Late April severe weather risk ~Mon thru next Mon 4/24-5/01

[Stebo]

2 minutes ago, brettjrob said:

Up-too-late musings: situations where the models break into these camps regarding frontal positions are among the most frustrating short-range problems in convective forecasting. The event wxmann alluded to (2013-04-17) definitely comes to mind. When you look at the past 4-6 runs of the ECMWF, and especially considering that it only got more bullish about lifting the WF well into OK on tonight's run, it's hard to believe it will be 100-150+ mi. off on synoptic-scale features at this range (e.g., if the HRW runs verify on the frontal position at 00z Sat). It is, after all, the elite global model.

It's true that in the 2013 event, the global models were embarrassingly overzealous about lifting the WF (which pretty much ended up just being a CF). The Euro was bad, and the GFS was just disastrous, trying to pull it up to I-70 in KS. But that day also had legitimately chilly air trying to ooze down the southern High Plains. This one is still a tough call, to me, because it's not like the airmass progged over W OK and the TX PH during the day is even cold. Overall, it's a very different synoptic situation, with the only commonality being that the global models may be underestimating the push of cool air out west.

Ultimately, when the camps break down like this, the "cold air wins" camp virtually always has something to say about reality. So, for now, I'd go with a compromise that's not too far off from the 3-km NAM. Unfortunately, even that compromise is likely flawed enough to make the daytime threat pretty low key.

That April 2013 event is incomparable to this one, that one had a huge Canadian high dropping in bringing down the cold air. The high this time is about 10mb weaker and centered further west, the cold push will not be significant comparatively.

[thewxmann]

7 minutes ago, andyhb said:

On the mesoscale. We're looking much more at the synoptic scale here when you refer from the Mid-Mississippi/Ohio Valleys back towards the Plains.

If you're looking for something that could veer the low level jet like the NAM has, it would be the passage of the lead 500 mb shortwave to the north on Thursday evening/Friday morning encouraging early pressure falls.

April 2013 (I'm assuming you're talking April 17) was an intense, anafrontal, surging cold front. None of guidance has anything like that on Friday regardless of the extent of the warm frontal retreat/LLJ characteristics/etc. You're going to have to do a bit better than that IMO if we're talking similar setups.

Well the ECMWF and GFS have that lead 500mb shortwave as well. And we've seen instances like March (or April?) 2009 in the SE where a batch of intense convection eats up the LLJ for areas a few states away. It can happen.

Might have been April 17, 2013. I remember GFS showing a retreating WF all the way to KS and a full-blown outbreak, whereas the NAM kept that front down in SW OK, which ended up being right. It's not the same thing but we also have a boundary (much more diffuse this time) draped over N TX/ S OK. So they aren't completely different. Either way, it's an example of the NAM being better than the globals <48 hr out. I have yet to see the reverse happen, at least on big banner severe setups.

[thewxmann]

3 minutes ago, Stebo said:

That April 2013 event is incomparable to this one, that one had a huge Canadian high dropping in bringing down the cold air. The high this time is about 10mb weaker and centered further west, the cold push will not be significant comparatively.

We don't have a strong Canadian high but we also don't have a strong lee low.

[Stebo]

Just now, thewxmann said:

We don't have a strong Canadian high but we also don't have a strong lee low.

Depends what model you are looking at, the globals are strong enough, the NAM is a hot mess. You have already decided which camp you are in.

[thewxmann]

10 minutes ago, brettjrob said:

Ultimately, when the camps break down like this, the "cold air wins" camp virtually always has something to say about reality. So, for now, I'd go with a compromise that's not too far off from the 3-km NAM. Unfortunately, even that compromise is likely flawed enough to make the daytime threat pretty low key (at least W of the Arklatex/Ozarks).

This x100. If you look closely at the surface maps, you'll see that even on Friday morning there's some kind of a boundary left behind by Thursday's s/w and north of that boundary the sfc flow is weak and easterly. Due east flow north of a boundary with delayed/weak WAA is always a red flag to me. I've seen way too many instances where the LLJ gets shut down around those kind of boundaries (remember that epic AR high risk bust on 5/1/2010?). I don't know what exactly that physical process is, but something is wonky with the low-level fields on Friday and it wouldn't be the first time global models don't pick that up correctly.

