one potential "bust" mode. 

This is a convection contaminated sounding

[Bob's Burgers]

 

[jpeters3]

34 minutes ago, Upper Level LOL said:

This is a convection contaminated sounding

How do you figure?  There is only a saturated layer at low levels.

Also, this saturated layer is prevalent everywhere (not just in the example I showed).  The presence of saturation in a sounding does not necessarily imply convective contamination.

[crossbowftw3]

11 minutes ago, jpeters3 said:

How do you figure?  There is only a saturated layer at low levels.

Also, this saturated layer is prevalent everywhere (not just in the example I showed).  The presence of saturation in a sounding does not necessarily imply convective contamination.

Lines on the left give the big hint. This sounding is excessively containinated  

[jpeters3]

6 minutes ago, crossbowftw3 said:

Lines on the left give the big hint. This sounding is excessively containinated  

The lines on the left show ascent related to warm air advection (not convection).  Also, you'll notice that the sounding looks early identical to those in the post subsequent to mine.

 

Now this (attached) is an "excessively" convectively contaminated sounding.  Note the saturation through most of the column and strong ascent at mid-to-upper levels that does not coincide with vertically decreasing warm air advection (as QG theory would suggest for WAA induced ascent).

 

[Quincy]

15 minutes ago, jpeters3 said:

How do you figure?  There is only a saturated layer at low levels.

Also, this saturated layer is prevalent everywhere (not just in the example I showed).  The presence of saturation in a sounding does not necessarily imply convective contamination.

It’s a convectively contaminated sounding at 23z that’s near a squall line associated with a cold front. The NAM shows multiple rounds and areas of warm sector storms, to the east and several hours earlier.

I don’t think low-level lapse rates are a big issue here. The models show a relatively uncontaminated warm sector around 18-21z. You can get QLCS tornadoes even with the surging cold front, but I’d be more focused on the warm sector and any pre-frontal trough. 

[jpeters3]

2 minutes ago, Quincy said:

It’s a convectively contaminated sounding at 23z that’s near a squall line associated with a cold front. The NAM shows multiple rounds and areas of warm sector storms, to the east and several hours earlier.

I don’t think low-level lapse rates are a big issue here. The models show a relatively uncontaminated warm sector around 18-21z. You can get QLCS tornadoes even with the surging cold front, but I’d be more focused on the warm sector and any pre-frontal trough. 

For the record, the ascent you are seeing at low levels in that sounding is associated with warm air advection - not "convective contamination."

Edit: I merely stated what the NAM NEST showed, not what "will happen."  But the NAM nest shows this warm signature broadly through the warm sector at various times and in regions sufficiently far from ongoing deep moist convection to avoid convective contamination.  Correspondingly the model does not show any CI in the warm sector.  We've seen this signature in the NAM nest for other would-be big events that busted.  So it's not something to be discounted because of incorrect notions about convective contamination.

 

[Upper Level LOL]

7 minutes ago, Quincy said:

It’s a convectively contaminated sounding at 23z that’s near a squall line associated with a cold front. The NAM shows multiple rounds and areas of warm sector storms, to the east and several hours earlier.

Other than that though.....

[jpeters3]

Here is a similar looking sounding from the warm sector, but showing a descent signature.  Same problem with a warm signature at low levels.

Often times wave like ascent/descent signatures form atop the boundary layer in the presence of strong low-level warm air advection.  What was probably shown in the previous sounding is the "up" branch of one of these signatures, and this one hows the "down" branch.

 

[jpeters3]

1 minute ago, Upper Level LOL said:

Other than that though.....

I'm sorry but you are simply incorrect.  I didn't spend 10 years in in college studying this subject for nothing...

[Quincy]

9 minutes ago, jpeters3 said:

For the record, the ascent you are seeing at low levels in that sounding is associated with warm air advection - not "convective contamination."

Edit: I merely stated what the NAM NEST showed, not what "will happen."  But the NAM nest shows this warm signature broadly through the warm sector at various times and in regions sufficiently far from ongoing deep moist convection to avoid convective contamination.  Correspondingly the model does not show any CI in the warm sector.  We've seen this signature in the NAM nest for other would-be big events that busted.  So it's not something to be discounted because of incorrect notions about convective contamination.

 

Uh, that’s a sounding from hour 59 in NW Mississippi. I see plenty of warm sector CI by 19-20z. (Including cells directly over the 33.82, -88.65 data point at 23z if you look closely. 

