Severe Weather Threat in Dixie Alley/OV Feb 27 - Mar 1

[thewxmann]

Wtf, the 0Z NCEP suite is even more strung out with the 500mb trough.

One thing to watch is the possibility of a weak shortwave swinging out from MX during the day tomorrow, in advance of the main system. The kink in the 700mb height fields indicates that this subtle wave will be located in northern AR around 0Z tomorrow. If the cap is weak enough then, it might be enough to initiate storms around the 0-3Z timeframe well east of the dry line, in the MS River Valley. And if the boundary layer remains well-mixed (and it will most likely be considering the strength of the low-level windfields), those storms may produce tornadoes. I feel like this is the show to watch, although I am known to be wrong.

On the other hand, it would seem that some of the NCEP guidance makes that piece of energy "take control" after 6Z, which shears out the main wave... this seems somewhat unrealistic given the strength of the main vort max. I feel like this is a very subtle case of convective feedback, although I would wait for tomorrow's 12Z Euro guidance tonight for confirmation. If today's 12Z Euro is more correct, a new line of storms should consolidate later in the night and possibly hold tornadic potential, again depending on the strength of the EML, mixing of the boundary layer, and storm mode. These storms will need to be watched as they head into the TN Valley Monday morning... that area is a tornado magnet in the late morning/early aftn hours (e.g. Gallatin, 4/11(?)/08, Murfreesboro).

*Edited for dates of 12Z Euro references

[CUmet]

The new NAM is pretty similar to the 12Z ECMWF through Monday morning except the EC surface low was about 4-5 mb deeper over SE MO. It looks pretty scary for the mid-South for late Sunday night into Monday morning.

The NAM and ECMWF have gradually come around to the GFS's idea of a strung-out low, however, although they both still have it stronger than the GFS. In my view, this lowers the tornado threat somewhat, as initiation will come relatively late, and the surface winds will veer as the low devolves into a surface trough. It seems like this is the most probable surface evolution, although I'm always leery of this being one of those events that reverts to a backed surface wind field at the last second.

Something that is being hinted at by the SPC WRF, NAM, and GFS precip fields is the idea of some storms developing in the warm sector within the LLJ across northern AR/southern MO into southern IL between 00-06z. These storms would follow along with the relatively-backed flow to the NE. Overall though, this looks like one of those cases that, although a large number of severe reports and quite a few tornadoes will likely result from this, if we had a slightly-different trough evolution and subsequent surface reflection...this would be a much, much bigger event especially in the tornadic sense.

[CUmet]

Wtf, the 0Z NCEP suite is even more strung out with the 500mb trough.

One thing to watch is the possibility of a weak shortwave swinging out from MX during the day tomorrow, in advance of the main system. The kink in the 700mb height fields indicates that this subtle wave will be located in northern AR around 0Z tomorrow. If the cap is weak enough then, it might be enough to initiate storms around the 0-3Z timeframe well east of the dry line, in the MS River Valley. And if the boundary layer remains well-mixed (and it will most likely be considering the strength of the low-level windfields), those storms may produce tornadoes. I feel like this is the show to watch, although I am known to be wrong.

On the other hand, it would seem that some of the NCEP guidance makes that piece of energy "take control" after 6Z, which shears out the main wave... this seems somewhat unrealistic given the strength of the main vort max. I feel like this is a very subtle case of convective feedback, although I would wait for the 12Z Euro guidance tonight for confirmation. If yesterday's 12Z Euro is more correct, a new line of storms should consolidate later in the night and possibly hold tornadic potential, again depending on the strength of the EML, mixing of the boundary layer, and storm mode. These storms will need to be watched as they head into the TN Valley Monday morning... that area is a tornado magnet in the late morning/early aftn hours (e.g. Gallatin, 4/11(?)/08, Murfreesboro).

Wow, we simultaneously had the same thought.

[thewxmann]

Wow, we simultaneously had the same thought.

Sweet, it's a good sign when I am agreeing with you, haha.

[tornadotony]

I may be in a minority on this one, but even with the more strung-out solution, I'm not terribly concerned about veering of the sfc winds substantially decreasing the tornado threat. Even with a 200 degree sfc wind, which is what the GFS (most strung-out) model is painting, the hodographs are still very large and looping, rather classic hodos for significant tornadoes.

