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March 17-18 Severe Weather Event


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2 hours ago, jpeters3 said:

I think the main question is storm mode.  CAMS seem to be trending toward a classic southeastern US "grunge fest" with widespread nonsupercellular convection in the warm sector, and a QLCS along the front.  This could therefore pan out like 5-20-19 in the plains, with an impressive environment but a lack of tornadic supercells.

Main question in the SE usually is storm mode given lack of more obvious boundaries, but I'm not so sure that 12z CAM suite was as "grungey" as it would appear, especially a couple of the WRFs. The aggregate/average of them as depicted by the 12z HREF would likely be a significant severe event, with both supercells and bowing line segments.

The big question is what does the storm mode look like post-21z when the low-level shear increases significantly. If there are still supercells ongoing by 00z and into the nocturnal period, watch out. The pseudo-cold core regime nearer to the upper low in AR/MO is also worth watching should it be able to destabilize sufficiently.

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http://www.meteo.psu.edu/fxg1/NARR/2014/us0428.php

You want broad trough , not a compact ejecting upper low.  Look for waves to rotate around the trough to spark an outbreak. 

http://www.meteo.psu.edu/fxg1/NARR/2011/us0427.php

Look for s/w rotating around broad trough axis. 

 

gfs_z500_vort_us_7.png 

Tomorrow's system is a southern stream upper low that remains  an upper low and not broad.  In the back of your head you should look for what can go wrong. Typically wind fields will be veer back veer, or speed of the system is not ideal. Also usually cloud cover with these southern stream vorts doesn't allow sufficient build up of 03 km cape. 

https://gyazo.com/4e21b68ed657c4990f86f67510a89971 Veer back veer noted around 2 -3 km . 

 

Summary looks for storms to be on going in morning, but move north of the warm front. Storms should take off after 23z Se arkansas and move quickly east in a messy band into north Alabamy overnight. 1 or 2 tornadoes and swaths of wind are likely.  Hail generally under 2.5 inches. 

 

 

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9 minutes ago, andyhb said:

Main question in the SE usually is storm mode given lack of more obvious boundaries, but I'm not so sure that 12z CAM suite was as "grungey" as it would appear, especially a couple of the WRFs. The aggregate/average of them as depicted by the 12z HREF would likely be a significant severe event, with both supercells and bowing line segments.

The big question is what does the storm mode look like post-21z when the low-level shear increases significantly. If there are still supercells ongoing by 00z and into the nocturnal period, watch out. The pseudo-cold core regime nearer to the upper low in AR/MO is also worth watching should it be able to destabilize sufficiently.

I'm not arguing against a significant severe event.  Given the volatility of the environment, QLCS tornadoes, and perhaps some isolated supercell tornadoes, are likely if the current CAMs play out.  But they aren't showing a classic tornado outbreak by any means. 


An issue in these types of events is that there is no forcing mechanism that generates sub-cloud updrafts that are wide enough to make supercells in the warm sector.  So everything that forms in the warm sector forms from smaller boundary layer thermals, and ends up being shallow and grungy.  This was probably an issue on 5-20-19.

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7 minutes ago, jpeters3 said:

I'm not arguing against a significant severe event.  Given the volatility of the environment, QLCS tornadoes, and perhaps some isolated supercell tornadoes, are likely if the current CAMs play out.  But they aren't showing a classic tornado outbreak by any means. 

No one said you did.

I'm just saying that I don't think the 12z CAMs were really a trend one way or another towards a really muted event ala 5/20/2019. If anything, the HRRR being a little more bearish than some of the WRFs might increase my confidence in a cleaner evolution, lol.

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4 minutes ago, andyhb said:

No one said you did.

I'm just saying that I don't think the 12z CAMs were really a trend one way or another towards a really muted event. If anything, the HRRR being a little more bearish than some of the WRFs might increase my confidence in a cleaner evolution, lol.

I wouldn't call this bearish on a cleaner evolution.  The lack cohesive of UH tracks in the warm sector suggests shallower (than supercells), nonsteady convection.

18Z-20210316_HRRRSE_prec_radar-18-48-10-100.gif

 

NAM nest, for reference, is quite similar:

18Z-20210316_NAMNSTSE_prec_radar-18-48-10-100.gif

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Just now, jpeters3 said:

I wouldn't call this bearish on a cleaner evolution.  The lack cohesive of UH tracks in the warm sector suggests shallower (than supercells), nonsteady convection.

I was referring to the 12z runs here. The 18z was definitely less favorable, although there still seems to be some attempts at warm sector initiation.

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11 minutes ago, jpeters3 said:

I'm not arguing against a significant severe event.  Given the volatility of the environment, QLCS tornadoes, and perhaps some isolated supercell tornadoes, are likely if the current CAMs play out.  But they aren't showing a classic tornado outbreak by any means. 