I would normally advise against detailed sounding analysis of the 59hr 3km NAM, but FWIW:

 

[jpeters3]

10 minutes ago, Quincy said:

Uh, that’s a sounding from hour 59 in NW Mississippi. I see plenty of warm sector CI by 19-20z. (Including cells directly over the 33.82, -88.65 data point at 23z if you look closely. 

I would normally advise against detailed sounding analysis of the 59hr 3km NAM, but FWIW:

 

The sounding was simply posted as an example of a signature that shows up broadly in both the NAM and NAM nest.   My post contained the proper "words of caution" that should be attached to any analysis of any model solution for any severe weather even 3 days out.  And the fact is that this solution does not show a tornado outbreak.  I see a lot of warm air advection driven junk to the east part of the warm sector, and then a squall line along the cold front.  The main region of the warm sector with the highest parameter values>

Also, the sounding was taken sufficiently far head of the squall line to avoid convective contamination.  If you were indeed correct and the sounding were contaminated, there would be a deep region of saturation and ascent through the troposphere (like in the second sounding I posted).  There is in fact no such signature.  It's possible that there is some model representation of low-leevel convection (again related to the warm air advection) showing up.

In any case, whether or not the sounding i posted is "contaminated" does not even negate my original point.  This model solution is showing a potential bust mode in terms of a widespread severe weather outbreak.  This is not something that can really be argued with.

Multiple soundings from both the NAM and NAM nest have been posted here, or are available for you to look at yourself, that show relatively poor 0-3 km lapse rates.  This has been a bust mechanism for events in the past, and could be one here.

 

Also, the "cells" you point out in the warm sector in the vicinity of this sounding most likely represent shallow (not deep) convection.

[Bob's Burgers]

 

[ILMRoss]

Couple of thoughts;

-A weird wildcard to keep in mind is that model accuracy has gone down because of the COVID-19 outbreak. Why? Commercial planes are equipped with sensors that collect data that is fed into models. We're not talking hurricane hunters grade stuff here, but enough to increase our sampling to improve model outputs. Model verification has dipped significantly since the onset of the outbreak, and less air traffic is probably the reason why. Now- obviously models are still robust and should be trusted, but they may swing a little more/have a bit less consistency than usual.

Some NC specific things: 

850MB winds will be whipping- the default to expect is a squall line with some embedded spin-ups. There are two ways this becomes something fiercer:

-The model trend has been ticking this thing slower and slower. The slower this goes, the more juiced the atmosphere can get ahead of the storms, and put far east places (New Bern, Edenton, E-City) in range for stronger storms/stronger tornado risk. 

-The flip side to the above is Sunday's storms evolving into an MCS that rockets past guidance, and is offshore the NC Coast by midmorning Monday, allowing the atmosphere over Eastern NC to recover. 

I think that the ceiling for NC on Monday is an enhanced risk; but keep an eye on things. Not a good idea to take your eye off the ball in high-ceiling events like this.

 

 

[Quincy]

6.8 C/km is the average 0-3km lapse rate for significant tornadoes: 

https://www.spc.noaa.gov/publications/mosier/2018-JOM1.pdf

Taken verbatim, the NAM shows 6-6.5 C/km lapse rates with around 100 J/kg 3CAPE through much of the risk area. 

Given the wind profiles, that’s definitely supportive of strong, long-track tornadoes.

Maybe if the low-level lapse rates were steeper we’d be talking about a historic event. I would not go there yet, but I think to argue against a tornado outbreak at this stage is a stretch. 

[jpeters3]

5 minutes ago, Quincy said:

6.8 C/km is the average 0-3km lapse rate for significant tornadoes: 

https://www.spc.noaa.gov/publications/mosier/2018-JOM1.pdf

Taken verbatim, the NAM shows 6-6.5 C/km lapse rates with around 100 J/kg 3CAPE through much of the risk area. 

Given the wind profiles, that’s definitely supportive of strong, long-track tornadoes.

Maybe if the low-level lapse rates were steeper we’d be talking about a historic event. I would not go there yet, but I think to argue against a tornado outbreak at this stage is a stretch. 

I should have been mores specific.  What I was referring to is the buoyancy at low-levels for a lifted parcel, which is on the low side because of the mentioned warm layer blow 300 mb.  This is something that has been shown in research to inhibit tornadogenesis.  You'll notice that there is almost 0 CAPE below ~ 2.5 km in these soundings.

Also, the average 0-3 km lapse rate for violent tornadoes (per the preprint you posted) is 7.2 C/km, with a lower bound at 6.5 C/km.  So this lapse rate would fall within the lower range of violent tornadoes.

