2016-04-26 EVENT: TX/OK/KS

[Jeff Duda]

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Thank you, everyone, for participating. Let's keep the discussion going!

 

[Jeff Duda]

I'm gonna go on the record and say the way things are trending, I don't see this being a big event. Still a setup well worth chasing, but no outbreak.

I was expecting to see the 12Z NAM come in and plaster the dryline region in W OK/C KS with the high moisture that the GFS was, for some reason, cutting out in several previous runs. Instead I see the NAM prog a more neutrally tilted and westward placed trough axis well over C CO at 00Z with the eastern edge of the "acceptable" (defined here as 40 kts or larger) 500 mb flow just barely into W OK with a nose covering most of W KS. One might think, fine, we'll just accept somewhat lesser shear right along the dryline; at least the moisture will be there (unlike what the GFS had been saying). Nope, the NAM has rather shallow moisture, and only a very limited axis above the surface. I also see some capping concerns. It looks very likely there will be an EML in place over the central and southern Plains which is great for getting extreme instability (assuming the moisture is there), but there is a bit of a capping inversion. It does appear surface winds will be backed enough to provide some pretty solid convergence, so I don't think the cap will prevail, but the lack of signal in the precip and mid-level temperature fields in the 12Z NAM is kind of telling.

Also, the difference on the synoptic scale between the NAM and GFS this far out does not offer high hopes in terms of certainty of this event. The GFS puts the trough noticeably farther east, but also continues to take a huge chunk out of the moisture along the dryline just off the surface. I have been suspecting since Wednesday that this was the result of inadequacies in the GFS' PBL physics scheme. Seeing the 12Z NAM makes me think I'm right, but again, the 12Z NAM didn't restore what the GFS removed.

Finally, the 15Z SREF does not prog an outbreak either. It struggles to maintain mid-60s dews east of the dryline, and suggests 500 mb flow collapsing as the day wears on such that shear will end up only being adequate rather than extraordinary. Low-level shear also doesn't appear to be incredible, but again, adequate for tornadoes. It has a precip signal, but it doesn't suggest the ingredients for sig tors are highly likely to align, at least for a long time.

Given the current weather on this Saturday afternoon, I'm tending to think this toned down forecast may be accurate. The main reason is looking at moisture and wind over the Gulf. A ridge remains in place over the northwestern Gulf, basically continuing to restrict moisture return. The only onshore flow right now is in deep south Texas and northern Mexico, where it also was yesterday. Also, it's not like the Gulf is completely loaded either. There aren't even 60s dews right at the coast in most places. I know this is still 3.5 days away, and there is an intervening trough to get past, but this evolution makes me want to think the more conservative forecast of moisture quality will verify.

 

[Stan Rose]

As Jeff mentioned, timing differences are disconcerting but the 00Z NAM is holding tight with its more western solution. Surface winds seem to have some better backing too in the latest run--I suspect lack of llvl shear will not be an issue, and there's a boatload of bulk shear and more than adequate CAPE. Would like to see the low centers a bit farther south--am somewhat skeptical of the moisture quality that far north--and therefore a lack of negative tilt in the trough, but overall, looking at the big picture I could see a regional outbreak coming out of this setup.

 

[Joe Pudlik]

Regarding the NAM, and some other guidance, the VBV profiles showing up are a bit of a concern as well.

 

[Greg Blumberg]

Agreeing with Jeff et al. regarding the wind profile. The key is to look at the ensembles to highlight any uncertainty with respect to this forecast. Ensembles may help with not just the storm scale/mesoscale forecast, but also the synoptic scale forecast. For example, the two key synoptic scale ensembles (NAEFS and SREF) are indicating that there are timing issues associated with the shortwave trough that is progged to be the source of enhanced lift and shear for Tuesday's event. Some members have the 500 mb flow a little more meridional than some of us would like (see attached SREF map), which is contributing to the cold advection between 500-700 mb and veer/back/veer wind profile (see attached SREF sounding). Such a profile can rob you of better storm-relative helicity or create an environment with strong synoptic scale lift and a wind pattern that is more conducive to more storm coverage rather than discrete cells. Contrary to the hype this event has brought forth, this forecast (like many of at this lead time) is not locked in solid for a significant tornado outbreak, although current public forecasts are more than justified for highlighting the potential for severe weather. Many of the ingredients are present for such an event, but there are always failure modes for events like these.

