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Severe Event March 25th 2021


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SPC finally updated lol... here is teh disco

Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1228 PM CDT Wed Mar 24 2021

   Valid 251200Z - 261200Z

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE LOWER MISSISSIPPI VALLEY/MID-SOUTH INTO THE TENNESSEE VALLEY AND
   SOUTHEAST...

   ...SUMMARY...
   The potential for an outbreak of severe storms including several
   long-track strong tornadoes, scattered large to very large hail, and
   severe/destructive winds, will exist Thursday into Thursday evening
   across a portion of the lower Mississippi Valley, Tennessee Valley,
   and Southeast.

   ...Synopsis...
   A shortwave trough with attendant 70-100 kt+ mid-level jet will
   eject northeastward from the southern Plains across the lower/mid MS
   Valley on Thursday, and continue over the OH Valley and lower Great
   Lakes region Thursday night. At the surface, a weak low over
   northeast TX at the beginning of the period Thursday morning is
   expected to likewise develop northeastward to the mid MS Valley
   through the day while deepening. A warm front attendant to this
   cyclone will likely lift northward across the lower MS
   Valley/Mid-South and TN Valley towards the lower OH Valley by
   Thursday evening.

   ...Lower Mississippi Valley and Southeast...
   Storms will likely be ongoing at the start of the period from
   northeast TX into southern AR and northern MS/AL and TN. This
   activity will likely be supported by a southerly low-level jet and
   related warm advection. These storms should shift northward through
   the morning, allowing for destabilization via diurnal heating to
   occur across the lower MS Valley. Rich low-level moisture with at
   least mid to upper 60s surface dewpoints (perhaps lower 70s), should
   advect northward across parts of the lower/mid MS Valley, TN Valley,
   and Southeast by Thursday afternoon. This increasing moisture,
   coupled with diurnal heating, will likely support MLCAPE of
   1500-2000 J/kg developing over a fairly broad portion of these
   regions.

   A pronounced low-level jet (50-60+ kt) will overlie much of this
   region as well, as large-scale ascent from the ejecting shortwave
   trough overspreads the lower/mid MS Valley vicinity. Strongly
   veering and strengthening wind profiles from the surface through mid
   levels will support supercells. 0-1 km SRH along/south of the
   surface warm front will likely reach 300-500 m2/s2, and low-level
   mesocyclones capable of producing strong, long-track tornadoes
   appear likely with any supercells that can form. Scattered large
   hail, some of which may be 2+ inches in diameter, and
   severe/destructive winds also appear probable, both with supercells
   and any bowing line segments that can develop. At this time, the
   best potential for strong tornadoes appears to extend from parts of
   central/northern MS into western/middle TN and central/northern AL,
   mainly Thursday afternoon and evening as storms move generally
   northeastward. Concern does exist regarding the effect of storms
   forming during the morning on the development of instability, and
   potential for destructive interference from too many storms
   developing across the warm sector at the same time. Still, given the
   very favorable environment forecast across this region and the
   potential for numerous supercells, an upgrade to High Risk for
   multiple strong to potentially violent, long-track tornadoes may be
   needed in a later outlook update.

   ...Ohio Valley...
   Fairly widespread precipitation should occur over a majority of the
   lower OH Valley through the day. This should temper diurnal
   destabilization to some degree, but weak instability will probably
   still develop along/east of the surface low and northward-moving
   warm front. At least an isolated severe threat may develop late
   Thursday afternoon into the evening as the southerly low-level jet
   strengthens over this region and low-level moisture increases. The
   potential for a low-topped line of storms capable of producing
   damaging wind gusts and tornadoes will exist given the strength of
   the low-level flow and related shear. These storms will likely be
   tied to a cold front as it tracks northeastward through the evening
   and overnight hours.
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1 minute ago, DanLarsen34 said:

Some key context on the language used in this outlook. It’s very rare to see the mention of violent tornadoes in a day two outlook. 
 

Kind of crazy in hindsight that 4-27-2011 wasn’t on this list. 

I went back to read the April 14th, 2012 D2 discussion and it doesn't mention the word "violent" in the right context. In fact, the discussion seems to suggest that violent tornadoes may not be likely.
 
 
 
 
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22 minutes ago, Bob's Burgers said:
I went back to read the April 14th, 2012 D2 discussion and it doesn't mention the word "violent" in the right context. In fact, the discussion seems to suggest that violent tornadoes may not be likely.
 
 
 
 

Great catch. Just went back and read the discussion and it certainly seemed like Jewell was walking back the southern high risk a bit. It mentioned that lapse rates were marginal for a typical plains violent tornado day. That was the context in which that word was used.

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37 minutes ago, Bob's Burgers said:
I went back to read the April 14th, 2012 D2 discussion and it doesn't mention the word "violent" in the right context. In fact, the discussion seems to suggest that violent tornadoes may not be likely.
 
 
 
 

Are you telling me someone misinterpreted the data on Twitter?

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3 minutes ago, GoAPPS said:

Are you telling me someone misinterpreted the data on Twitter?

It’s not really a misinterpretation, the query that he was running just wasn’t nuanced enough to pick up the difference. The fact that one additional day should be excluded, only makes the wording for tomorrow even more unusual. Certainly seems we could be on the verge of a historic event. 

