April 13 Severe Threat - TX, OK, LA, ARK, KS, MO, IA

[OUGrad05]

I don't yet see a thread for this event and it straddles the lines between several states, I figured I would go ahead and kick things off.  The severe threat for April 13th appears to be increasing for the eastern half of Oklahoma, Northeast Texas, Northwest Louisiana, much of Arkansas, Missouri and Iowa, possibly Kansas as well.  

A trough is expected to eject into the plains Friday morning and move across N KS and S NE through the day.  A reasonably sharp dryline will run south through the central and southern plains.  Exact timing and location of these features is still very much TBD with some significant differences between the NAM, GFS and Euro.  It does appear increasingly likely that all modes of severe weather will be possible, at least initially.  Speed shear and moderate instability is more than adequate to support supercell structures and strong updrafts.  There are many details for this system yet to be determined and the spread in model solutions is significant.  

A few things that could be problematic for sustained supercells is the lack of directional shear, a bit of veer-back showing in the profiles and what may turnout to be a relatively uncapped warm sector over much of the area.  It's possible that we have a convective mess on our hands and then perhaps linear storm modes as the day progresses.  

There have been persistent but weak signals that this system would drift back west with time, the NAM is significantly further west than the other models, it's hard for me to jump on board with NAM solutions after getting burned more than once, but if convergence occurs it could put a lot of Eastern OK in play...not particularly great chase country.  

[jojo762]

12Z NAM NEST shows a rather volatile environment by as early as 19-20Z along most if not all of the dryline from east-central KS down into southeastern OK... appears to have an oddly tough time initiating sustained convection, I'm not too sure what to make of that given that intense sustained convection is almost a given with 1500-3000J/KG CAPE and bulk shear of 60-80kts.

[OUGrad05]

7 minutes ago, jojo762 said:

12Z NAM NEST shows a rather volatile environment by as early as 19-20Z along most if not all of the dryline from east-central KS down into southeastern OK... appears to have an oddly tough time initiating sustained convection, I'm not too sure what to make of that given that intense sustained convection is almost a given with 1500-3000J/KG CAPE and bulk shear of 60-80kts.

Agreed, but NEST is sometimes odd with convection...still 48hrs to go.

[andyhb]

The NAM 3 km seems to always have problems generating organized convection in environments that both support initiation and subsequent strong convection. I wouldn't worry too much about it.

If anything, I'd be paying attention to the fact that it isn't showing a rapid evolution to a squall line.

[hlcater]

Oddly enough, the lack of initiation is interesting because, at least in my experience, the 3km NAM seems to usually initiate too many storms and then go linear. We've seen this happen even during events that clearly favor supercell structures. The 3km NAM doesn't seem the best at handling initiation and really convective evolution at all. With that out of the way, I see storms initiating discrete/semi discrete before wind vectors nearly parallel(especially up north) to the boundary and the VBV in spots promotes linear development by the evening. If the winds aloft were more zonal, not only would that make the shear more perpendicular, it would also increase directional shear—but I doubt that happens.

[Quincy]

The 12z Euro shows a similar scenario to the NAM, based on limited precipitation fields, suggesting that there may very well be a number of discrete/semi-discrete cells in eastern Kansas and eastern Oklahoma, immediately ahead of the dryline. I wonder if the veer-back-veer signature in the wind fields may be a factor in limiting updraft strength/longevity.

Either way, it does seem like the squall line scenario is increasingly unlikely, but it should be noted that the NAM still erodes most of any lingering capping by early afternoon along the dryline, so there's no reason to believe there wouldn't be convection.

[Drz1111]

Man, if anything the divergence b/w the GFS and the rest increased with the 18Z run.

[bjc0303]

38 minutes ago, Drz1111 said:

Man, if anything the divergence b/w the GFS and the rest increased with the 18Z run.

GFS is very often too progressive with feature placement. 

[jojo762]

Given what the Euro and NAM are showing for precip fields - I.e., discrete supercells for several hours after CI - in addition to the progged parameters in place, Friday could be a fairly significant event over quite a large area. Long-tracking tornadoes seem to be a distinct possibility to me with storm motions likely between 50-60mph. I get that there is still some details to iron out, such as potential warm sector morning junk convection that could really impact instability, but I wouldn't be surprised to see the SPC go with a day 2 MDT (for ANY  or all of the three hazards) in either outlook tomorrow.