Also, nobody is saying there isn't going to be a tornado outbreak (myself included).  All I said is that the NAM and NAM Nest are not showing solutions that are consistent with an outbreak.  This does not mean one won't happen, but it's not something that should be ignored in lieu of wishcasting ether.

[Quincy]

2 minutes ago, jpeters3 said:

I should have been mores specific.  What I was referring to is the buoyancy at low-levels for a lifted parcel, which is on the low side because of the mentioned warm layer blow 300 mb.  This is something that has been shown in research to inhibit tornadogenesis.  You'll notice that there is almost 0 CAPE below ~ 2.5 km in these soundings.

Also, nobody is saying there isn't going to be a tornado outbreak (myself included).  All I said is that the NAM and NAM Nest are not showing solutions that are consistent with an outbreak.  This does not mean one won't happen, but it's not something that should be ignored in lieu of wishcasting ether.

There are failure modes for sure. If there’s too much convection too early, that would also throw things off.

To get a major outbreak, the needle really has to be threaded just right. Low-level instability is one limiting factor.

I’m mostly concerned because global and mesoscale models alike show CI in the unstable/highly sheared warm sector, which is what you’d expect to see during an outbreak.

How high will the ceiling be? We’ll see. I’ve been burned for being too bullish on occasion. 

All it takes is one...

Let’s hope that for the sake of the local residents, this does perform below expectation.  

[jpeters3]

43 minutes ago, Quincy said:

There are failure modes for sure. If there’s too much convection too early, that would also throw things off.

To get a major outbreak, the needle really has to be threaded just right. Low-level instability is one limiting factor.

I’m mostly concerned because global and mesoscale models alike show CI in the unstable/highly sheared warm sector, which is what you’d expect to see during an outbreak.

How high will the ceiling be? We’ll see. I’ve been burned for being too bullish on occasion. 

All it takes is one...

Let’s hope that for the sake of the local residents, this does perform below expectation.  

I do agree that the ceiling for this event is a high end tornado outbreak.

[jojo762]

After looking at various SHARPpy 12z NAM sounding locations across MS/AL from 18z to 00z on Sunday, i've made a few observations...

1. The wind profile has excellent turning with height from about 18-20z'ish across most of Mississppi and Alabama.

2. After 20z wind profiles progressively get uglier across much of the moderate-risk area; yes ESRH is still very impressive with values of 300-550m2/s2 prevalent across most of the threat area, but still the low/mid-level profile from about 2-7km starts to get a little less-than-ideal looking.

3. I think this could mean that potential warm sector supercells might start to get a bit grungy in the mid/later afternoon hours... Adding to wind profile issues, a lot of soundings are pretty darn saturated, so HP supercells could be pretty prevalent...

 

It is probably too early to give a prediction but here is my two-cents based off of what I have seen... Altogether, I think early in the event several if not numerous supercells are going to be ongoing across Louisiana, Mississppi, and Arkansas... Some may produce long-tracking strong/violent tornadoes, and given the moist PBL some might be quite large... after a couple/few hours, storms might start to get clustery/HP looking and a tornado threat will persist, but it won't be anything like it was in the early afternoon... eventually we should end up with a large QLCS along the cold front, with severe-wind producing clusters of former supercells in Alabama or Georgia. The dry-slot could be a mitigating factor for clustery/HP modes in the later afternoon, but that remains to be seen.

 

[Quincy]

6 minutes ago, jojo762 said:

After looking at various SHARPpy 12z NAM sounding locations across MS/AL from 18z to 00z on Sunday, i've made a few observations...

2. After 20z wind profiles progressively get uglier across much of the moderate-risk area; yes ESRH is still very impressive with values of 300-550m2/s2 prevalent across most of the threat area, but still the low/mid-level profile from about 2-7km starts to get a little less-than-ideal looking.

Not sure I agree with this. Even from 21-00z, when the mid to upper level wind profiles become rather unidirectional, low level shear remains highly favorable for tornadic supercells.

Point forecast soundings over most of MS and vicinity exhibit a classic, large, sickle shaped hodograph in the lowest 1-2km. Pre-frontal/warm sector wind profiles out ahead of the shortwave maintain even larger hodographs longer. 

[jpeters3]

Lots of quibbling here over details this far out.  At this point, I think we can all agree that we have the potential for a VERY high end event.  But like many recent would-be high events that did not pan out, the current guidance suite has also shown us a few ways that this could bust.  Maybe it will bust, maybe it won't?  Obviously too early to responsibly make a call either way.

[MUWX]

One of the Birmingham NWS mets was on twitter saying that the NAM 3KM rarely produced realistic convection in the SE.