Feb. 28 Severe Wx Event

[bjc0303]

This might actually be a pretty volatile day. Upper level troughing slowly makes its way east with strengthening wind profiles aloft yielding deep layer shear well above 50 knots. 

Large, rather unfocused pressure falls early on lead to broad warm air advection, giving way to a very broad warm sector spanning several regions. As the day progresses an elongated low pressure develops, backing flow somewhat to S/SW to even southerly as the eve approaches. With middle 60s dewpoints beneath lapse rates ranging from 7-8 K/km, strong deep layer shear, and increasing low level flow/shear, a pretty volatile environment may form especially over most of Arkansas. initiation would likely be driven by mesoscale boundaries such as confluence zones or enhanced areas of warm air advection. 

 

Soundings like the following can be found throughout most of extreme SE OK and AR: 

While 68 dewpoints probably won't materialize, 63-65 may very well verify which does not hamper the thermodynamics much.

[Jim Martin]

Here is the sounding near Shreveport, Louisiana Tuesday Evening.

[MattPetrulli]

Eerie to note one of the analogs is March 1, 1997. 16 strong tornadoes with 3 F4s. A lot of long trackers.

[Jim Martin]

The Little Rock, Arkansas 4k NAM sounding for Tuesday Evening. If this is realized, watch out.

[Quincy]

While the instability/shear overlay is fairly impressive for late February over the Arkansas vicinity, no focused area of forcing seems apparent until well after dark. Confidence is not high in 22-01z initiation over AR, but if such a scenario was realized, there could be a significant supercell or two.

The 4km NAM does show a bit of a VBV signature in the 1-3km AGL layer around AR, while a stout cap makes initiation over MO/IL even more questionable. 

Given the parameter space, it may be wise to wait a few more model runs to get into specifics, as it could be another big or bust type setup. 

I could see a situation where robust storms develop overnight or early Wednesday, as sizable CAPE advects northeast and the LLJ really ramps up. 

[bjc0303]

25 minutes ago, Quincy said:

While the instability/shear overlay is fairly impressive for late February over the Arkansas vicinity, no focused area of forcing seems apparent until well after dark. Confidence is not high in 22-01z initiation over AR, but if such a scenario was realized, there could be a significant supercell or two.

The 4km NAM does show a bit of a VBV signature in the 1-3km AGL layer around AR, while a stout cap makes initiation over MO/IL even more questionable. 

Given the parameter space, it may be wise to wait a few more model runs to get into specifics, as it could be another big or bust type setup. 

I could see a situation where robust storms develop overnight or early Wednesday, as sizable CAPE advects northeast and the LLJ really ramps up. 

Regarding veer-back... there have been some fairly significant tornado episodes featuring backing aloft. As posted in another thread, the effects of backing aloft are not well understood, and simulations have had mixed results at best.

 

I think focus for ascent would have to come from broad WAA and any confluence zones that arise. perhaps further west into OK there may be a window for initiation prior to arrival of the cold front.. but given the parameter space involved any supercells would be pretty significant  

[Jim Martin]

This afternoon's run of the SREF with the Significant Tornado Ingredients...

[andyhb]

There's a pretty strong consensus amongst the global models right now to fire off discrete convection by 21z in W AR/SW MO and move it eastward towards the Mid South by 00z. Given the parameter space in place there with plenty of instability, impressive deep layer shear and degree of hodograph length/curvature in the low levels, those would almost certainly mature into supercells with a rather substantial tornado threat and perhaps last into the later evening as the LLJ intensifies even more. That would be problematic to say the least.

[MattPetrulli]

The NAM's parameters 

[Jim Martin]

To what Matt was referring to. West of Little Rock Tuesday Night off the 0z NAM.

[MattPetrulli]

2 hours ago, Jim Martin said:

To what Matt was referring to. West of Little Rock Tuesday Night off the 0z NAM.

Again, thank you 

[jojo762]

Beginning to becoming very concerned for Eastern Oklahoma and Western Arkansas late Tuesday afternoon into Tuesday night. Model consensus is for parameters to be pretty juiced up. Believe it was mentioned earlier, but the nighttime tornado threat could be relatively enhanced as even more moist and unstable air advects into the region during the night time hours. 