[Drz1111]

1 hour ago, bjc0303 said:

GFS is very often too progressive with feature placement. 

Oh sure, and if i didn’t have to be in court on Friday I’d be chasing my lawyer ass all over the plains.  But it’s just striking how big the difference is.

[Quincy]

I'm not really sold on the long-track tornado potential. The low level shear is decent and while low-end for long-track tornadoes, could be supportive, if there weren't other red flags to be concerned about.

Yes, storm motions should be on the speedy side, on the order of 50 knots or faster and most of the SRH will be confined to the lowest kilometer, but the latter is overshadowed by unidirectional winds in the 2-6km layer, not to mention some backing in the upper levels and some forecast soundings showing kinky (subtle VBV) signatures in the mid-levels. The deep layer shear vector is out of the SW and the dryline orientation should largely be north-south, so there is some crossover, but it's not as perpendicular as you would like to see for a long-track tornado event. The wind profile seems to suggest that any discrete cells may have trouble maintaining organization for extended periods of time.

Also looking at analogs, it's actually a bit surprising how low the tornado probabilities are. Using a NAM-based analog, only about 20-30% of the top 15 analogs had at least one tornado report within a 110km of a gridpoint via CIPS. Furthermore, less than 10% of the top analogs had at least one long-track tornado event.

There's little doubt that there is some "enhanced" tornado potential given the degree of low-level shear, but the setup doesn't look ideal for long-lived, tornadic supercells. Farther south across the threat zone, into eastern Oklahoma and eastern North Texas, the mid and upper level wind profiles are somewhat better and there will be more instability there, but capping and weaker forcing may work to offset that.

[jojo762]

0-1km SRH of 250-350m2/s2 is only decent and "low-end" for tornadoes? No real definitive science behind the idea that VBV legitimately limits tornado potential, just a multitude of flawed (largely limited by LLVL shear/moisture, and storm interactions or MCSs) setups that failed to live up to hyped-up expectations, mostly resulting in it all getting subjectively blamed on a concept that isn't exactly well understood attm. Computer simulations have largely rejected the idea that VBV tempers mesocyclones or tornadogenesis. But guess we will put that to the test here with a setup that appears to have ample low-level shear (50kt 850 flow and modestly backed surface winds) and moisture and *seems* to be aiming toward a mainly discrete/semi-discrete mode.  

[Quincy]

27 minutes ago, jojo762 said:

0-1km SRH of 250-350m2/s2 is only decent and "low-end" for tornadoes? No real definitive science behind the idea that VBV legitimately limits tornado potential, just a multitude of flawed (largely limited by LLVL shear/moisture, and storm interactions or MCSs) setups that failed to live up to hyped-up expectations, mostly resulting in it all getting subjectively blamed on a concept that isn't exactly well understood attm. Computer simulations have largely rejected the idea that VBV tempers mesocyclones or tornadogenesis. But guess we will put that to the test here with a setup that appears to have ample low-level shear (50kt 850 flow and modestly backed surface winds) and moisture and *seems* to be aiming toward a mainly discrete/semi-discrete mode.  

Which model has that much 0-1km SRH? The 3km NAM has 100-200 m2/s2 immediately ahead of the dryline (storm-scale environments lead to locally, slightly higher values) and I said that was lower-end for long-track tornadoes. VBV can limit updraft organization, as updrafts get sheared and have shorter residence times than you'd see with a larger hodograph. Mean wind fields that are unidirectional or sloppy (VBV) in the 2-6km layer are not often associated with regional tornado outbreaks.

Tornadoes? Yes, there will probably be multiple, but there is not much data to suggest there will be numerous long-track tornadoes, just as the analog data alone suggests a questionable threat of a significant or widespread tornado event. There is a reason for that. We've seen better wind profiles struggle to produce any tornadoes, so we'll see.