 

[Trey Thee]

VBV is the biggest fly in the ointment as I see it. It seems moisture return will be adequate but the wind profiles are going to be somewhat limiting. As mentioned above storm relative helicity, while adequate for tornadoes is hardly indicative of widespread major outbreak at this point. The system is now fully onshore, model samples should start reflecting this with the 12Z runs I would suspect.
Another potential issue...the 00Z Euro has the low weakening throughout the day and retrograding back into NM. Going from 992mb at 0Z on the 26th, to 995 at 00Z on the 27th.
On the upside? It seems overnight some of the displacement between upper air and surface features has become somewhat more aligned...VBV is still bothersome in OK.

 

[JamesCaruso]

12Z NAM and GFS have both shifted the warm front / triple point up toward northeastern Nebraska. Don't like the near southerly 500mb flow up there. Dryline in OK has more southwesterly 500mb flow across it but not the greatest convergence along it and LLJ is shunted into eastern OK. Both areas have high SIGTOR values per NAM.

 

[Ethan Schisler]

I don't care too much for the latest developments of this system. The low pressure appears to be occluding too soon, mid-level flow somewhat meridonal further north near the triple point, which could support a messier storm mode. In addition, the low level jet doesn't appear as focused, organized, or strong down the dry line, I'm thinking this is owing to a more disorganized cyclone. With that said, I think the threat further north near the triple point which latest 12z NAM has in E/SE Nebraska appears the most ominous from a tornadic standpoint.The best thing going for this system might be looking at things from a thermodynamic standpoint, quite robust CAPE and low level moisture values. With the variability of guidance, probably several more runs before we can nail down an exact target location.

 

[chrisbray]

I don't know if I am reading too much into this, but the last two runs of the GFS have shoved the trough and attendant surface low way farther north than previous runs. It also closes off the upper low. I mean, the triple point moved almost an entire state north. This much inconsistency this 2 and a half days out is making me nervous

 

[Trey Thee]

I don't know if I am reading too much into this, but the last two runs of the GFS have shoved the trough and attendant surface low way farther north than previous runs. It also closes off the upper low. I mean, the triple point moved almost an entire state north. This much inconsistency this 2 and a half days out is making me nervous

Seems reasonable to me. Given all the other issues with the system, run-to-run consistency, surface vs upper air dynamics being displaced, VBV profiles, etc...
I like the 30% enhanced, seems reasonable to me. If they upgrade to MDT for day 2 I would suspect thats due to the hail threat which looks really impressive. While I think they'll be some tornadoes, the risk of EF3 or greater tornadoes, long-track, etc appear to be pretty limited right now. I think we could see a lot of quick spin-ups to EF1/EF2, stay on the ground 5, 7 or 10 minutes and then recycle, repeat....
At this point I'm not convinced we're due for some sort of epic outbreak.

edit: the Euro just caved to the more northern solution for the low. The 12Z vs the 00Z run has the low 300ish miles further north. What a mess. That will change the synoptic pattern...for those (myself included) who assumed the Euro was going to get the placement of the low correct...

 

[chrisbray]

Seems reasonable to me. Given all the other issues with the system, run-to-run consistency, surface vs upper air dynamics being displaced, VBV profiles, etc...
I like the 30% enhanced, seems reasonable to me. If they upgrade to MDT for day 2 I would suspect thats due to the hail threat which looks really impressive. While I think they'll be some tornadoes, the risk of EF3 or greater tornadoes, long-track, etc appear to be pretty limited right now. I think we could see a lot of quick spin-ups to EF1/EF2, stay on the ground 5, 7 or 10 minutes and then recycle, repeat....
At this point I'm not convinced we're due for some sort of epic outbreak.

Which would be fine to me. I don't need a violent outbreak, but I am not about to commit a 10 hour drive out there for a messy mode condusive to squall lines. They way it has all been trending is making this a very difficult choice, and if we do go, picking a target still seems way up in the air. With these shear vectors the warm front play looks to be less than stellar as storms will likely cross the boundary even if they are right movers. Meanwhile, the dryline shear vectors could lead to messy storm interference as they don't move too far off the line.