Strong low-level shear (200-300m2/s2 0-1KM SRH, 50+ kt LLJ), effective bulk shear of 70-90kts, moderate-strong low-level cape and moderate MLCAPE... A good recipe for tornadoes.

The VBV showing up in some soundings it a bit of a buzz kill, but the potential impacts of that will only be known on the day of, some soundings also show varying degrees of a low-level inversion which will be something to pay attention to too... In addition, also concerned by the lack of any notable convergence or focused forcing mechanism until well after dark (GFS does not seem to care about this and produces numerous storms)... As Quincy mentioned, it could be one of those situations where we get just one or two supercells, but they would likely be monsters, but again that is dependent on any potential structural impacts from VBV.

[bjc0303]

For people continually pointing to veer back in soundings: https://ams.confex.com/ams/28SLS/webprogram/Paper300986.html

 

Highly suggest checking this presentation out from SLS 

[nwburbschaser]

Out of all my concerns with this set up, VBV is at the bottom of the list. I've heard this several times now and I get this feeling that people are overly obsessed with it after 4/26 last year.

[bjc0303]

2 minutes ago, nwburbschaser said:

Out of all my concerns with this set up, VBV is at the bottom of the list. I've heard this several times now and I get this feeling that people are overly obsessed with it after 4/26 last year.

I was one until researching it. 

 

To sum it up: exact effects of veer-back profiles are kind of unknown still with simulations providing mixed results. the linked presentation actually suggests stronger updrafts for the backed profiles vs the 'normal' one, and storm mode was not really impacted. 

 

If anything certain configurations may impact storm mode, but in my opinion veer back profiles tend to be observed with highly amplified systems like we saw on 4/26 which tend to produce strong large scale ascent promoting rapid storm development and upscale growth. additionally low level shear on 4/26 was very, and I repeat, VERY marginal. 

[Hoosier]

13 minutes ago, bjc0303 said:

For people continually pointing to veer back in soundings: https://ams.confex.com/ams/28SLS/webprogram/Paper300986.html

 

Highly suggest checking this presentation out from SLS 

Thanks for posting.  I may be wrong but I swear I remember SPC even mentioning VBV in some forecast outlooks in the past. Seems like it's not so automatic as far as hurting tornado chances.

[nwburbschaser]

1 minute ago, bjc0303 said:

I was one until researching it. 

 

To sum it up: exact effects of veer-back profiles are kind of unknown still with simulations providing mixed results. the linked presentation actually suggests stronger updrafts for the backed profiles vs the 'normal' one, and storm mode was not really impacted. 

 

If anything certain configurations may impact storm mode, but in my opinion veer back profiles tend to be observed with highly amplified systems like we saw on 4/26 which tend to produce strong large scale ascent promoting rapid storm development and upscale growth. additionally low level shear on 4/26 was very, and I repeat, VERY marginal. 

Yes, that was an issue last year on a few occasions, and VBV was more of a symptom of a bigger problem. A system like the 4/26 one was doomed from the start, but VBV took the blame. 

This isn't a situation like that at all. 

[bjc0303]

4 minutes ago, Hoosier said:

Thanks for posting.  I may be wrong but I swear I remember SPC even mentioning VBV in some forecast outlooks in the past. Seems like it's not so automatic as far as hurting tornado chances.

Jared Guyer of SPC was there as well. He made a remark that, anecdotally speaking, it seemed that low level backing (above the surface/below 500mb) seemed to impact tornado probabilities. It isn't clear however. I can't remember if Parker's simulation altered backing at different levels or not. 

 

If anything forecasters currently consider backing aloft as it relates to storm mode. That is well understood. whether backing aloft impacts structure or storm dynamics/supercell pressure perturbations...we don't know but I am doing a study on it for capstone! 

[nwburbschaser]

3 minutes ago, Hoosier said:

Thanks for posting.  I may be wrong but I swear I remember SPC even mentioning VBV in some forecast outlooks in the past. Seems like it's not so automatic as far as hurting tornado chances.

I think 5/24/11 had some VBV going on and it was still able to produce several sig tors. I know that day was somewhat messy which may be VBV related, but it still produced. 