Tornadogenesis is most concerned with low-level shear, but in order for long-lived supercells, the mid and upper level support would need to be improved.

Back to analogs again as well, it's not just the CIPS analogs. Look at forecast soundings up and down the dryline. There are very few matching analogs that were associated with significant tornado events/outbreaks.

[Stebo]

9 minutes ago, Quincy said:

Which model has that much 0-1km SRH? The 3km NAM has 100-200 m2/s2 immediately ahead of the dryline (storm-scale environments lead to locally, slightly higher values) and I said that was lower-end for long-track tornadoes. VBV can limit updraft organization, as updrafts get sheared and have shorter residence times than you'd see with a larger hodograph. Mean wind fields that are unidirectional or sloppy (VBV) in the 2-6km layer are not often associated with regional tornado outbreaks.

Tornadoes? Yes, there will probably be multiple, but there is not much data to suggest there will be numerous long-track tornadoes, just as the analog data alone suggests a questionable threat of a significant or widespread tornado event. There is a reason for that. We've seen better wind profiles struggle to produce any tornadoes, so we'll see.

Tornadogenesis is most concerned with low-level shear, but in order for long-lived supercells, the mid and upper level support would need to be improved.

Back to analogs again as well, it's not just the CIPS analogs. Look at forecast soundings up and down the dryline. There are very few matching analogs that were associated with significant tornado events/outbreaks.

If you look ahead of that there is a convergent band about 50 miles ahead of the dryline and ahead of that the 0-1km shear is much higher, both on 3km and 12km NAM... Also much higher than 100-200 in AR/OK/TX.

As for the 2-6km winds if they are unidirectional that is fine if the speed is increasing which it is. There are a few pockets of VBV but it is no where near as bad as previous model runs from yesterday for example.

[jojo762]

10 minutes ago, Quincy said:

Which model has that much 0-1km SRH? The 3km NAM has 100-200 m2/s2 immediately ahead of the dryline (storm-scale environments lead to locally, slightly higher values) and I said that was lower-end for long-track tornadoes. VBV can limit updraft organization, as updrafts get sheared and have shorter residence times than you'd see with a larger hodograph. Mean wind fields that are unidirectional or sloppy (VBV) in the 2-6km layer are not often associated with regional tornado outbreaks.

Tornadoes? Yes, there will probably be multiple, but there is not much data to suggest there will be numerous long-track tornadoes, just as the analog data alone suggests a questionable threat of a significant or widespread tornado event. There is a reason for that. We've seen better wind profiles struggle to produce any tornadoes, so we'll see.

Tornadogenesis is most concerned with low-level shear, but in order for long-lived supercells, the mid and upper level support would need to be improved.

Back to analogs again as well, it's not just the CIPS analogs. Look at forecast soundings up and down the dryline. There are very few matching analogs that were associated with significant tornado events/outbreaks.

I never said "numerous long-track tornadoes," I simply stated that having long-track tornadoes seemed like a distinct possibility. 

But what are you talking about exactly?The NAM NEST has numerous point forecast soundings along much of the dryline with 0-1KM SRH of 200-300m2/s2 by 21Z, and the 12NAM is even better (reference the 00Z sounding for KC in the other thread). 

Im not anticipating a "major tornado outbreak." I am just significantly more optimistic than I would be more with such a SSW mid/upr flow system because of model precip fields implying discrete supercells, in addition to low-level winds being VERY impressive (50kts by 21Z is very impressive for the plains). Analogs aren't exactly the end-all of severe weather forecasting.

Predictable pessimism lol.

[Radtechwxman]

I do agree with points of both Quincy and jojo. Yes I do love that models are showing a broken storm mode instead of a solid squall. That is def much better for tornadogenesis. However I know there is no concrete studies of vbv profiles on tornadoes but with vbv in past setups I have definitely seen it impact longevity of storm organizaton with storms getting outflow dominant due to lack of better venting aloft. But each setup is unique. We do have great wind shear, a deepening system for a change, and ample moisture and cape. The orientation this trough comes in at will be critical to how this event pans out. The trends have improved with wind profiles and I am cautiously optimistic on more tornado potential. As tempting as the southern play is, I'm more keen to play closer to the low in this type of setup

[bjc0303]

Seeing a bimodal threat on Friday. Confluence axis sets up east of the dry line, probably in association with strong low-level jet axis. Convection initiating here would be capable of all hazards, though buoyancy could be in question, specifically buoyancy axis/width.

