2015-05-16 EVENT: TX, OK, KS, NE, SD, IA, MO,MN

[Royce Sheibal]

THE BIG DAY! Prepare for a wall of text!

I know you're all excited. Friday will likely be an appetizer for what will probably be the best day of the year on Saturday. Multiple targets, good cape, good bulk shear, questionable hodographs? Let's talk models.

SPC's ensemble just came out for Saturday. My personal favorite probability they run is 1000 cape + 40 kts bulk shear + precip. 70% by 21z N KS and 50% from S OK to the SD border. These are good #'s. SPC's 30% on 4 day is going to be a Moderate tomorrow, most likely. Depending on morning convection, it could be more than just a moderate day. Keep an eye on ML LCL's, which are forecast to be below 750m! for the entire zone. Low bases mean more helicity from surface based flow and outflow interactions. We could see some really high 0-1km SRH in localized areas, even if forecasted hodo's aren't impressive. Also: HP's..Be Advised...

12Z NAM is not a fan of the central plains for Sat. Sig tor values are peaked in SW IA and OK, but most of KS and NE is covered with ongoing precip and cloud debris, as well as significant drying at the lower levels behind the MCS's. The models appear to be mixing in a lot of dry southwestern air behind the MCS's, despite hitting 70 Td in Nebraska on Friday. NAM is forecasting a full fledged outbreak on Friday in Nebraska as well, so it will be interesting to see how things pan out overnight. NAM 4km is not at Saturday yet, but it's been my baby the last 2 years, so we'll see what she says tomorrow.

12Z GFS has been predicting ongoing precip in the central plains region as well, which will continue to complicate things. The GFS has, however, shown areas of increased helicity in central KS along the dry line as well as another possible target out in IA/MO or even up north into Minnesota, for you easterners out along the warm front there is also some solid vorticity. Another possible target is down to the south in Wichita Falls where cape is bigger, but cap becomes more of a question. 850's are screaming outta the south everywhere.

So what does this all mean? Well, we are still 4 days out, and forecasts may diverge greatly, but the general placement of the front, moisture, and shear are all lining up pretty well. Hodograph's aren't great, and I'm seeing a lot of veer/back/veer shenanigans. But any area that can get some sun between the AM precip is going to be juicy.

Consider the following, and please do comment:
Super far north target: Central MN into W IA: 1500 cape, shear along the WF, and likely ahead of the AM precip
Middle of the road target: SC Neb: Cape may rebound in the dry slot behind morning precip, shear is good near the dryline as the low gets a little negative tilt, also NUTS DPVA.
South Target: Wichita, KS into NW OK: Better Cape, OK Shear, early convection is an issue
Super south target: Wichita Falls, TX: Best Cape, OK Shear, Strong cap may work to your advantage to keep convection down until it breaks after 21z. Meanwhile bases are still forecast to stay low enough for tors.

 

[Matt Hunt]

I'm not very confident in Saturday at all, and likely going to sit this one out. Looks like some of the same issues that we had last Saturday, only amplified further. Morning convection will wipe out the unstable air, and the 12z NAM is showing meager instability under 1000 CAPE everywhere north of Oklahoma. There is a narrow corridor of 2000+ CAPE in western Oklahoma down into north Texas where the LLJ may be able to bring some of that moisture back in. However, it's narrow, and the VBV hodos are even more pronounced this Saturday. With that, NE storm motions, and the lack of a cap, I'm thinking QLCS rather than isolated supercells.

 

[Andy Wade]

Why do people love negatively tilted waves? I know there are good qualities: they don't bring iron caps, tend to propagate slowly enough for multi-day events, and have a little extra curvature divergence aloft. But they seem to come with their own set of problems. Meridional upper flow decreases directional shear and contributes to veer-back profiles. Given any meridional initiating boundary, it favors messy storm modes. The same thing that prevents cap busts--the lack of a pristine EML plume streaming in directly off the high terrain--can translate to mediocre lapse rates and unwanted convection. All of those problems are great candidates to short-circuit Saturday. In particular, the signal for widespread junk precip is pervasive and realistic. I think we wait another week for a (final?) shot at something big.