[Stebo]

I may be in a minority on this one, but even with the more strung-out solution, I'm not terribly concerned about veering of the sfc winds substantially decreasing the tornado threat. Even with a 200 degree sfc wind, which is what the GFS (most strung-out) model is painting, the hodographs are still very large and looping, rather classic hodos for significant tornadoes.

Yeah I am with you on this one, there is a lot of speed shear in the lowest km coupled with modest to moderate directional shear. We don't always need things to be SE-S-SW to have a good tornado threat.

[Hoosier]

Yeah I am with you on this one, there is a lot of speed shear in the lowest km coupled with modest to moderate directional shear. We don't always need things to be SE-S-SW to have a good tornado threat.

Add me into your guys' camp. The low level speed shear is, in a word, tremendous. The 00z GFS has 0-1km SRH in excess of 500 at Evansville tomorrow night...this from the more veered model. As long as the low levels can stay mixed, I'm afraid there could be big problems.

[Stebo]

Add me into your guys' camp. The low level speed shear is, in a word, tremendous. The 00z GFS has 0-1km SRH in excess of 500 at Evansville tomorrow night...this from the more veered model. As long as the low levels can stay mixed, I'm afraid there could be big problems.

Yeah this like any other early season outbreak will have one limiting factor, and that is surface based instability. The key will be seeing how much the Models are under doing it as they always under do early season instability.

[Kevin Askew]

[Chicago Storm]

DAY 1 CONVECTIVE OUTLOOK

NWS STORM PREDICTION CENTER NORMAN OK

1141 PM CST SAT FEB 26 2011

VALID 271200Z - 281200Z

..THERE IS A MDT RISK OF SVR TSTMS OVER SRN IL

EXTREME SWRN

IND...FAR W KY...WRN TN...SERN MO...CNTRL/NRN AR AND EXTREME ECNTRL

OK...

...THERE IS A SLGT RISK OF SVR TSTMS FROM THE SRN PLAINS TO THE

LOWER OH VALLEY/MIDWEST...

..SYNOPSIS

POSITIVE-TILT MID-LEVEL TROUGH LOCATED OVER THE GREAT BASIN AND SWRN

STATES IS EXPECTED TO MIGRATE ENE TODAY AND TONIGHT AS UPSTREAM

IMPULSE DIGS OFF BRITISH COLUMBIA COAST. THE WEAKER NRN PORTION OF

THE TROUGH WILL EJECT FASTER...REACHING THE MID-MS VLY THIS EVENING

AND THE MIDWEST TONIGHT. SFC REFLECTION WILL DEVELOP ALONG A NWD

MOVING WARM FRONT FROM CNTRL MO TO NRN IND BY TONIGHT.

MEANWHILE...COLDER AND STRONGER SRN PARTS OF THE TROUGH... CURRENTLY

APPROACHING THE SRN CA COAST...WILL OPEN INTO A WAVE AND MOVE INTO

THE CNTRL/SRN PLAINS THIS EVENING/TONIGHT. WEAK CYCLOGENESIS WILL

OCCUR WITH THIS FEATURE OVER THE OZARKS AS A DRYLINE MIXES EWD INTO

ERN OK AND NE TX BY TONIGHT.

..SRN PLAINS TO THE MID-SOUTH/MIDWEST

BI-MODAL SVR EVENT IS LIKELY LATER THIS AFTERNOON THROUGH THE

OVERNIGHT WITH DAMAGING WINDS...TORNADOES AND LARGE HAIL LIKELY OVER

A LARGE REGION.

FIRST CONCERN WILL REVOLVE AROUND THE AFOREMENTIONED WEAK LEAD

IMPULSE THAT WILL MOVE INTO THE MIDWEST BY TONIGHT. VERY STRONG

SWLY FLOW OF 65+ KTS WILL TRANSPORT SFC DEW POINTS OF 58+F NWD TO

SRN EDGE OF THE SNOWPACK...ROUGHLY ECNTRL MO TO SRN IND BY

MID-EVENING. MODEST LARGE-SCALE HEIGHT FALLS WILL CONTRIBUTE TO

MOISTENING/ASCENT OVER A LARGE REGION THIS AFTERNOON AND EVENING

WITH MLCAPES 500-1000 J/KG FROM THE LOWER OH VLY AND MID-SOUTH.