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42 minutes ago, DanLarsen34 said:

Some key context on the language used in this outlook. It’s very rare to see the mention of violent tornadoes in a day two outlook. 
 

Kind of crazy in hindsight that 4-27-2011 wasn’t on this list. 

what I'm more interested in is how often the word "violent" is used in the 1730z d2 outlook that didn't precede an outbreak with at least one ef3+. 

I thought for sure they did last week, but it looks like they used the terms "strong" and "intense" instead.  I have to wonder if they're just using those terms interchangeably with violent or if their word use really is that specific.

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3 minutes ago, ATDoel said:

what I'm more interested in is how often the word "violent" is used in the 1730z d2 outlook that didn't precede an outbreak with at least one ef3+. 

I thought for sure they did last week, but it looks like they used the terms "strong" and "intense" instead.  I have to wonder if they're just using those terms interchangeably with violent or if their word use really is that specific.

By definition, “strong” refers to EF2+, “violent” refers to EF4+.

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4 hours ago, andyhb said:

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.

Seems to be fairly common for the ARW-based CAMs (HRRR, HRW-ARW, HRW-NSSL) to be more impressive-looking with discrete supercells/UH in these types of scenarios compared with the NMM-B based CAMs (3km NAM, NMM-B), with the HRRR and NAM usually on opposite ends of that spectrum. The HRRR is probably the most skillful of all the CAMs, and it tends to generate attention because it creates the most realistic looking storms, but obviously it is also prone to generating supercells that don't actually happen. If an event does turn out to be a supercell-driven tornado outbreak, though, the HRRR is probably the most likely to get it right. 

The problem is that there really hasn't been a clean supercell-driven tornado outbreak recently, so we don't know what the current generation of CAMs should look like with that type of event. April 12 of last year was a pretty big outbreak, but the mode was very messy, with only a couple of truly discrete warm sector supercells (which produced big-time tornadoes). Most of the CAMs (even the HRRR) generally forecast that the 04-12-20 event would have a messy mode, which probably prevented it from ever being upgraded to High Risk. 

For tomorrow, the northern extent of the high-end potential remains quite uncertain due to the potential effects of early/ongoing convection. However, the southern extent of the threat will also be a big deal, in terms of potential impact. It could be a high-impact event regardless, but if the Jackson, MS to Birmingham corridor gets involved, the potential outcomes become a lot worse than if it stays mostly north of there.  

 

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9 minutes ago, ATDoel said:

and intense refers to EF+3?  Very confusing, especially since last week their hatched area indicated a 10% chance of an EF2-EF5 tornado.

Not saying I like the terminology, just saying I’ve always been of the understanding that there is a specific distinction between those terms among meteorologists.

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Just now, ATDoel said:

what I'm more interested in is how often the word "violent" is used in the 1730z d2 outlook that didn't precede an outbreak with at least one ef3+. 

Well, just looking at those 4 dates mentioned, all of them DID have an EF3 or higher. 4/14/12 had 5 EF3s and 1 EF4 (yes, I know "violent" was being used in a different context for that D2 outlook, but I'm including it anyways). 4/24/10 had 3 EF3s and 2 EF4s, as did 3/01/07. 4/07/06 had 2 EF3s. My source is Wikipedia lol but it's not bad for information on tornado outbreaks.

3 minutes ago, ATDoel said:

and intense refers to EF+3?  Very confusing, especially since last week their hatched area indicated a 10% chance of an EF2-EF5 tornado.

Also, about this, I believe you are correct about "intense" referring to EF3+ tornadoes. As for the hatched area thing, you are also correct, in that the hatched area represents a 10% or greater chance of an EF2-EF5 tornado. Anything EF2+ would verify their outlook from a probability standpoint. Usually, what differentiates their use of "strong" vs. "intense" vs. "violent" when it comes to their actual technical discussion is based on what the forecaster views the potential severity/ceiling of the event as. A setup that could conditionally produce sigtors will look different from a setup that screams more specifically higher-end tornado potential, but the probabilistic graphics don't communicate that, so you might see a different choice of words in the discussion to indicate that thinking from a human forecaster's perspective. Mesoscale accidents can happen that could produce an EF4+ on a "lower-end" (kind of an oxymoron) significant tornado day, but in general, you could probably tell a setup that could produce a couple EF2s from one that could produce several EF3-EF4+ tornadoes.

All of this has little to do with tomorrow from an actual meteorological analysis, but I hope this answers your questions

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18z HRRR is a bloodbath :axe:

EDIT: this is not to say that I expect it to verify. The failure mode of crowded warm sector has been greatly discussed, and is still a very valid concern, and something that we won't know for sure until tomorrow. That isn't to say that the HRRR should totally be tossed.. but yeah

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I too worry about high levels of storm traffic in the warm sector. EML is impressive early on but doesn’t really result in much residual CINH by 18z. I wonder if this allows 69 updrafts to initiate all at once and just ram into each other and impede the ability for deep rotating mesocyclones to develop.

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2 minutes ago, hlcater said:

I too worry about high levels of storm traffic in the warm sector. EML is impressive early on but doesn’t really result in much residual CINH by 18z. I wonder if this allows 69 updrafts to initiate all at once and just ram into each other and impede the ability for deep rotating mesocyclones to develop.

SPC did mention their concern for "destructive interference from too many storms in the warm sector" in their 1730 OTLK... so this is on their mind as well

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