 

Other is the dry line and dry line/cold front intersection and farther north. All hazards possible here too, only upscale growth more likely. In terms of tornado potential, I like E KS/W/NW MO most of all where favorable parameters overlap with potential to remain discrete longer than Iowa and adjacent regions. Farther south into OK storm coverage less certain, and could be slightly less favorable wind profiles for tornadoes.

[jojo762]

00Z NAM NEST fires off storms by 18-19z along the I35 corridor (Believe BJC0303 mentioned that this would probably happen), with the atmosphere already primed for severe storms. Regardless of how "significant" the tornado threat ends up being (the latest run pops some very impressive parameters across SE KS/E OK/W AR), it is going to be a very long and very active day for severe storms.

One thing, in addition to other potential "limiting tornado threat factors" that have been mentioned, is that the critical angles are very unremarkable for friday (mostly between 45 and 60 degrees), meaning that storms could have a bit of a hard time ingesting purely stream-wise vorticity. Likewise, intense supercells have a knack for being able to augment their surrounding environment... So if we do indeed get numerous scattered mature supercells on friday, then all bets are off. 

[Radtechwxman]

12 minutes ago, bjc0303 said:

Seeing a bimodal threat on Friday. Confluence axis sets up east of the dry line, probably in association with strong low-level jet axis. Convection initiating here would be capable of all hazards, though buoyancy could be in question, specifically buoyancy axis/width.

 

Other is the dry line and dry line/cold front intersection and farther north. All hazards possible here too, only upscale growth more likely. In terms of tornado potential, I like E KS/W/NW MO most of all where favorable parameters overlap with potential to remain discrete longer than Iowa and adjacent regions. Farther south into OK storm coverage less certain, and could be slightly less favorable wind profiles for tornadoes.

Couldn't agree more man. Great write up. Our system seems to be just off shore. Should get good sampling soon and hopefully really help to hone in on target area and storm modes

[bjc0303]

2 minutes ago, jojo762 said:

00Z NAM NEST fires off storms by 18-19z along the I35 corridor (Believe BJC0303 mentioned that this would probably happen), with the atmosphere already primed for severe storms. Regardless of how "significant" the tornado threat ends up being (the latest run pops some very impressive parameters across SE KS/E OK/W AR), it is going to be a very long and very active day for severe storms. One thing in addition to other potential "limiting tornado threat factors" that have been mentioned is that the critical angles are very unremarkable for tomorrow (mostly between 45 and 60 degrees), meaning that storms could have a bit of a hard time ingesting purely stream-wise vorticity. Likewise, intense supercells have a knack for being able to augment their surrounding environment... So if we do indeed get numerous scattered mature supercells on friday, then all bets are off. 

Well, streamwise vorticity, even in the lowest km of the atmosphere, simply goes into the updraft to feed/maintain the midlevel mesocyclone. Your greatest contribution to the low-level mesocyclone comes from storm-scale processes, specifically the streamwise vorticity generated along the FFD of the storm.

Recent literature suggests environments of primarily crosswise vorticity can tend to disrupt and disorganize the low-level mesocyclone. Other research suggests that this isn't as important as previously thought. 

[jojo762]

The nearly 150 mile difference in dryline placement between the NAM and GFS in KS/MO at HOUR 48 is truly mind boggling. 

[andyhb]

27 minutes ago, jojo762 said:

The nearly 150 mile difference in dryline placement between the NAM and GFS in KS/MO at HOUR 48 is truly mind boggling. 

And the NAM will win this battle 100 times out of 100 because of the GFS' bias.

[bjc0303]

Yep. Nothing terribly shocking here. Just how the GFS operates. 

