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Severe Event March 25th 2021


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As could be expected the 00z HRRR depicts a mixed bag. Yes, there will likely be many storms in the warm sector as should be anticipated in the deep south when there is weak/no CINH and large CAPE, but there will also be many dominant supercells. Pretty classic for a "big" setup in this region. Almost certainly looking at a sizable Moderate Risk here in about 3.5 hours.

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One thing to note (as was mentioned by @Bob's Burgers), the 00 UTC HRRR run extends the outbreak well up into the MS and OH river valleys.  This HRRR run is fairly close to a worst-case super-outbreak esq scenario.  However, I am still somewhat skeptical of storm mode throughout the main warm sector in MS/AL, despite the HRRR showing apocalypse.  There may be hints in this solution of an ambient messy storm mode, and an upscale evolution akin to 4-12-20.

 

Freaky CAM solutions aside, I still highly doubt a day 2 High will happen, nor do I think it is warranted.


Edit: NAM nest shows a similar scenario for MS/AL, and also extends the outbreak a bit further north (albeit not to the extent of the HRRR).  One thing to note is both models show fairly quick stabilization into the early morning on Friday, suggesting that a prolific nocturnal event is questionable.  

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00z CAM run down:

HRRR: Plenty of supercells, but also a fair amount of junk in the warm sector.

NSSL-WRF: Numerous discrete supercells traverse the warm sector... Much less junkvection.

WRF-NMMB: Most aggressive with junk in the warm sector. Maybe some tornadic supercells, but looks awfully junky.

WRF-ARW: Much like the WRF-NSSL is fairly aggressive developing numerous discrete supercells across the warm sector.

NAM NEST: Numerous discrete supercells across the warm sector.

 

Pretty substantial agreement in CAM output that a significant threat will evolve on Thursday... Perhaps the biggest question relates to northward progression of the warm sector with both the HRRR and NAM, as well as the globals being substantially further north than most other CAM guidance was.

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4 minutes ago, jojo762 said:

00z CAM run down:

HRRR: Plenty of supercells, but also a fair amount of junk in the warm sector.

NSSL-WRF: Numerous discrete supercells traverse the warm sector... Much less junkvection.

WRF-NMMB: Most aggressive with junk in the warm sector. Maybe some tornadic supercells, but looks awfully junky.

WRF-ARW: Much like the WRF-NSSL is fairly aggressive developing numerous discrete supercells across the warm sector.

NAM NEST: Numerous discrete supercells across the warm sector.

 

Pretty substantial agreement in CAM output that a significant threat will evolve on Thursday... Perhaps the biggest question relates to northward progression of the warm sector with both the HRRR and NAM, as well as the globals being substantially further north than most other CAM guidance was.

You gotta wonder what the next few hours of both the ARW and NSSL WRF would look like; both look like they are forming a lot more storms at 0z when the run ends. 

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1 hour ago, Bob's Burgers said:

SPC might have to bring the Slight all the way to Dayton Ohio

Given that several models show intensification of the low pressure over Illinois and Indiana (eg. GFS, Euro, 0z HRRR), I would definitely agree with that sentiment, if primarily out of concern for damaging straight-line winds (and while not related to SPC criteria, perhaps for non-thunderstorm winds too, especially in IN/OH, in the event of a particularly-strong low).  Some of these models also have lower-60s dew points getting north of the Ohio River, even as far north as the I-70 corridor (Indianapolis-Dayton-Columbus) in some runs.  SPC even hinted at QLCS potential for the Ohio Valley (KY/IN/OH) as well.

If more destabilization and warmer temperatures/dews (than what models currently show) can get that far north, I would be concerned for a more widespread severe thunderstorm outbreak than what is currently expected.  It appears as if that all comes down to the amount of "junk" precipitation that forms, as less would mean more supercell (and tornado) potential and potentially a greater spatial extent to any potential outbreak.  If things stay "junky," then whatever tornado potential remains might appear confined to MS/AL, but less junk would increase potential in MS/AL plus bring more severe storm action into the TN and OH valleys.

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4 hours ago, JasonOH said:

I should have clarified more. The EML is not enough (at least on the soundings I’ve grabbed) to provide substantial capping.  The EML isn’t quite warm enough at its base

Agreed on that point. Though I think the EML in and of itself is actually sufficient, or at least sufficient enough to hold through 18z if it didn't erode quite so quickly and instead allowed for surface heating to overcome it.

