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Thursday will be a regional severe wx outbreak. @CheeselandSkies I think SPC was waiting for American guidance to come toward the ECMWF. It was stubbornly slow this time. Regardless of how we got here, we are at hatched ENH Day 3 now.

First thing I noticed with 12Z guidance is that 500/200 mb winds are forecast WSW. Some of the sloppy South days those winds are SSW. From the SW is plenty of turning. From the WSW as forecast offers robust turning with height. No strange 700 mb winds, smooth forecast hodographs. LLJ is still forecast S or SSW, correct for South severe, even on the squirrelly GFS runs. Winds of course strengthen with height too. 

Warm sector looks a little bigger than last week defined by northward extent of the synoptic warm front. That does not necessarily mean intense severe weather farther north. That'll depend on the outflow boundary situated south of the warm front, and influenced by midday rain. Undisturbed warm sector soundings should have a little EML and lots of instability.

Background pattern is there for a severe weather outbreak. Mesoscale details remain up in the air as usual Day 3, including the size of the region impacted.

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27 minutes ago, nrgjeff said:

Thursday will be a regional severe wx outbreak. @CheeselandSkies I think SPC was waiting for American guidance to come toward the ECMWF. It was stubbornly slow this time. Regardless of how we got here, we are at hatched ENH Day 3 now.

First thing I noticed with 12Z guidance is that 500/200 mb winds are forecast WSW. Some of the sloppy South days those winds are SSW. From the SW is plenty of turning. From the WSW as forecast offers robust turning with height. No strange 700 mb winds, smooth forecast hodographs. LLJ is still forecast S or SSW, correct for South severe, even on the squirrelly GFS runs. Winds of course strengthen with height too. 

Warm sector looks a little bigger than last week defined by northward extent of the synoptic warm front. That does not necessarily mean intense severe weather farther north. That'll depend on the outflow boundary situated south of the warm front, and influenced by midday rain. Undisturbed warm sector soundings should have a little EML and lots of instability.

Background pattern is there for a severe weather outbreak. Mesoscale details remain up in the air as usual Day 3, including the size of the region impacted.

Broad troughing like we're seeing with this event Thursday usually bodes poorly for the Deep South and TN Valley. The shear profiles are better, less VBV, storms move off the hodograph... etc. The deep, super amplified negative tilt trough is way overrated. 

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MRX seems to think the real question is surface instability that any discrete cells could take advantage of. Is that determined by just sunshine warming the boundary layer? They also mention a 850 warm layer limiting instability. So my take is that for Cumberland plateau and areas east a lot depends on how quickly the atmo recovers after the AM warm front? Clouds and rain = more stability and more likely that everything consolidates into a line before it gets here? 

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1 hour ago, Holston_River_Rambler said:

MRX seems to think the real question is surface instability that any discrete cells could take advantage of. Is that determined by just sunshine warming the boundary layer? They also mention a 850 warm layer limiting instability. So my take is that for Cumberland plateau and areas east a lot depends on how quickly the atmo recovers after the AM warm front? Clouds and rain = more stability and more likely that everything consolidates into a line before it gets here? 

See what the models show today.Euro6z went back to the west,probably just slower but the west shift shows a better LLJ

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10 minutes ago, jaxjagman said:

See what the models show today.Euro6z went back to the west,probably just slower but the west shift shows a better LLJ

ECMWF Deterministic FORECAST FOR: TYS    LAT=  35.82 LON=  -83.98 ELE=   981

                             06Z MAR24
PRESSURE LEVELS  SFC  1000   925   850   700   600   500   400   300   200      
WED 06Z 24-MAR 28001 17001 18010 19031 21022 21016 23031 23046 23069 24112      
WED 09Z 24-MAR 13004 13003 17013 20031 23022 22021 24026 24070 22064 24113      
WED 12Z 24-MAR 13004 12002 17016 20030 22026 23032 24023 24063 22072 24097      
WED 15Z 24-MAR 19002 18002 18014 21024 22034 24022 25023 25046 22074 24077      
WED 18Z 24-MAR 21009 21007 21015 21016 24032 24023 25026 26042 24072 24085      
WED 21Z 24-MAR 22008 22006 22014 21018 25031 25021 26030 26043 24061 24091      
THU 00Z 25-MAR 25004 23004 23017 20021 25020 24017 26028 26041 24065 25092      
THU 03Z 25-MAR 18004 18004 20018 21019 23015 21018 25022 25035 24068 25092      
THU 06Z 25-MAR 18004 16003 19017 20017 21018 21025 23023 24032 25075 26093      
THU 09Z 25-MAR 11004 10003 17014 19021 22026 21029 21030 25047 25091 25114      
THU 12Z 25-MAR 10005 09003 15015 19023 22032 22030 23033 25055 24074 24108      
THU 15Z 25-MAR 11006 11003 15019 19034 22043 23048 24054 24064 24069 24081      
THU 18Z 25-MAR 16003 16002 18018 20043 24048 24051 25060 25066 24062 24079      
THU 21Z 25-MAR 16008 16006 18025 21040 23037 23051 24062 24066 25077 25095      
FRI 00Z 26-MAR 15007 16004 17024 19054 23067 23072 24074 25069 24069 24073      
FRI 03Z 26-MAR 18012 19008 19032 20060 23069 24077 24082 24087 24084 23074      
FRI 06Z 26-MAR 21015 21010 22035 22057 24061 24074 24088 24098 24105 24087      
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After looking at several models it appears to me that both TN and AL are in the crosshairs for this system? My big question further north into TN based off the atmosphere being able to sufficiently recover. Those STP values are crazy high but does anyone else think they are a bit over done?