[brettjrob]

8 minutes ago, thewxmann said:

Either way, it's an example of the NAM being better than the globals <48 hr out. I have yet to see the reverse happen, at least on big banner severe setups.

This event aside, I'll adamantly agree on this point. The NAM gets way too much crap from the severe community in general. It has its biases and flaws, like any NWP, but in that 48-72 h range it quite often is onto something when it diverges significantly from the globals.

[andyhb]

5 minutes ago, thewxmann said:

Well the ECMWF and GFS have that lead 500mb shortwave as well. And we've seen instances like March (or April?) 2009 in the SE where a batch of intense convection eats up the LLJ for areas a few states away. It can happen.

Might have been April 17, 2013. I remember GFS showing a retreating WF all the way to KS and a full-blown outbreak, whereas the NAM kept that front down in SW OK, which ended up being right. It's not the same thing but we also have a boundary (much more diffuse this time) draped over N TX/ S OK. So they aren't completely different. Either way, it's an example of the NAM being better than the globals <48 hr out. I have yet to see the reverse happen, at least on big banner severe setups.

Considering the LLJ here is much more under the influence of the low developing further to the west, I really don't see that as much of a hinderance here. In addition, the convection to the east is generally scattered (and it should be, given the lack of more synoptic scale forcing for ascent that far east and height rises through the day).

Ideally, you'd want stronger pressure falls extending eastward from the developing lee low by Friday mid afternoon to enhance the low level gradient, in turn strengthening the low level jet and also leading to more isallobaric backing.

[thewxmann]

6 minutes ago, Stebo said:

Depends what model you are looking at, the globals are strong enough, the NAM is a hot mess. You have already decided which camp you are in.

What are you talking about? They're all a hot mess. Just because I side with the NAM doesn't mean I haven't seen all the models.

[thewxmann]

3 minutes ago, andyhb said:

Considering the LLJ here is much more under the influence of the low developing further to the west, I really don't see that as much of a hinderance here. In addition, the convection to the east is generally scattered (and it should be, given the lack of more synoptic scale forcing for ascent that far east and height rises through the day).

Ideally, you'd want stronger pressure falls extending eastward from the developing lee low by Friday mid afternoon to enhance the low level gradient, in turn strengthening the low level jet and also leading to more isallobaric backing.

Not until after 0Z. That's the crux of the problem. 

[andyhb]

To be clear, I'm not exactly arguing on the bullish or bearish side here. I just think picking the NAM and then raising cases where it was right only on the bearish side (especially when the synoptic setups in general were wholly different than Friday) is a bit iffy as far as logic is concerned.

May 1, 2010 is also a pretty iffy case to bring up since that was a massive, amplified upper trough with plenty of contamination of the warm sector/etc. There's a reason that thing yielded the historic flooding in Tennessee.

[1900hurricane]

I've heard good things about the NCAR ensembles this year so far, so I decided to give them a peek for Friday. It's at the very tail end of the run, but four out of the ten members have convective initiation along the dryline by 00Z Saturday. What I found to be curious is that all for members firing off convection are doing so further south than I would have expected.

 

[Quincy]

FWIW, which may not be much, here's the 36hr HRRRX (SBCAPE) for 21z FRI:

[1900hurricane]

The 12Z NAM had a fairly marked increase in developing the western branch in the low level jet by 00Z Friday.

 

[Superstorm93]

FWIW, the 12km/3km NAMs have the best parameter space completely contaminated on Saturday.

Looks like a pretty strong signal for widespread convective contamination at the very least with that meridional jet and impressive UL divergence. No matter how many tornado emojis Reed Timmer puts out, or what the TorCon is, I think the deadliest issue on Saturday will be flash flooding for portions of AR/MO. PWATS around 2" and training storms will be a huge issue. 

 

[Hoosier]

15 hours ago, jojo762 said:

Don't want to beat this drum... But add this to the growing list of SPC 10%+ Tornado probability  busts this year. (Mostly due to being aggressive on days with tremendous ingredients but questionable storm-modes)

It's tough.  I can point to some fairly recent QLCS cases that were prolific producers (as in dozens of tornadoes) in my region, like 4/19/2011 and 6/30/2014, but I'm not sure how to pick out which QLCS cases are likely to produce a lot vs. very little.  Admittedly I am not up on any recent research in that area.