[jojo762]

 Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1259 AM CDT Thu Apr 12 2018

   Valid 131200Z - 141200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM PARTS OF
   WESTERN MISSOURI SOUTHWARD INTO NORTHEAST TEXAS/NORTHERN
   LOUISIANA...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS EXTENDING FROM
   SOUTHWEST IOWA SOUTHWARD ACROSS EAST TEXAS/NORTHERN AND CENTRAL
   LOUISIANA...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   ENHANCED AND SLIGHT RISK AREAS...AND EXTENDING SOUTH ACROSS THE
   WESTERN GULF COASTAL REGION...

   ...SUMMARY...
   Strong/severe storms are expected to develop Friday afternoon and
   continue into the overnight hours -- centered over a zone extending
   from Missouri southward across the Arklatex region.

   ...Synopsis...
   A strong upper trough initially over the Rockies will shift
   gradually eastward into the central U.S., surrounding a deepening
   closed low moving across Kansas/Nebraska through the second half of
   the period.

   At the surface, a low progged to reside in the vicinity of southeast
   Nebraska at the start of the period will gradually approach/reach
   northwest Missouri, while a strong/trailing cold front sweeps
   quickly eastward/southeastward across the central and southern
   Plains.  By the end of the period, a warm front should extend
   eastward from the low across the Midwest, while the cold front is
   progged to reside in the vicinity of the mid and lower Mississippi
   Valley.  The advancing front will contribute to development of
   strong/severe storms Friday afternoon and evening.

   ...Mid and lower Missouri Valley southward across the Arklatex...
   An increasingly favorable environment for severe storms is forecast
   to evolve during the day Friday, ahead of the advancing cold front. 
   As the deepening upper system shifts slowly eastward, a very strong
   deep-layer wind field will overspread the evolving warm sector. 
   Diurnal heating combined with low-level moistening beneath cooling
   mid-level temperatures will result in moderate destabilization
   during the afternoon, with up to 1500 J/kg mixed-layer CAPE over
   northern portions of the risk area ahead of the front, and 2000 to
   2500 J/kg progged to evolve from the Arklatex region southward.

   Operational as well as CAM output suggest isolated storm development
   will occur by late afternoon over the eastern Kansas/western
   Missouri/southwest Iowa area, with more widespread convection
   developing across Arkansas/east Texas/northern Louisiana.  Storm
   mode farther north appears likely to remain cellular -- with the
   environment supportive of strong/rotating updrafts.  As such, large
   hail is expected, along with a few damaging gusts.  In addition, the
   risk for a couple of tornadoes is also evident, with overall severe
   risk only limited by what appears likely to remain sparse storm
   coverage.

   Farther south, storm mode remains more questionable/less certain --
   especially as storms become more widespread.  Guidance agrees that
   an eventual line or band of storms -- though with attendant severe
   risk -- will eventually evolve as the front advances across the
   Arklatex through the evening.  Initially however, more isolated
   storm mode may be maintained during the afternoon.  Given 
   veering/increasing flow with height, and greater CAPE expected than
   areas farther north, very large hail, damaging winds, and potential
   for a few tornadoes exists -- one or two of which may be strong. 
   Uncertainty with respect to evolution of storm mode, and where the
   greatest risk may ultimately evolve, precludes an upgrade to
   moderate risk at this time.  However, given the anticipated
   background thermodynamic and kinematic environment, higher-end
   potential evident across the Arkansas and Arklatex vicinity suggests
   potential for a later outlook upgrade as details become more clear.

   Overnight, storms across Missouri should diminish in
   coverage/intensity given a narrow warm/moist sector expected ahead
   of the front.  However, the aforementioned upscale growth into a
   northeast-to-southwest band of storms from the Mississippi Delta
   region southwest into east Texas will likely be accompanied by
   severe risk well into the overnight hours.

   ..Goss.. 04/12/2018

[jojo762]

Fairly good discussion from SPC. Not exactly following what Goss is saying in regard to storm coverage further north being a problem vs further south... Also definitely surprised that the slight risk and enhanced risk were not pushed a bit further west given what we have been seeing from the NAM and Euro (i.e., the dryline still being within 25 miles of the I35 corridor at 21Z, and just on the westernmost portion of the SLGT risk by 00Z), believe we'll probably see those changes in the 1730Z update, along with an upgrade to MDT for any of the potential hazards. Night folks.