PRIMARY QUESTION WILL BE IF THE WARM SECTOR CAP WILL WEAKEN ENOUGH

TO SUPPORT DEEP MOIST CONVECTION. THERE WILL CERTAINLY BE SUPPORT

FOR INCREASING BANDS OF ELEVATED TSTMS ALONG/N OF I-70...BUT CONCERN

IS FOR ANY DISCRETE AND/OR CLUSTERS OF MORE SFC-BASED STORMS IN THE

DEEPENING MOIST/MORE UNSTABLE AIR MASS ACROSS THE LWR OH

VLY/MID-SOUTH...PRIMARILY LATE AFTERNOON/EVENING. THESE STORMS WILL

BE EMBEDDED IN AN ENVIRONMENT CHARACTERIZED BY LARGE-CURVED

LOW-LEVEL HODOGRAPHS AND STRONG DEEP-LAYER SHEAR.

TORNADOES...DAMAGING WINDS WILL BE LIKELY WITH THESE STORMS

/PARTICULARLY IN NRN PORTIONS OF MDT RISK/...ALTHOUGH LARGE HAIL

WILL BE MORE LIKELY N OF THE WARM FRONT IN CNTRL PORTIONS OF IL/IND.

THIS ACTIVITY WILL DEVELOP/TRANSLATE ENE INTO THE MID-OH VALLEY/KY

COALFIELDS BY 12Z MONDAY WITH A LOWER SVR PROBABILITY WITH EWD

EXTENT.

MEANWHILE...A LATE NIGHT SURGE IN THE SEVERE THREAT WILL OCCUR

FARTHER SW AS SRN EXTENT OF THE EJECTING TROUGH ARRIVES LATE AT

NIGHT. EXPECT ANY REMAINING CAP TO WEAKEN WITH A GRADUAL INCREASE

IN STORMS ALONG/E OF THE DRYLINE/PRE-FRONTAL TROUGH FROM THE MO

OZARKS SWWD INTO ERN OK/NE TX AFTER 06Z. THESE STORMS WILL LIKELY

GROW QUICKLY UPSCALE INTO A QLCS BY 09Z WHILE ACCELERATING ENE

TOWARD THE LWR OH VLY/MID-SOUTH 09-12Z. GIVEN POSITIVE-TILT NATURE

TO THE TROUGH...DEEP-LAYER WIND FIELDS WILL BE COMPARATIVELY MORE

UNIDIRECTIONAL AND ORIENTED MORE PARALLEL TO THE INITIATING BOUNDARY

THAT THE ENVIRONMENT EARLIER IN THE NIGHT. AS SUCH...BOWING LINE

SEGMENTS WILL BE FAVORED WITH POSSIBLE SIGNIFICANT CORRIDORS OF WIND

EVOLVING. THERE IS ALSO A POSSIBILITY OF EMBEDDED

TORNADOES...ALTHOUGH FORECAST HODOGRAPHS SHOW A VEERING-BACKING

PATTERN WITH HEIGHT THAT MAY BE DETRIMENTAL IN STORM STRUCTURES

NORMALLY CONDUCIVE FOR SIGNIFICANT TORNADOES. NONETHELESS...GIVEN

NEARLY 100 KTS OF MID-LEVEL WINDS EJECTING NEWD AND FAVORABLE

BUOYANCY/ FORCING...STILL EXPECT A BUSY SEVERE WEATHER NIGHT AHEAD

ACROSS THE MID-SOUTH AND LWR OH VALLEY AND THE MDT RISK HAS BEEN

MAINTAINED.

[Chicago Storm]

[Kevin Askew]

.

[tornadotony]

If I were at SPC tonight, this is what my categorical map would look like.

Red = moderate risk, Green = slight risk

A couple things of note...

1) Slight risk area would reach pretty well into IL/IN for hail in the WAA regime. Nice MUCAPE across the area should support vigorous updrafts rooted above the sfc.