[The Iceman]

Pretty interesting line of supercells in alabama/georgia line. one storm with decent rotation between clayton and midway. the tor warned storm nw of hurtsboro seems to be losing it's couplet but it also looked pretty decent a few frames ago. pretty active for early afternoon. new tor warned cell north of geneva has a pretty nice couplet on it as well. haven't seen any confirmed tornados but seems like any cells that have been getting going are turning. there's more tor action than yesterday already.

 

Edit: cell in brundidge bares watching as well with some broad rotation being indicated in it.

[monsoonman1]

Not in the subforum but likely a significant tornado near Junction City GA. Very deep TDS with debris at least up to 10k ft.

Edit: could be wrong but looks like debris up to 20k ft.

[The Iceman]

Tornado is confirmed near Junction City. rain wrapped as well.

[mob1]

4 minutes ago, monsoonman1 said:

Not in the subforum but likely a significant tornado near Junction City GA. Very deep TDS with debris at least up to 10k ft.

Edit: could be wrong but looks like debris up to 20k ft.

As defined a debris signature as you'll ever see. 

[pbrussell]

Pretty weird stuff going on with the radar scans and that storm currently. Looks like another pronounced debris signature around salem, but no other evidence to be found with velocity/reflectivity in that area. I'd write it off as radar contamination or something, but given what just took place earlier...

[David Reimer]

3 minutes ago, pbrussell said:

Pretty weird stuff going on with the radar scans and that storm currently. Looks like another pronounced debris signature around salem, but no other evidence to be found with velocity/reflectivity in that area. I'd write it off as radar contamination or something, but given what just took place earlier...

Depending on the amount of debris lofted earlier we may be seeing some of it return down. 

[pbrussell]

1 minute ago, David Reimer said:

Depending on the amount of debris lofted earlier we may be seeing some of it return down. 

Kind of what I was thinking. Must have been a fairly intense tornado if that's the case.

[1900hurricane]

The ECMWF has repeatedly fired convection on what appears to be some sort of weak effective warm front or some other gradient boundary in ESE Texas (from about Beaumont to Crockett) Saturday afternoon. I'm not really sure why it's there and where it comes from, but it has been a consistent feature for the past few runs. If something like that were to transpire, that could be something that enhances otherwise largely uniderectional shear, which is always concerning in the presence of CAPE of that magnitude. It'll be interesting to see if that feature remains persistent and/or if other guidance (such as the CAMs one they're in range) begin to pick it up as time progresses.

[jojo762]

Think we can all agree at this point that tomorrow really isn't looking too spectacular, especially when compared to what it once looked like. Looking like a day, currently (things can still shift and change drastically on the day of), where a few/couple supercells go up by late afternoon/early evening with very large hail and a few tornadoes, perhaps a strong tornado or two as well. Wouldn't be surprised to start out with just a 5% TOR, perhaps a 10% TOR with a hatched area later on as details become clearer... but doubt we get a MDT risk driven by tornado probabilities. 

[MattPetrulli]

19 minutes ago, jojo762 said:

Think we can all agree at this point that tomorrow really isn't looking too spectacular, especially when compared to what it once looked like. Looking like a day, currently (things can still shift and change drastically on the day of), where a few/couple supercells go up by late afternoon/early evening with very large hail and a few tornadoes, perhaps a strong tornado or two as well. Wouldn't be surprised to start out with just a 5% TOR, perhaps a 10% TOR with a hatched area later on as details become clearer... but doubt we get a MDT risk driven by tornado probabilities. 

I feel as if cap will win tomorrow. But assuming the scenario a cell goes up and sustains itself in AR area with a lot of high end paramters and we got a different animal on our hands. Not a tornado outbreak, or even a localized by any means. But if a  cell can sustain itself in the 00-03z timeframe it will need to be watched very closely. KLZK seems to be concerned about this in their CWA. 