[bjc0303]

moderate seems aggressive as hell, but to each their own. 

[OUGrad05]

4 hours ago, jojo762 said:

Fairly good discussion from SPC. Not exactly following what Goss is saying in regard to storm coverage further north being a problem vs further south... Also definitely surprised that the slight risk and enhanced risk were not pushed a bit further west given what we have been seeing from the NAM and Euro (i.e., the dryline still being within 25 miles of the I35 corridor at 21Z, and just on the westernmost portion of the SLGT risk by 00Z), believe we'll probably see those changes in the 1730Z update, along with an upgrade to MDT for any of the potential hazards. Night folks.

If you look at the overall trends on the NAM you'll see that it's been trending east over the last 4 or 5 model runs, it's especially evident at 00Z Saturday.  If storms go off between 1 and 4pm then that threat area will be back to the West a good amount, if it's a 5 to 6pm initiation time frame storms will take a little time to mature and be moving toward or already be in the enhanced risk.  I guess my point is SPC is hedging a bit based on current model guidance and timing of the system.  We could see things move back to the west some with the midday update.

[MattPetrulli]

LZK doesn't even mention the word tornado in their afd

 Factors will be
coming together, strong lift, instability and surging moisture to
bring the potential for severe storms to the area. Large hail and
damaging winds will be the main threats, while some shear will be
present. SPC does have much of AR in an Enhanced or Slight risk of
severe storms. Model CAPE values do reach over 1000, LI values
reach -3 to -5, and precip water values reach 1.5 to near 2
inches. Jet dynamics will be strong with a low level jet around 50
knots. The main surge of convection appears in the west in the
early evening, then moving through AR through midnight in Friday
morning. Also, some heavy rain will be possible, with amounts of 3
to 5 inches in the southeast the highest, while 1 to 3 over most
of central AR. Localized flooding will be possible.

[Radtechwxman]

Yea I was in total disbelief of that. Wichita didn't really either and their far eastern forecast area is def at risk

[jojo762]

EAX had a on-point and more in-depth discussion on friday than almost every WFO.

Quote

Surface based convective initiation location and timing will be
dependent on where the low level moisture boundary is located
Friday afternoon. Most models agree that the warm front will be
north of the forecast area, in Iowa, but there is still some
differences in the location of the dryline/cold front, which will
be located somewhere likely in eastern Kansas Friday afternoon.
Wherever that north/south oriented moisture boundary is located
will likely be the starting point for afternoon/evening surface
based convection. The general thought with this event is that the
north/south oriented moisture boundary will be located in the
vicinity of TOP/EMP/LWC. Would expect convective initiation on
that boundary to take place around 21z as the warm capping
inversion erodes. Given the longitudinal proximity to the KC
Metro, as well as storm motions to the north/northeast around 30
to 40 knots, it would not take long before these storms reached
the urban corridor. Hi-resolution models have picked up on the
time frame for likely strong/severe storms, and have indicated
that storm mode might be more cellular, as opposed to widespread
linear nature, at least at first. This would lend to more of a
discreet rotating updraft scenario as opposed to a linear
scenario. Given potential ML CAPE pushing 1500 J/KG along with 50
to 60 kts of deep layer shear, the ingredients will be present for
some supercellular type structures. Low level shear will also be
ample, given south/southeast surface winds and gradual veering
with height. NAM soundings (06z-180402) indicate 0-1 SRH pushing
into the 200+ m2/s2 range, which would be plenty to warrant
concern for one or more of these rotating updrafts to produce a
tornado. While predicting widespread severe weather with tornadoes
is a bit hasty at this juncture, with plenty of meantime for
things to change it is probably important for communities to
start preparing for the first severe weather event of the season.
Storms will generally move from southwest to northeast through the
evening and into the overnight hours, gradually weakening after
sunset due to boundary layer decoupling. While boundary layer
decoupling is expected by Friday night, it may take a while to
fully stabilize the low levels, due to surface winds sustaining
20-25 mph, with a strong low level jet persisting well into the
night. By late Friday night storms should move completely out of
the area.