[bjc0303]

4 minutes ago, nwburbschaser said:

I think 5/24/11 had some VBV going on and it was still able to produce several sig tors. I know that day was somewhat messy which may be VBV related, but it still produced. 

Probably a combination of backing aloft and storm coverage with very strong divergence aloft.

[weatherextreme]

New day 2

 

 

 

Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1257 AM CST Mon Feb 27 2017

   Valid 281200Z - 011200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM CENTRAL
   AND NORTHERN ARKANSAS INTO SOUTHEASTERN MISSOURI AND ADJACENT
   PORTIONS OF SOUTHERN ILLINOIS...WESTERN KENTUCKY...AND WESTERN
   TENNESSEE...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE ARKLATEX
   NORTHEASTWARD THROUGH THE LOWER AND MIDDLE MISSISSIPPI VALLEY AND
   INTO THE OHIO VALLEY...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   SLIGHT RISK...

   ...SUMMARY...
   Severe thunderstorms will be possible on Tuesday and Tuesday night
   from eastern portions of Oklahoma and Texas northeastward into parts
   of the lower and middle Mississippi Valley and Ohio Valley.

   ...Synopsis...
   Model guidance is in good agreement in showing a larger-scale
   mid-level trough axis moving from the Great Basin/southern CA region
   eastward to the Great Plains late Tuesday night.  A very strong
   500-mb speed max over the Chihuahuan Desert will move downstream
   within a belt of southwesterly flow and be located over the middle
   MS and lower OH Valleys and intensify to 100+ kt by early Wednesday
   morning.  In the low levels, a broad and strengthening southwesterly
   low-level jet will extend from the northwest Gulf Coast
   northeastward into the lower Great Lakes.  The primary surface low
   will develop eastward from NE/IA vicinity eastward to the lower
   Great Lakes during the period.  Concurrently, a warm front will
   advance northward through the OH Valley as a cold front pushes
   southeastward through the lower MO Valley and through eastern OK
   overnight.

   ...ArkLaTex and eastern OK northeastward into the MS Valley and OH
   Valley...
   This remains a complicated forecast and there remains considerable
   uncertainty regarding several factors described below, some of which
   are conditional, that will have large influence on the unconditional
   severe probabilities.

   Low-level moisture will continue to stream poleward within a ribbon
   of strong south-southwesterly flow as a warm front advances
   northward from the lower MO and lower OH Valleys to the southern
   Great Lakes.  Lowest 100-mb mean mixing ratios of 10-13 g/kg will
   translate to 58-62 degrees F dewpoints over the OH Valley and the
   middle 60s reaching as far northeast as the MO Bootheel and
   encompassing areas farther southwest over the ArkLaTex.  A cirrus
   canopy is likely to overspread a large portion of the enlarging
   warm/moist sector and partially modulate heating.  A capping
   inversion located in the 850-700 mb layer will likely delay storm
   development until late in the day across AR and areas downstream
   over the MS/OH River confluence.  By late afternoon/early evening,
   there is uncertainty whether storms will initiate along a
   pre-frontal confluence/850-mb moisture axis bisecting AR from
   southwest to northeast and into southeastern MO.  

   Models show only weak convective inhibition but this area will
   largely be on the eastern edge of 500-mb height falls beginning to
   tease the area.  In other words, only weak forcing for ascent
   primarily in the form of isentropic lift/warm-moist advection, is
   expected with any possible diurnally-driven development. 
   Deterministic models including the ECMWF, UKMET, GFS, and to a
   certain extent the ARWKF, show warm-sector activity developing over
   AR and moving downstream towards the MS/OH River confluence.  The
   NAM and NMMB models are largely void of storm development with this
   scenario and are uncharacteristically "dry".  It appears
   increasingly probable that a cluster of storms develops within this
   corridor.  The supercell-wind profile, moderate buoyancy (1000-1750
   J/kg MLCAPE), and 300-400 m2/s2 effective SRH would favor
   surface-based supercells with all hazards possible, including the
   conditional possibility for a strong tornado(s).