An issue in these types of events is that there is no forcing mechanism that generates sub-cloud updrafts that are wide enough to make supercells in the warm sector.  So everything that forms in the warm sector forms from smaller boundary layer thermals, and ends up being shallow and grungy.  This was probably an issue on 5-20-19.

The issue on 5/20 was that the primary wave lagged and deep large scale lift never really overspread the warm sector like it was projected to on the HRRR. I remember distinctly that there were no shortage of updrafts but they lacked better support to really root and take hold. Additionally concerns with the EML not really eroding(again due to lag in trough) and smoke keeping temps down by 2-3 degrees. The thing that is problematic tomorrow in this regard is that track of the trough encourages large scale lift overspreading the warm sector by 21z which should encourage OWS development. However, like you, I too wonder if this activity struggles to produce anything more than brief tornadoes as there is a distinct lack of enlargement/curvature in the 0-3km hodographs until the LLJ ramps up. At which point it becomes a question of what do the storms look like when it does? There's no ifs ands or buts about the environment between 00-03z, but do discrete supercells still exist at that hour to take advantage of it? I personally would hold at a large moderate risk until that question can be answered.

 

I'll also add that for some reasons, CAMs ALWAYS struggle to produce UDH swaths in dixie. I'm not sure why, but I have noticed it.

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6 minutes ago, andyhb said:

I was referring to the 12z runs here. The 18z was definitely less favorable, although there still seems to be some attempts at warm sector initiation.

In looking at the 12z (I hadn't before), they do look more favorable for discrete storms.

For reference, here is the 12Z HRRR.  This looks quite a bit more "outbreakish"

 

12Z-20210316_HRRRSE_prec_radar-24-48-10-100.gif

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5 minutes ago, hlcater said:

The issue on 5/20 was that the primary wave lagged and deep large scale lift never really overspread the warm sector like it was projected to on the HRRR. I remember distinctly that there were no shortage of updrafts but they lacked better support to really root and take hold. Additionally concerns with the EML not really eroding(again due to lag in trough) and smoke keeping temps down by 2-3 degrees. The thing that is problematic tomorrow in this regard is that track of the trough encourages large scale lift overspreading the warm sector by 21z which should encourage OWS development. However, like you, I too wonder if this activity struggles to produce anything more than brief tornadoes as there is a distinct lack of enlargement/curvature in the 0-3km hodographs until the LLJ ramps up. At which point it becomes a question of what do the storms look like when it does? There's no ifs ands or buts about the environment between 00-03z, but do discrete supercells still exist at that hour to take advantage of it? I personally would hold at a large moderate risk until that question can be answered.

Honestly, 5-20 is still a bit of a head scratcher for me.  I understand the issue with large scale lift, but large scale lift doesn't trigger convection.  It simply makes the environment more favorable by steepening lapse rates and potentially reducing CIN.

An interesting result that trending in recent research is that, for CI to really go, you need wide enough sub-cloud updrafts.  Because entrainment-driven dilution of updrafts scales inversely with updraft width, wider updrafts can go deeper because they are less hindered by entrainment.  In the absence of other forcing mechanisms (like OFBs, terrain, front, dryline), the size of sub-cloud updrafts scales with the PBL depth.  In the case of 5-20, the PBL was < 1 km deep, whereas the "threshold" sub-cloud updraft width for making sustained deep updrafts is generally larger than 1 km (in radius).  So my hypothesis is that the PBL convection just couldn't cut it on that day.  I have meant to write a paper on this, but i never seem to have the time.

I wonder if a similar issue will come to play here.

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Just now, jpeters3 said:

Honestly, 5-20 is still a bit of a head scratcher for me.  I understand the issue with large scale lift, but large scale lift doesn't trigger convection.  It simply makes the environment more favorable by steepening lapse rates and potentially reducing CIN.

An interesting result that trending in recent research is that, for CI to really go, you need wide enough sub-cloud updrafts.  In the absence of other forcing mechanisms (like OFBs, terrain, front, dryline), the size of sub-cloud updrafts scales with the PBL depth.  In the case of 5-20, the PBL was < 1 km deep, whereas the "threshold" sub-cloud updraft width for making sustained deep updrafts is generally larger than 1 km (in radius).  So my hypothesis is that the PBL convection just couldn't cut it on that day.  I have meant to write a paper on this, but i never seem to have the time.

Based on my experience being out there in SW OK as storms tried to develop, this was the case. Every attempt became a turkey tower, and part of it was likely due to all of the smoke in the warm sector limiting insolation (and the capping being stronger than the models had).

