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Severe Event March 25th 2021


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May have been a little bit too early to put a thread out given there's no SPC risk area yet, however 06z GFS and 00z Euro both are pretty interesting solutions for the deep south Thursday with some timing and location differences to work out and this setup definitely deserves watching.

sfctd_b.conus.png

sfctd_b.conus.png

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Day 4 risk area put out by SPC

spcd4prob.conus.png

 ...DISCUSSION...
   ...Day 4/Thu - Eastern TX to the Central Gulf Coast States...

   An upper shortwave trough over the Rio Grande/northern Mexico will
   strengthen and become negatively tilted as it ejects northeast
   across the Arklatex through Thursday evening, and to the Ohio Valley
   by Friday morning. Intense shear will overspread the south-central
   and southern U.S. ahead of the trough. Furthermore, strong low-level
   warm advection will result in a broad warm sector ahead of a
   deepening surface low and eastward-advancing cold front from the
   Sabine Valley eastward across the central Gulf coast/TN Valley
   vicinity.

   Differences in the evolution of the surface low across the lower MS
   and OH Valleys are still apparent in medium-range guidance. This is
   mainly resulting in uncertainty in the position of the surface low
   and cold front Thursday morning, and how far east each of these
   features progresses by Friday morning. As a result, changes in
   severe probabilities, especially on the western and northeastern
   edges, are likely in the coming days. Nevertheless, weak to moderate
   instability will overlap with favorable shear parameters and an
   overall supportive pattern for severe convection. A couple of rounds
   of severe storms could be possible, as some warm-sector development
   may occur across the Lower MS Valley before a QLCS develops along
   the surging cold front during the evening/nighttime hours. All
   severe hazards will be possible with discrete warm-sector
   supercells. Potential for damaging gusts and tornadoes will becoming
   preferential with any upscale development along the cold front.
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12z NAM depicts a high-end environment (much more volatile than what we had on 3/17 with the verified high risk) across much of Louisiana and Mississippi on Thursday. Given good agreement amongst models on this, fully anticipate at least an ENH risk from SPC with the D3 outlook. 

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The LLJ problem is why I was skeptical of the high risk until the overnight models Tuesday into last Wednesday (when it seemed like less of an issue, at least according to the CAMs) and the 45% "let's tie the 4/27/11 probs over the exact same area" always seemed like an odd choice to me.

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16 minutes ago, CheeselandSkies said:

The LLJ problem is why I was skeptical of the high risk until the overnight models Tuesday into last Wednesday (when it seemed like less of an issue, at least according to the CAMs) and the 45% "let's tie the 4/27/11 probs over the exact same area" always seemed like an odd choice to me.

For what it's worth a 45% did verify in Alabama. A large high risk area verified for much of central Alabama and eastern Mississippi. 

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34 minutes ago, StormySquares said:

For what it's worth a 45% did verify in Alabama. A large high risk area verified for much of central Alabama and eastern Mississippi. 

From a probabilistic standpoint, the 45% contour did verify. However the language used in the convective outlook was too strong (i.e. several long track, violent tornadoes) for the actual event. The shear profiles for Thursday are more supportive of a violent tornado than last week.

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1 minute ago, Bob's Burgers said:

From a probabilistic standpoint, the 45% contour did verify. However the language used in the convective outlook was too strong (i.e. several long track, violent tornadoes) for the actual event. The shear profiles for Thursday are more supportive of a violent tornado than last week.

I agree. It doesn't feel right that last Wednesday had the same probs as 4/27 or 5/24. This Thursday looks pretty impressive. 

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If this is the 75-hour forecast for the STP parameter, then I must say that this might be a situation with several EF1+ tornadoes. As was mentioned before, the NAM and GFS have 70 kt to 80 kt winds at 500mb, with 70 degree dew points in Mississippi.

k5OPCUa.png

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The 0Z GFS has a very ripe environment for Thursday afternoon into Thursday evening, as do all the OZ runs so far. Timing and features will shift, but verbatim both the NAM and GFS have loaded gun soundings for parts of Mississippi rather early on Thursday. 