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1 hour ago, Chip said:

After looking at several models it appears to me that both TN and AL are in the crosshairs for this system? My big question further north into TN based off the atmosphere being able to sufficiently recover. Those STP values are crazy high but does anyone else think they are a bit over done?

Always that possibility

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A broad trough in the south-central US (not the Southeast) with vigorous wave ejecting our way is exactly the comparison case to high end events in the South. We have discussed such a weather pattern in other threads. Key is that 200/500 winds are WSW vs SSW which happens with sloppy teardrops. LLJ then responds to short-wave, maximizing both speed and directional wind shear.

18 hours ago, Bob's Burgers said:

Broad troughing like we're seeing with this event Thursday usually bodes poorly for the Deep South and TN Valley. The shear profiles are better, less VBV, storms move off the hodograph... etc. The deep, super amplified negative tilt trough is way overrated. 

Apparently the SPC has faith in the entire warm sector destabilizing between the outflow (MS/AL early) and warm front (Tenn maybe KY). Multiple boundaries are available to enhance low level shear. If morning rain abates somewhat by midday, maximum afternoon destabilization will be realized. In addition to the WF and OFB west-east boundaries, two north-south troughs are forecast. One comes out of Arkansas with the Pac front. Other develops in Mississippi and/or Alabama (maybe up into Tenn) which is common South Climo.

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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.

   ..Gleason.. 03/24/2021
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Here is the wording from the SPC... take I get is game on if we get sun in the morning tomorrow... if we get sun high risk could verify
9C767250-21EA-4C33-B414-09C55523142D.thumb.png.a1e020985d686f6b5570d9324755a2f4.png

We don’t really even need the sun to be honest. The biggest question is how much trash will be in the warm sector.


.
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6 minutes ago, VOLtage said:

I’m wondering, if the current trends hold, more of East Tennessee could end up in an enhanced risk.

Honestly I hope we wake up to lots of mess on the radar to hamper this event. This could be historic if this were to verify. Trust me you don’t want this to verify! 

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1 hour ago, VOLtage said:

I’m wondering, if the current trends hold, more of East Tennessee could end up in an enhanced risk.

Agreed, somewhat nervous here if a cell escapes its way into the valley like 12z HRRR insisted upon. Definitely a decent tornado risk with any supercell from 00-03z across most of the valley imo.

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From the outflow boundary south is pretty much high risk material. SPC rightly holds off because overnight and morning trends are required to discern location. Outflow will start Corinth, MS to say Cullman, AL. Approximate I just reference bigger towns. Outflow OFB should lift toward Tennessee, but might not quite get here. Instability will be greatest south of the OFB, with just right EML 800-700 mb temps, promoting discrete cells. Also more WSW wind at 500/200 creates additional turning with height.

Warm front is another question precluding high risk Day 2. It's forecast to lift through Tennessee, perhaps well into Kentucky. Zone south of the warm front WF and north of the OFB is the biggest question mark. Do we blob fest like last week? Blobs prevent intense severe weather. NAM has blobs. Do we destabilize as the crazy HRRR forecasts and go supercells? Perhaps something in between like the 12Z ARWs have. One of each ARW version is closer to the NAM and HRRR respectively. Said in between area has SW winds 500/200 mb which is robust, but not WSW nuts. I'm not feeling high risk Tenn. Moderate is enough though.

CAVEAT for West and Middle Tennessee. Reaching maximum destabilization (if realized) would tap into incredible surface and deep helicity. 

So we have questions on placement and size of any potential high risk area. Well I always have questions and like to fade things. Getting more difficult to do so Thursday.

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