 

[John Farley]

It seems to me that the dryline is looking a little more diffuse on this morning's NAM run, and the models aren't breaking out much precipitation until after 0Z. I am concerned that this could mean a largely after-dark event. The NAM does really ramp up the significant tornado parameter between 0Z and 3Z. Whether this translates into sunset tornadoes or an after-dark event is unclear, at least to me. Also, the NAM seems to be forecasting the development of some kind of secondary low around the OK panhandle in the evening, with the heaviest precipitation actually forecast in that area rather than farther east. Just east of that feature, the low-level directional shear is pretty decent, but the location of this feature seems to be behind the dryline. Not too sure what to make of this. Overall it seems like a rather messy situation, though one with some decent potential, no doubt. I like Chris face a 10+ hour drive, so still undecided about chasing this one.

 

[Tom Dulong]

I don't know if I am reading too much into this, but the last two runs of the GFS have shoved the trough and attendant surface low way farther north than previous runs. It also closes off the upper low. I mean, the triple point moved almost an entire state north. This much inconsistency this 2 and a half days out is making me nervous

I have been noticing this as well. It is likely due to increased impact on our Tuesday low system by the next one upstream. I went into SPC's Violent Tornado webpage http://www.spc.noaa.gov/exper/outbreaks/outbreaks-regions.php# and looked all outbreaks of >= 4 violent tornadoes within a 3-day period. I only found one event (May 2, 1942) that could be an analog for what we will be facing (e.g., back-to-back upper lows of similar paths/intensity that directly impact the OK/TX/KS region in the course of a few days). In the analog event, no tornadoes occurred with the leading system on April 30th while 8 tornadoes, including one violent one, were recorded on May 2nd. From available data, it appears availability of low level moisture was the difference in the impact from the two lows.

So...based on latest model runs and analog cases...I would not expect a tornado outbreak, especially with violent tornadoes...for Tuesday. However, we should keep a wary eye on the next low coming in for something more significant (but that is for another thread).

 

[Trey Thee]

I have been noticing this as well. It is likely due to increased impact on our Tuesday low system by the next one upstream. I went into SPC's Violent Tornado webpage http://www.spc.noaa.gov/exper/outbreaks/outbreaks-regions.php# and looked all outbreaks of >= 4 violent tornadoes within a 3-day period. I only found one event (May 2, 1942) that could be an analog for what we will be facing (e.g., back-to-back upper lows of similar paths/intensity that directly impact the OK/TX/KS region in the course of a few days). In the analog event, no tornadoes occurred with the leading system on April 30th while 8 tornadoes, including one violent one, were recorded on May 2nd. From available data, it appears availability of low level moisture was the difference in the impact from the two lows.

So...based on latest model runs and analog cases...I would not expect a tornado outbreak, especially with violent tornadoes...for Tuesday. However, we should keep a wary eye on the next low coming in for something more significant (but that is for another thread).

Not a huge fan of basing things off analogs especially older analogs when additional parameters aren't necessarily available or accurate.

Your point overall about the system being overhyped is valid at this point. Everything points to a high end hail day with perhaps numerous smaller or quick spin up type tornadoes. I'm not seeing much that would indicate support for strong, long track tornadoes. SRH 0-1km of 200 m2/s2 is adequate for tornadoes but pretty poor for a moderate to high end outbreak type event. There is just way too many small issues, add them up all and it's pretty inhibitive of a significant outbreak. That could change pretty quickly though, so it certainly warrants keeping a close eye on. I'm likely chasing since it'll be an enhanced risk or an MDT at day 1 for N Central OK.