 

[brettjrob]

17 minutes ago, jojo762 said:

Think we can all agree at this point that tomorrow really isn't looking too spectacular, especially when compared to what it once looked like. Looking like a day, currently (things can still shift and change drastically on the day of), where a few/couple supercells go up by late afternoon/early evening with very large hail and a few tornadoes, perhaps a strong tornado or two as well. Wouldn't be surprised to start out with just a 5% TOR, perhaps a 10% TOR with a hatched area later on as details become clearer... but doubt we get a MDT risk driven by tornado probabilities. 

Pretty much. I would lean strongly toward a 5% initial outlook based on where things stand. We're finally seeing some convergence between the two "camps" that were discussed last night. Looks like the warm front should set up near or perhaps a county SE of I-44 in N TX and OK. The million dollar questions remain: (1) daytime CI, and (2) how rapidly low-level shear can increase in the evening. In general, it just looks like the low-level wind field will take a few hours too long to recover fully from the lead shortwave tonight. By 03z, it looks beautiful, of course (obviously aided by the nocturnal LLJ, but still a monumental change from 00z due partially to further recovery). The hi-res guidance is fairly split on CI before dusk, but I'll lean bullish based on the steadfast signal from global models.

This is all focused on the OK/TX region; as Matt mentions above, the threat for daytime storms in a respectable environment may well be greater in AR... just ugh.

[jojo762]

14 minutes ago, brettjrob said:

Pretty much. I would lean strongly toward a 5% initial outlook based on where things stand. We're finally seeing some convergence between the two "camps" that were discussed last night. Looks like the warm front should set up near or perhaps a county SE of I-44 in N TX and OK. The million dollar questions remain: (1) daytime CI, and (2) how rapidly low-level shear can increase in the evening. In general, it just looks like the low-level wind field will take a few hours too long to recover fully from the lead shortwave tonight. By 03z, it looks beautiful, of course (obviously aided by the nocturnal LLJ, but still a monumental change from 00z due partially to further recovery). The hi-res guidance is fairly split on CI before dusk, but I'll lean bullish based on the steadfast signal from global models.

Yep... IF any discrete or semi-discrete supercells remain surface-based and near the warm front after dark in Oklahoma and Arkansas, it would appear that the tornado threat would be fairly substantial. Several CAMs point to a solution with numerous discrete/semi-discrete supercells blowing up along the boundary after dark, most notably the 12Z HWR-NMMB and HRW WRF-ARW, 12Z 3km NAM also showed something similar, all before convection increases and everything becomes messy. 

I'm currently thinking we get a supercell or two to form in the "sweet-spot" in N TX/SW OK near the dryline-WF intersection sometime around dusk (give or take an hour), and additional supercells and clusters will develop later on along a large portion of the warm-front in OK/AR and perhaps into southern Missouri. 

[jojo762]

Today is such a waste of a great windfield across W OK/SW KS. Currently an elevated supercell in Kearny Co. KS with ~95kt G2G couplet lol Granted it's several thousand feet up, but still... sigh.

[jojo762]

1 minute ago, jojo762 said:

Today is such a waste of a great windfield across W OK/SW KS. Currently an elevated supercell in Kearny Co. KS with ~95kt G2G couplet lol Granted it's several thousand feet up, but still... sigh.

Speak of the devil 

BULLETIN - EAS ACTIVATION REQUESTED
Tornado Warning
National Weather Service Dodge City KS
550 PM CDT THU APR 27 2017

The National Weather Service in Dodge City has issued a

* Tornado Warning for...
  Southern Kearny County in southwestern Kansas...

* Until 630 PM CDT
    
* At 550 PM CDT, a severe thunderstorm capable of producing a tornado
  was located 4 miles southeast of Kendall, or 12 miles west of
  Lakin, moving east at 20 mph.

  HAZARD...Tornado and ping pong ball size hail. 

  SOURCE...Radar indicated rotation. 

  IMPACT...Flying debris will be dangerous to those caught without 
           shelter. Mobile homes will be damaged or destroyed. 
           Damage to roofs, windows, and vehicles will occur.  Tree 
           damage is likely. 

* This dangerous storm will be near...
  Lakin around 625 PM CDT. 
 

[Santa Clause]

Agree with jojo, think we see ENH stay, and 5 for TOR probs. They'll adjust accordingly from there.