2) As CUmet and Jim noted, a subtle shortwave appears to be keyed upon in tonight's models runs ahead of the main system. If this feature exists and sparks convection in the warm sector tomorrow night, I would expect that convection to be trouble. Well into the warm sector, there will not be much capping in place. Subtle forcing should keep amount of convection under control, and a good combination of instability and shear would support discrete supercells and a dangerous tornado potential. If this shortwave is noted in obs tomorrow morning or afternoon (wind profilers, soundings, etc.) and the models continue to suggest convection in this region earlier in the night, I would even go so far as to suggest a high risk area be placed across nrn AR, SE MO, W KY, and S IL to account for this, as this thread would compound upon the threat posed by the main system later in the night.

3) Convection will not begin along the dryline prior to 00z. I do, however, think that ern OK and cntl/N AR are posed a very serious severe weather and tornado threat by storms off of the dryline after about 03z, when the main disturbance comes in to provide a sufficient lifting source for updrafts to break the cap, as well as cooler mid-level temps. And here's where things get interesting. With this storm being a tad weaker than initially forecast, some would think that reduces the tornado threat. I honestly think contrary. I think it increases it, at least for this mode of the outbreak. With a slightly weaker system, the forcing is still there to break the cap, but convection is not going to develop in overwhelming fashion and line out quickly. We will see quite a few hours, I believe, of discrete to semi-discrete convection across E OK into AR. Even with slightly veered sfc winds, hodos remain large and looping, and with significant instability in place, I think the distinct threat for long-tracked, deadly tornadoes will be present across that area.

Overall, a very tricky forecast. A moderate risk sounds good for this, at least right now. Still many question marks that need to be figured out, but I think the potential ceiling for damage and especially for loss of life, sadly, is pretty high.

[Hoosier]

Interesting that the tornado probs aren't hatched.

[Stebo]

Interesting that the tornado probs aren't hatched.

Yeah I am thinking they might wait til 12z runs are in before doing that. Although one could argue it being needed with this outlook.

[Thundersnow12]

Interesting that the tornado probs aren't hatched.

I would of though that was a given but thats just me.

was hoping for a hail threat up here like some runs a few days ago showed but looks like it will remain to the south.

[Hoosier]

Yeah I am thinking they might wait til 12z runs are in before doing that. Although one could argue it being needed with this outlook.

Well, I can understand keeping the overall tor probs a little tame at least for now but the sig tor parameters look very favorable.

[Chicago Storm]

was hoping for a hail threat up here like some runs a few days ago showed but looks like it will remain to the south.

You would like the 0z SPC WRF.

[Thundersnow12]

You would like the 0z SPC WRF.

Image me lol

[Hoosier]

I would of though that was a given but thats just me.

was hoping for a hail threat up here like some runs a few days ago showed but looks like it will remain to the south.

Although MUCAPE drops off that far north, I could still see at least a sub-severe hail threat there.

[Stebo]

I would of though that was a given but thats just me.

was hoping for a hail threat up here like some runs a few days ago showed but looks like it will remain to the south.

Yeah I am kind of surprised how subdued the risks are compared to their 4km wrf, would argue at least a marginal hail threat even up here.

[Chicago Storm]

Image me lol

[Thundersnow12]

not bad at all! going to be tough to get golf balls banging on my deck like last year but I will take any hail.

going to be fun to see the radar explode with convection tomorrow night. Hoping for the sake of people down there that the tornado threat doesn't end up as bad as it can be....but geez does it have potential.

[thewxmann]

Lol, the probs and the diction in the disco suggest that SPC didn't rly want to go MDT, but was obligated to out of continuity and the potential loss of life associated with the nocturnal threat.

Despite the SRH, I cannot help but believe that the lack of directional shear will temper the magnitude of the event. In such a high speed shear environment my experience has shown me that lack of backing makes for anemic supercells with struggling rotation. (Not to mention the hodograph kink in the low-mid levels implied in the discussion.) Especially when 500 flow is out of the WSW rather than more veered to the west.

[Stebo]

Lol, the probs and the diction in the disco suggest that SPC didn't rly want to go MDT, but was obligated to out of continuity and the potential loss of life associated with the nocturnal threat.