 

[Michael Snyder]

Seems reasonable to me. Given all the other issues with the system, run-to-run consistency, surface vs upper air dynamics being displaced, VBV profiles, etc...
I like the 30% enhanced, seems reasonable to me. If they upgrade to MDT for day 2 I would suspect thats due to the hail threat which looks really impressive. While I think they'll be some tornadoes, the risk of EF3 or greater tornadoes, long-track, etc appear to be pretty limited right now. I think we could see a lot of quick spin-ups to EF1/EF2, stay on the ground 5, 7 or 10 minutes and then recycle, repeat....
At this point I'm not convinced we're due for some sort of epic outbreak.

edit: the Euro just caved to the more northern solution for the low. The 12Z vs the 00Z run has the low 300ish miles further north. What a mess. That will change the synoptic pattern...for those (myself included) who assumed the Euro was going to get the placement of the low correct...

Just a question here, did you mean the Warm Front being further north? Looks like the surface low is a tad deeper (996mb area larger) and a hair further west. But overall the cyclone being a bit more broad to the NE.

The Euro has some precip starting on the dryline in OK around 18z, and then really firing off by 00z.

But to agree with others here I think you might have one cell go bonkers on the dryline, and not quite sure to make of the TP yet with the lack of consistency.

 

[Trey Thee]

Just a question here, did you mean the Warm Front being further north? Looks like the surface low is a tad deeper (996mb area larger) and a hair further west. But overall the cyclone being a bit more broad to the NE.

The Euro has some precip starting on the dryline in OK around 18z, and then really firing off by 00z.

But to agree with others here I think you might have one cell go bonkers on the dryline, and not quite sure to make of the TP yet with the lack of consistency.

Warm front appeared further north, the low itself was positioned quite a bit further north as well. But I don't have access to hour by hour euro info. It's in 24 hr increments so I'm comparing runs but it's not apples to apples because of the 12hr spacing between the runs.

 

[Stan Rose]

In the EC and NAM, there are two upper low centers--one in SE CO and the other in SE WY. The GFS does not have this structure. The GFS will be wrong, as it has been with virtually every major storm this winter... Overall the pattern today looks more favorable for outbreak than it did yesterday (trough is looking a lot more negatively tilted!) despite the NAM failing to initiate things earlier, as it was doing yesterday. I'll look more closely at the 00Z runs.
ETA: not seeing any VBV in central OK soundings...

 

[Manuel Salgado]

As I am unable to leave town until 11am on Tuesday at the earliest, I was considering a chase of the southern target south of the Red River. But at this point, I'm not very keen on even making the drive out to the area near Wichita Falls due to the VBV being pretty ugly. There will be plenty of energy but I have a feeling we're going to see a QLCS develop rather quickly. At this point the setup down here reminds me a lot of April 24th last year. We got a few discrete cells early in Texas on that day, but things quickly congealed into a line after that. The wind profiles have been consistently getting worse down here for two days so at this point I am more inclined to sit it out rather than chase the southern area.

 

[Kelton Halbert]

I posted this discussion to Facebook earlier, but I figured it might be good to share it here too.

I was a bit confused as to why the NAM4km wasn't convecting at all given the high thermodynamic parameter space and extremely low LCL/LFC heights along the dryline. This is not an assessment of the validity of the forecast itself (I actually think something is fishy with the NAM4km solution at 850), but more of an investigation into why we're not seeing widespread convection being forecast by the model. If you would like to see the comments and discussion going on there too, which is also quite rich, the link is here: https://www.facebook.com/stormchaserkelton/posts/983784018385352


Initially, looking at the depth of the dryline circulation by taking soundings from either side of the dryline, the depth of the mixing is more than sufficient to bring a parcel to it's LFC. This can be approximated by looking at the top of the boundary layer (dewpoint trace of const. mixing ratio) on the dry side. Clearly, it's well above the LFC on the moist side. Typically this favors convective initiation, but this is not the case.



Looking at the 850mb and 700mb forecast maps, I believe the reason to be due to a boundary orthogonal component of the wind to the dryline at both levels. My understanding is that this can serve to do two things: 1) it can have influences on dry air entrainment into budding updrafts, which is typically not good, and 2) it greatly reduces the residence time of the updraft on the dryline, meaning that an updraft may not be in the area for ascent long enough to reach it's LFC.



Unfortunately, all the parameters in the world mean nothing if you can't get a parcel to it's LFC. Thankfully (whatever that means?) the NAM is much more southerly with the 850mb wind. If you want an active severe day, that's what you're going to need to look for.