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1 hour ago, PackGrad05 said:

WRAL just said they have noticed a trend with “storm energy” (probably CAPE) being higher east of Raleigh.   Said they are watching to see if the level 3 area is trimmed back tomorrow.    

IT will be trimmed back, I dont think we really get into the humid and moist airmass.  This is just like winter, we miss the meat of the storm here in the Triangle.  I rather be in Fayettenam or Lumberton to SC for this one if I want severe thunderstorms

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Don’t really ever post in this forum, just the New England one or Tropical weather ones. However with this set up tomorrow anyone saying there WILL or WILL NOT be ef4 + tornadoes tomorrow need to check themselves. I understand this is a forum for discussing weather but definitive statements either way aren’t going to hold much weight. What was it the Ef-3 Nashville tornado that hit on a slight risk day during nighttime? This environment is much more volatile, but doesn’t mean it actually produces. End of the day tornados are a process on such a micro basis no one should be using a model run to assess the intensity of future tornadoes tomorrow. Just my two cents, I hope everyone stays safe tomorrow.

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8 minutes ago, KoalaBeer said:

Don’t really ever post in this forum, just the New England one or Tropical weather ones. However with this set up tomorrow anyone saying there WILL or WILL NOT be ef4 + tornadoes tomorrow need to check themselves. I understand this is a forum for discussing weather but definitive statements either way aren’t going to hold much weight. What was it the Ef-3 Nashville tornado that hit on a slight risk day during nighttime? This environment is much more volatile, but doesn’t mean it actually produces. End of the day tornados are a process on such a micro basis no one should be using a model run to assess the intensity of future tornadoes tomorrow. Just my two cents, I hope everyone stays safe tomorrow.

Yes, making blanket statements about tornado intensity the day before is unadvisable.  Recall that the forecast parameters did not exactly scream EF-5 on 5-22-11 (Joplin) or 5-20-13 (Moore).  But it happened.

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10 minutes ago, KoalaBeer said:

Don’t really ever post in this forum, just the New England one or Tropical weather ones. However with this set up tomorrow anyone saying there WILL or WILL NOT be ef4 + tornadoes tomorrow need to check themselves. I understand this is a forum for discussing weather but definitive statements either way aren’t going to hold much weight. What was it the Ef-3 Nashville tornado that hit on a slight risk day during nighttime? This environment is much more volatile, but doesn’t mean it actually produces. End of the day tornados are a process on such a micro basis no one should be using a model run to assess the intensity of future tornadoes tomorrow. Just my two cents, I hope everyone stays safe tomorrow.

The overall background state for sure will allow for significant tornadoes, how many and what magnitude who knows

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WRT the talking points about 5/20/19 and tomorrow... As has been discussed to great extent, the capping on 5/20/19 turned out to be "just enough" to withhold substantial warm sector convection (so the sub-cloud updraft point mattered in that case), but tomorrow will feature near zero to zero capping... As I believe @hlcarter has referenced already. 

Obviously there is the contrarian argument to be made that that complete lack of capping could lead to a wide array of junkvection in the open warm-sector -- as has been potentially hinted at times by some guidance.

Regardless of nebulous mesoscale details about warm-sector forcing and residual capping, it feels obvious to me that there is inevitably going to be at least a couple sporadically tornadic open warm-sector supercells tomorrow...

To me the main questions comes down to organizational longevity and how intense they're able to become given some concern about potentially sloppy low/mid-level wind profiles. Particularly the 850mb-600mb layer could ultimately hold back storm organization at times, or even most of the event. This goes for both semi-discrete supercells along the main forcing line in Arkansas in the afternoon, and any potential open warm-sector supercells across Mississippi/Alabama as well. Lastly,  what I am most confident in is that a substantial QLCS event will almost certainly occur much of tomorrow night and into the early morning hours on Thursday -- much like we saw several times last year.

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46 minutes ago, Avdave said:

IT will be trimmed back, I dont think we really get into the humid and moist airmass.  This is just like winter, we miss the meat of the storm here in the Triangle.  I rather be in Fayettenam or Lumberton to SC for this one if I want severe thunderstorms

I'm surprised they went with the enhanced for such a large area so far in advance.  Seems it would've been better to go with a large slight and then increase to enhanced as needed.   Unless they wanted to raise awareness early.

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27 minutes ago, crossbowftw3 said:

The overall background state for sure will allow for significant tornadoes, how many and what magnitude who knows

Yup pretty much my point and like jpeters said Moore and Joplin weren’t exactly screaming Ef5 potential on those days. 