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00z GFS unsurprisingly is more progressive and faster with Thursday's system than all other guidance -- however it does not appear to matter as all 00Z guidance that is in depicts what would likely be a significant severe weather and tornado outbreak across several southeastern states.

A substantial plume of low-level moisture and >7C/KM lapse rates will combine to generate a large area of 1000-2500J/KG of CAPE south of the warm front. Meanwhile an intense 80-95kt 500mb jet should overspread the warm sector throughout the afternoon and evening while a 50-70kt low-level jet is cranking through much of the day. Do not need STP, EHI, or Supercell composite to tell you that this is not an ideal scenario for folks living in these areas.

My main caveats are potentially early initiation, widespread junkvection, etc... All the usual caveats for this area -- really...

 

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23 minutes ago, Snowstorm920 said:

You do have to wonder about the lack of a CAP Thursday and how that might allow junkvection to fire and contaminate the warm sector. Besides that, environmental parameters are off the charts. A potentially very potent setup.

CWASPnam212F066.png

Where did you find that on CIPS?

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Not bad for an 18Z sounding. As is always the case, details won't be ironed out till early Thursday morning, but there's a lot to work with here. One standout difference from the last event as that the storm will likely be strengthening as opposed to occluding, and the overall trajectory of the LP might be more conductive for a cleaner storm mode. 

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Well, safe to say this could be a pretty big event. Convective evolution/mode concerns notwithstanding, there seems to be a consensus across all of guidance for hodographs supportive of long-lived supercells with long tracked tornadoes as early as 15-18z in LA/MS. The degree of low-level moisture is on the high side for this time of year, and I wonder if there may be some subsidence in the late morning following the passage of a potential lead shortwave that could help to suppress junk convection.

Any discrete storm in the warm sector that manages to root in that boundary layer by early afternoon is likely going to be a problem. Now it's a question of what wrinkles the mesoscale throws in (and whether the NAM comes around more to the potent solutions suggested by the globals, particularly the UK and Euro).

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45 minutes ago, andyhb said:

Well, safe to say this could be a pretty big event. Convective evolution/mode concerns notwithstanding, there seems to be a consensus across all of guidance for hodographs supportive of long-lived supercells with long tracked tornadoes as early as 15-18z in LA/MS. The degree of low-level moisture is on the high side for this time of year, and I wonder if there may be some subsidence in the late morning following the passage of a potential lead shortwave that could help to suppress junk convection.

Any discrete storm in the warm sector that manages to root in that boundary layer by early afternoon is likely going to be a problem. Now it's a question of what wrinkles the mesoscale throws in (and whether the NAM comes around more to the potent solutions suggested by the globals, particularly the UK and Euro).

I'll be honest, I've been slightly stunned at the moisture/temps expected in this which gives me actually more confidence than what we had last week. We've had a fishing/camping trip planned for sometime out by Columbia, SC so to see mid to upper 80s this weekend shows the dynamic nature of this system. 

I know I am certainly a hobbyist in this field but the thing that gives me quite a bit of pause is the strength of the low shown on the Euro. Last week we were dealing with a system at 998 MB I believe at its strongest, while the Euro shows a rapidly deepening low for this week that may very well go sub-990. I'd say the vast majority of outbreaks I've dealt with in the past had this very feature, a strong deepening low sparking off overachieving cells that spawn violent tornadoes. 

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2 hours ago, andyhb said:

Well, safe to say this could be a pretty big event. Convective evolution/mode concerns notwithstanding, there seems to be a consensus across all of guidance for hodographs supportive of long-lived supercells with long tracked tornadoes as early as 15-18z in LA/MS. The degree of low-level moisture is on the high side for this time of year, and I wonder if there may be some subsidence in the late morning following the passage of a potential lead shortwave that could help to suppress junk convection.

Any discrete storm in the warm sector that manages to root in that boundary layer by early afternoon is likely going to be a problem. Now it's a question of what wrinkles the mesoscale throws in (and whether the NAM comes around more to the potent solutions suggested by the globals, particularly the UK and Euro).

When the NAM has PDS TOR forecast soundings with jaw-dropping hodographs (and 5/4/2003 analogs) up the wazoo and it's one of the more subdued solutions... :yikes:

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