Despite the SRH, I cannot help but believe that the lack of directional shear will temper the magnitude of the event. In such a high speed shear environment my experience has shown me that lack of backing makes for anemic supercells with struggling rotation. (Not to mention the hodograph kink in the low-mid levels implied in the discussion.) Especially when 500 flow is out of the WSW rather than more veered to the west.

I am just not seeing how its that terrible, I mean we aren't talking 90 degrees of turning but there a good amount of directional shear coupled with very good speed shear, and the key of this is that most of the direction/speed shear is in the lowest km.

[Thundersnow12]

I am just not seeing how its that terrible, I mean we aren't talking 90 degrees of turning but there a good amount of directional shear coupled with very good speed shear, and the key of this is that most of the direction/speed shear is in the lowest km.

I would tend to agree with this but each event is different and will be sorted out in the mesocale details. With the speed shear through the roof it will be interesting to see how that plays out with the decent directional shear and if that plays a major role in getting strong low level rotation.

Will also be curious as to where the chasers head tomorrow evening as there are a few targets that are in play.

[Stebo]

I would tend to agree to agree with this but each event is different and will be sorted out in the mesocale details. With the speed shear through the roof it will be interesting to see how that plays out with the directional shear and if that plays a major role in getting strong low level rotation.

Will also be curious as to where the chasers head tomorrow evening as there are a few targets that are in play.

Yeah there are other mitigating factors in play here, to me shear isn't one of them.

[tornadotony]

I am just not seeing how its that terrible, I mean we aren't talking 90 degrees of turning but there a good amount of directional shear coupled with very good speed shear, and the key of this is that most of the direction/speed shear is in the lowest km.

The winds at the surface are, at most, 200 degrees (and that's being very generous on the GFS) (and on the most veered of the models). Directional shear is fine...this isn't a situation where the hodos are verging between looping and straight lines. These are healthy looping hodographs indicative of significant tornado potential.

[thewxmann]

I am just not seeing how its that terrible, I mean we aren't talking 90 degrees of turning but there a good amount of directional shear coupled with very good speed shear, and the key of this is that most of the direction/speed shear is in the lowest km.

The GFS is terrible... sfc flow is from the SW in many areas. The NAM is doable but not great. Both will not be as potent as a southerly surface wind. In both cases (moreso the GFS) there is a greater-than-not likelihood that significant tornadoes will not be the rule, despite the large helicity values. It's difficult to get potent, persistent supercell structures with a SW wind at the surface and a WSW wind at 500mb, we've seen this countless times in the past few years, even when sfc winds were from the SSW. In almost every high-end event winds back in the hours before tornadogenesis, even in those exceptions when the wind was from the WSW-SW at the surface. And in those exceptions the flow in the mid-upper levels was highly veered from the west, which will not be in the case here.

I'm not saying that a significant tornado isn't possible, but it isn't as likely with winds at the sfc from the SW or SSW. Regardless, it will make an impact, at the very least, on the potential ceiling of this event.

If you account for the QLCS-nature of the storms after 06Z, the likelihood of persistent supercell structures is even less.

I agree though that it's a tough forecast with lots of potential pitfalls.

[tornadotony]

The GFS is terrible... sfc flow is from the SW in many areas. The NAM is doable but not great. Both will not be as potent as a southerly surface wind. In both cases (moreso the GFS) there is a greater-than-not likelihood that significant tornadoes will not be the rule, despite the large helicity values. It's difficult to get potent, persistent supercell structures with a SW wind at the surface and a WSW wind at 500mb, we've seen this countless times in the past few years, even when sfc winds were from the SSW. In almost every high-end event winds back in the hours before tornadogenesis, even in those exceptions when the wind was from the WSW-SW at the surface. And in those exceptions the flow in the mid-upper levels was highly veered from the west, which will not be in the case here.

I'm not saying that a significant tornado isn't possible, but it isn't as likely with winds at the sfc from the SW or SSW. Regardless, it will make an impact, at the very least, on the potential ceiling of this event.

If you account for the QLCS-nature of the storms after 06Z, the likelihood of persistent supercell structures is even less.

I agree though that it's a tough forecast with lots of potential pitfalls.

The sfc flow on the GFS is SW in many areas...behind the convection. Let's try looking just ahead of it...

Large, looping hodo in SSW sfc flow...this is NE AR at 06z...