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26 minutes ago, jojo762 said:

WRT the talking points about 5/20/19 and tomorrow... As has been discussed to great extent, the capping on 5/20/19 turned out to be "just enough" to withhold substantial warm sector convection (so the sub-cloud updraft point mattered in that case), but tomorrow will feature near zero to zero capping... As I believe @hlcarter has referenced already. 

Obviously there is the contrarian argument to be made that that complete lack of capping could lead to a wide array of junkvection in the open warm-sector -- as has been potentially hinted at times by some guidance.

Regardless of nebulous mesoscale details about warm-sector forcing and residual capping, it feels obvious to me that there is inevitably going to be at least a couple sporadically tornadic open warm-sector supercells tomorrow...

To me the main questions comes down to organizational longevity and how intense they're able to become given some concern about potentially sloppy low/mid-level wind profiles. Particularly the 850mb-600mb layer could ultimately hold back storm organization at times, or even most of the event. This goes for both semi-discrete supercells along the main forcing line in Arkansas in the afternoon, and any potential open warm-sector supercells across Mississippi/Alabama as well. Lastly,  what I am most confident in is that a substantial QLCS event will almost certainly occur much of tomorrow night and into the early morning hours on Thursday -- much like we saw several times last year.

That's not necessarily true. I think capping will probably be sufficient through at least 17-18z to prevent widespread storm development in the warm sector courtesy of a bone dry EML arriving ahead of the trough. The primary difference that tomorrow has and something I think makes OWS convection more likely is that if you look aloft, there are substantial height falls over nearly the entire warm sector along with stout low level moist advection. These should work together to ensure the best possible shot at storms ahead of the primary band. As such, hodographs and strength of the low level jet late tomorrow afternoon will be something that needs to be monitored intensely given the possibility of (numerous?) discrete/semi-discrete rotating updrafts.

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12 minutes ago, hlcater said:

That's not necessarily true. I think capping will probably be sufficient through at least 17-18z to prevent widespread storm development in the warm sector courtesy of a bone dry EML arriving ahead of the trough. The primary difference that tomorrow has and something I think makes OWS convection more likely is that if you look aloft, there are substantial height falls over nearly the entire warm sector along with stout low level moist advection. These should work together to ensure the best possible shot at storms ahead of the primary band. As such, hodographs and strength of the low level jet late tomorrow afternoon will be something that needs to be monitored intensely given the possibility of (numerous?) discrete/semi-discrete rotating updrafts.

I'm referencing capping by mid-afternoon mostly. Good analysis. 

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Just now, jojo762 said:

I'm referencing capping by mid-afternoon mostly. Good analysis. 

the most unfortunate part of this all remains the idea that SPC hoists potential PDS watches by mid-afternoon, and very little winds up happening until the jet kicks in, lulling people into thinking not much would wind up occuring.

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1 minute ago, crossbowftw3 said:

the most unfortunate part of this all remains the idea that SPC hoists potential PDS watches by mid-afternoon, and very little winds up happening until the jet kicks in, lulling people into thinking not much would wind up occuring.

Spann has done a pretty good job of saying the primary threat for Alabama may not be until overnight. 

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2 hours ago, Upper Level LOL said:

This is needlessly scaremongering and doesn't fit the parameters we see. We're going to see a lot of low-end tornadoes since the warm sector has essentially no cap. This will prevent bigger storms from firing. The threat of tomorrow aren't EF4-EF5s, but a firehose of EF2's and EF3's. I hope your reputation among your friends and family doesn't suffer from crying wolf about this.

I have been forecasting longer than you have been probably been alive. Read my profile.  Just because I do not have a  meteorologist tag does not mean that I am not one.  I am retired. I earned my BS degree in physical geography/atmospheric sciences ( few schools had meteorology programs back then)  and when computers were not even in the regional offices and the  NGM to AVN to GFS model were the only models by paper and fax available. No internet and weather offices were at the airports with no regional offices. Everything came out from the supercomputers in Maryland and Oklahoma. Long range models were printed only once a week in paper newsletters. Prior to this, one relied mostly on personal experience and the satellite imagery  as dopplar radar was not even around until the late 70's and eraly 80's and only available in certain areas of the country.  Plus  many meteorologists were former Navy air traffic controllers in the 70's and 80's , since they went to school to read and understand the AVN models, which I was one.

This is not scaremongering and any one who downplays this unfolding situation has never seen this particular weather pattern evolve before.  I also said "could"  in my forecast too for the formation of  EF 4-5. tornadoes.  I have personally experienced  EF-3 tornadoes myself in 1997 west memphis tornado and an EF-3 near Philly in 1994.  Fuerthermore, a tornado emergency is usually reserved to be  issued when debris ball signatures are clearly detected on the radar.

My family trusts my weather predictions for 35 years. Can you say this for your forecasts?  I have no reputation to tarnish.

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