Less pessimistically, the warm sector is so expansive that some chunk of it seems likely to escape early convection. Maybe there's a sweet spot south of the synoptic forcing, like last weekend. Maybe the nuisance storms organize overnight into a progressive MCS and clear the region early. With rich low-level moisture, abundant shear, and strong forcing in place over such a wide area, there should be localized significant severe weather somewhere in spite of the limitations.

 

[JamesCaruso]

I haven't looked at the models in detail, still focused on clearing the decks at work before my chase trip next week. But it seems like something this synoptically evident has to bear fruit somewhere, even if it does not end up being quite the epic widespread outbreak that it otherwise could be if not for the limiting factors discussed. Where there's smoke there's fire, so to speak. Definitely wishing I could be out there, and in fact normally would fly out to the Plains on a Friday night to start a chase trip but unfortunately have some personal obligations this weekend so can't fly out until Sunday night. Couldn't possibly imagine sitting it out if I was out there!


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[JThumberger]

Why do people love negatively tilted waves?

This has been my question for a while as well. I've read books that essentially say you want a negative tilt trough because they're better for severe storms. And I guess the kicker here is that they say "severe storms"...and not "chaser desirable storms". I can see why a negative tilt trough favors increased severe thunderstorms, but at the same time, like you mention, I've never seen the benefit of them with regards to chasable storms. Every time I chase a negative tilt trough I wind up with VBV profiles and northward moving storms that are messy and never worth my time. Last Saturday did manage to produce some decent storms/tornadoes, but that seems to be the exception and not the rule in my limited experience.

 

[Brett Roberts]

Why do people love negatively tilted waves? I know there are good qualities: they don't bring iron caps, tend to propagate slowly enough for multi-day events, and have a little extra curvature divergence aloft. But they seem to come with their own set of problems. Meridional upper flow decreases directional shear and contributes to veer-back profiles. Given any meridional initiating boundary, it favors messy storm modes. The same thing that prevents cap busts--the lack of a pristine EML plume streaming in directly off the high terrain--can translate to mediocre lapse rates and unwanted convection. All of those problems are great candidates to short-circuit Saturday. In particular, the signal for widespread junk precip is pervasive and realistic. I think we wait another week for a (final?) shot at something big.

Negative tilt waves can have their pitfalls, but all things considered, they're probably associated with a higher probability of major-league tornado events for the Plains than the opposite. We've seen plenty of significant days with VBV profiles, meridional upper winds, etc. -- see 20 June 2011 and 24 May 2010, for example. Anecdotally, it seems to me that these issues are less problematic over the High Plains than they are farther east, on average -- in fact, most High Plains outbreaks are associated with some significant meridional component aloft. Strongly-backed low-level flow can easily help mitigate these problems, as was the case on the dates I just listed in addition to the prolific W KS outbreak on 23 May 2008, off the top of my head. The best troughs IMO are negative tilt but also broad and low in amplitude (see 3 May 1999 or 29 May 2004), where you get the dynamical benefits of the tilt but generally lack the kinematic pitfalls like meridional flow, VBV, etc.

I can see where negative-tilt could seem undesirable if you have a short-term memory, though: this year, the overwhelming failure mode has been morning convection and early initiation. This stands in stark contrast to much of the past 3-4 years, when capping, PBL mixing, and a lack of low-level backing were all more common failure modes -- through much of years like 2012 and 2014, for instance, chasers would've killed just to get one negative tilt wave over rich moisture. I would submit that the persistently strong subtropical jet carving SW-NE across N Mexico, transporting poor lapse rates aloft over the southern Plains, has been a more dominant and causal factor for this year's failure mode than a preponderance of negative tilt troughs (after all, most of the troughs before last week weren't negative tilt, and early convection still ruined a lot of days). That said, I won't argue that negative tilt waves aren't more likely to result in a "messy" convective evolution.