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day2otlk_0600.gif
 

SPC AC 240600

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0100 AM CDT Wed Mar 24 2021

   Valid 251200Z - 261200Z

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS OVER A LARGE
   PART OF MISSISSIPPI...PORTIONS OF CENTRAL AND NORTHERN ALABAMA AND
   ADJACENT AREAS OF WESTERN AND MIDDLE TENNESSEE...

   ...SUMMARY...
   A potential outbreak of severe storms including several long track
   strong tornadoes, large hail and damaging wind will exist Thursday
   into Thursday evening across a portion of the lower Mississippi
   Valley and Southeast States.

   ...Lower Mississippi Valley and Southeast States...

   Shortwave trough currently over the Southwest States will move from
   southern TX early Thursday through the lower MS Valley during the
   day and into the OH Valley overnight. Thursday morning a cold front
   will extend from the Great Lakes southwest to a weak surface low in
   northeast TX, then south into the western Gulf. In response to the
   ejecting negative-tilt shortwave trough, the cyclone is forecast by
   some models to undergo significant deepening as it develops
   northeast during the day. As this occurs, a cold front will
   accelerate through the lower MS Valley into the Southeast States,
   while a warm front initially from northeast TX into northern LA,
   central MS and southwest AL develops northward into the TN Valley.
   Some model differences do exist with the NAM being slightly less
   amplified and more progressive with resulting weaker cyclogenesis.

   It still appears likely that elevated storms will be in progress at
   the start of the period within the warm advection regime across
   northern MS, AL and TN. This activity is expected to continue
   lifting northward, allowing for destabilization from the south with
   time. Rich low-level moisture with upper 60s to near 70 F dewpoints
   will advect northward through warm sector contributing to moderate
   instability with up to 2000 J/kg MLCAPE. Discrete storms are
   expected to develop over the lower MS Valley as the surface layer
   destabilizes during the afternoon. The low-level jet will strengthen
   to 60+ kt over the lower MS and TN Valleys in response to forcing
   within exit region of the migratory mid-upper jet accompanying the
   shortwave trough. Vertical wind profiles with very strong shear and
   large 0-1 km hodographs (0-1 km helicity from 300-400 m2/s2) will
   support supercells with low-level mesocyclones capable of producing
   strong tornadoes and large hail. Some bowing structures are also
   likely. Activity will spread northeast through the lower MS and TN
   valleys during the day and into the evening. Additional storms may
   also develop along the cold front with linear structures capable of
   damaging wind.

   A secondary area of thunderstorm development might occur in vicinity
   of the ejecting vorticity maximum and near and just south of the
   surface low track from northern AR into southern MO. All hazards
   will be possible in this region, but threat is more conditional at
   this time given uncertainty regarding how much destabilization can
   occur. Will maintain this region in a SLGT for now, but continue to
   monitor for upgrade to higher probabilities in later updates.  

   ...OH Valley...

   Widespread rain and thunderstorms will occur over a large part of
   this region during the day. However, some destabilization should
   occur, especially during the evening fostered by a strengthening
   southerly low-level jet. Potential will exist for a forced line of
   storms to develop along the cold front, near and south of the
   surface low as it tracks northeast through this region during the
   evening and overnight. The primary threats will be damaging wind and
   possibly some QLCS tornadoes.

   ..Dial.. 03/24/2021
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Just now, Indystorm said:

Yes the HRRR runs are consistently extreme from AL and MS north to the Ohio River Valley.  How accurate were they a week ago for March 17?

By the time the event was 24-36 hours out the parameters were much less ominous than the ones for tomorrow.

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2 minutes ago, Cheeznado said:

By the time the event was 24-36 hours out the parameters were much less ominous than the ones for tomorrow.

Overall though, the HRRR did quite well wrt convective evolution. Better than most CAMs did. The HRRR doing better than most CAMs is quickly becoming the norm and not the exception, anyways.

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NAM NEST is much more toned down than the HRRR was. In fact it basically shows zero intense convection across most of the MDT risk area during its run. Not gonna buy that.

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5 minutes ago, jojo762 said:

NAM NEST is much more toned down than the HRRR was. In fact it basically shows zero intense convection across most of the MDT risk area during its run. Not gonna buy that.

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.  

Edit: In a probabilistic sense, we've seen the NAM nest solution play out numerous times over the past decades.  The HRRR solution, on the other hand, has played out only once (or maybe a handfull of times).  

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