   As stronger DCVA/500-mb height falls overspread the western
   periphery of the warm sector as the cold front begins to accelerate
   southeastward, scattered to numerous storms are forecast to develop
   across the middle MS Valley and farther south over eastern OK and
   northeast TX.  Strong deep-layer shear will aid in storm
   organization and upscale growth into one or more bands of storms
   potentially capable of damaging winds/hail with the stronger storms.
   The strong background low-level shear may support some tornado risk
   into the overnight associated with stronger supercells and/or QLCS
   mesovortices.  The frontal convection will probably reach the MS
   River and lower portion of the OH River by Wednesday morning.

[jojo762]

9 minutes ago, bjc0303 said:

Probably a combination of backing aloft and storm coverage with very strong divergence aloft.

Agree with the systems mentioned that VBV certainly wasn't the only issue.. But believe there were also a couple of events last year where it did likely cause an impact. Believe there was one in May, May 26th I think, where it likely did play some role. For example the 00Z DDC RAOB from that day was absolutely off the charts, and there were numerous Supercells in that area, but they never really were able to get going. Also notable that things got messy over the main area (C KS/NC KS), relatively early in the day. For reference that DDC RAOB had a 69 DP, ~2800 MLCAPE, 400+ M2/s2 0-1KM SRH, 160+ 0-1KM CAPE, etc, you get the point. http://www.spc.noaa.gov/exper/archive/event.php?date=20160526

[andyhb]

Well that new Euro run certainly does nothing to lessen the concern for AR and vicinity. Ramped the low level jet strength up by 10 kts from last night's 00z run valid 00z Wed. It's popping several discrete convective elements and moving them right into it at that time as well.

[Quincy]

32 minutes ago, andyhb said:

Well that new Euro run certainly does nothing to lessen the concern for AR and vicinity. Ramped the low level jet strength up by 10 kts from last night's 00z run valid 00z Wed. It's popping several discrete convective elements and moving them right into it at that time as well.

00z RGEM also spits out several discrete tracks between 21-00z across not only AR, but SE MO and far southern IL/IN as well. Not quite sure what the 4km NAM is on, with minimal CINH and little to no convection until overnight in that souped up environment  

It will be interesting to see how the HRRRX looks later today for a peek ahead. 

[Jim Martin]

Northeast of Little Rock, Arkansas Tuesday Night from the 12z NAM this morning.

[Jim Martin]

Strong tornadoes now a possibility from Northeast Arkansas through the Middle Mississippi River Valley tomorrow evening.

[Jim Martin]

From the Day 2 Outlook from the Storm Prediction Center...

   Given the 12Z ESRL HRRR and 12Z NAM 4km suggesting a broken band of
   discrete storms developing from east-central MO into central and
   northeast IL, the enhanced severe risk area has been expanded north
   across to now include more of eastern MO and into central IL, with a
   northward expansion of the slight and marginal risks as well. 
   Moderate instability and strengthening deep-layer and low-level
   shear will favor all severe hazards, with some indication for a
   strong tornado threat from parts of AR to southern IL along a
   strengthening low-level jet enhancing hodograph curvature.

[jojo762]

Not much, if any, support from the 12Z HI-RES cams for warm sector initiation tomorrow. NSSL-WRF being void of any convection at all in Arkansas is definitely concerning. Essentially is the RGEM and the global models vs. every other Hi-res model (NSSL-WRF, HRW NMMB, WRF-ARW, 3NAM, 4NAM, 12NAM, etc...)

[Quincy]

4 minutes ago, jojo762 said:

Not much, if any, support from the 12Z HI-RES cams for warm sector initiation tomorrow. NSSL-WRF being void of any convection at all in Arkansas is definitely concerning. Essentially is the RGEM and the global models vs. every other Hi-res model (NSSL-WRF, HRW NMMB, WRF-ARW, 3NAM, 4NAM, 12NAM, etc...)

NCAR ensembles are mixed with about 1/3 of the members showing robust convection over central AR by 00z Wed.

[bdgwx]

It is interesting that many CAMs are bearish on the warm sector convection and instead focus on the forced line in association with the frontal boundary. The NAM has an interesting feature where there is a strong northward theta-e surge in MO that only stops once it collides with the front just south of I-44. It's pretty impressive that the NAM and other CAMs are allowing to the instability to build like this in February with cloud cover in place.