 

[Jason McKittrick]

If I recall correctly I believe Rich Thompson, correct me if I am wrong, in his tornado forecasting workshop touched on a similar forecast complication which occurred in the days leading up to the April 14, 2012 event. He noted, as a reason not to rely totally on the models, the models did not forecast any warm sector convection for that forecast period. He gave an explanation however I do not recall the details at this time. Perhaps someone else remembers the specifics better that I. The science says moisture, conditional instability and a source of lift. All three will be present over a large part of the warm sector on Tuesday. My concern is the timing of the lift which could potentially make this a nocturnal event.


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[Stan Rose]

I think your number 1 is correct. The cap is pretty stout early on, and there is initiation, but there's a ton of dry air not that far up that is choking things off. As Jason points out, probably won't matter, timing will be more important as it still looks like a relatively late show. The 00Z guidance has raised some concerns, but I still expect significant severe.

 

[Randy Jennings]

I sat down tonight and hand drew a target area map based on the 20160426/21Z from the 20160425/00Z NAM and another one on the GFS. Spoiler alert – I’m not impressed. I was so unimpressed, I went to read the thread (hadn’t read it in a few days) before posting to see what I was missing before I made a fool of myself. Glad to see I’m not the only one who isn’t exited. Here goes:

The first thing that surprised me was how far north the front was. I had to cheat it south just to fit on my paper map of KS/OK/TX. Both GFS and NAM had the dryline reasonably close to the same spot. Next, I started to draw the CAPE and it was a pleasant find. GFS had 2000+ east of the dryline on my entire map. NAM did too except for a small pocket in SE TX. NAM also had 5000 CAPE in KS!

Everyone has been saying the cap may be the problem. Both the GFS and NAM had a narrow line of no CIN at 4 PM not too far east of the dryline with a larger uncapped region in central to south TX. NAM did have a little wider no cap region in northern KS. In the areas with cap, the GFS made it look breakable in all areas except DFW and east OK, and the NAM made it look breakable except for a narrow band from the DFW to OKC area (although it did brake in DFW when I went forward 3 hrs). I actually thought this was a good thing. The best storms come on days where the cap seems unbreakable and then finally does. I started to think my target area would be along 35 in KS or OK or west of 35 in TX.

I drew deep layer shear next. The NAM seemed to have a good area of deep shear all along the dryline and up to 100 miles east or so of the dryline. The GFS stopped the best deep layer shear in central TX and didn’t extend it into south TX. The GFS also had a wider area of deep layer shear in northern KS.

I tried to draw in low level shear next, and this is where things fell apart. I had to go all the way up to SFC-700 before I found 30+ knot shear. The first 1 or 2 km had around 10 or 15 knots shear. So I drew in the SFC-700 shear. GFS had a corridor from north central OK to the TX border then it when a little west of along the 35 corridor in OK and west of the 35 corridor in TX stopping in central TX. NAM had a very small area from south central OK to around Parker County TX.

I chase partner and I have been on planning taking off work and going out on Tuesday for about a week now. If I were leaving now, I guess I would head towards Wichita Falls, but I will defiantly revaluate that. I hate to take a day of work to chase hail and not see anything else.

 

[Randy Jennings]

Right after I posted I went to SPC and looked at the new day 2 outlook, and they couldn't disagree with me more. They are playing the triple point. I went to the 4K NAM (which I hadn't looked at yet) to see where I went wrong. 4K NAM does fire in KS at 20160426/21Z. It fires (better?) starting around 20160427/00Z in TX. What did look like a sure bet a few days ago, may be a frustrating chase for many of us. Am I too focused on low level shear? What am I missing? Maybe I need to go look at observations and soundings....

 

[Brandon Centeno]

I skimmed your post but I think a lot of people have taken models individually at face value which just isn't very smart to do. You want to take model guidance and apply meteorological theory to it. Having sat with spc people before, it's pretty incredible how much you can predict without any model data data at all.

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[Ben Holcomb]

The bottom line by your own post is extreme instability + strong jet/neg tilt + trigger mechanism. You don't need strong low level shear with 5000J/KG MLCAPE

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