As for Saturday, my expectations are tempered, too. Without morning convection, it looks like a solid MDT risk setup (for tornadoes) over much of the southern and central Plains, but that's not likely to be the reality. I think part of it is yet another case of poor timing, where the 500 mb height/vort map valid at 12z-18z Sat is what you'd like to see at 00z on a chase day. While I wouldn't be surprised to see overnight/early convection largely wreck the setup similarly to last Thu-Sat, I'd caution new chasers against assuming an outcome that catastrophic (to chase potential) every time overnight/early initiation is a problem. There are lots of big days in the vein of 5 May 2007 where destabilization is less than ideal as a result of early storms, but still plenty sufficient for tornadic storms closer to 00z. The MCS last Thursday morning put down very cold outflow for this time of year which spread far and wide across almost the entire southern Plains, and then could never really get dislodged before the trough cleared the area Sunday (partially due to additional MCSs, of course). That's pretty much the worst-case scenario and not all that common this late in the spring; something you'd more commonly see in March and April. Hopefully, even if Saturday's potential is mitigated by concerns over recovery and destabilization, it's not that bad.

 

[Brian McKibben]

New 00Z NAM looks much more promising for Saturday across Oklahoma. My original concern in the early morning convection. It actually starts out as convection in NM on Friday evening and works its way all the way to Arkansas. New 00z run shows about 200 miles of spacing between early convection (just east of I-35) and DL storms (Eastern TX panhandle). This should give enough room/time for the atmosphere to destabilize. Looking at about 1500 MLCAPE. While this isn't great, just a little more would be enough to get things done with the strong low level shear. LLJ cranks up to near 50 kt by 00z and easily 50+ by 03z.

Looking that the 4km NAM, it shows the precip over the TX panhandle Fri night as well. By Sat AM it doesn't look to be as robust so one can only hope that it dies out. If we can get some more clearing it could be a nice show.

 

[Jeff Snyder]

Negatively-tilted troughs can produce robust severe weather events owing to the strong ageostrophic response and flow that can occur when a big vort max swings though (think strong upper-level divergence and associated cooling aloft, strong pressure falls and isallobaric response in the low levels that backs winds in the low levels, etc.) . The significant curvature near the base of the trough (where air parcels turn from, say, moving to the east to moving to the north) and the low-level adjustments that occur can result in strong low-level shear in the warm sector. If sufficient moisture is in place and adequate CAPE can develop, a relatively large risk area can develop. Even from a purely hydrostatic standpoint, bringing low 500 mb heights southeastward over a moist warm sector implies the potential for steep lapse rates. One just needs to be careful not to get too far north to avoid the southerly (or southeasterly) mid- and upper-level flow often found downstream of negatively-tilted troughs. By pure pattern recognition, I really like those troughs that have a strong jet streak on the upstream (i.e., western) side of the trough; I don't like troughs (no matter if it's negatively tilted) where the strongest flow is on the downstream (i.e., eastern) side of the trough. @Jeff Duda referred to these as "front-loaded troughs" recently if I recall correctly. The trough on Saturday is more "back loaded" in terms of the main jet streak being upsteam of the trough axis.

The 12z NAM is trending a bit better, IMO, for Saturday. Note that I'm only looking at areas between I70 and I20, and I'm biasing towards western OK because I'd rather only have to drive 2-3 hours instead of 5-6 hours. All that being said, the latest 12z NAM now forecasts 500-250 mb flow to be more less meridional, with SW mid- and upper-tropospheric flow across most of western OK and western N TX. In turn, the really nasty S-shaped hodographs look a little less likely, although many hodographs still have a suboptimal shape. One of the main issues remains the potential for early-day convection to limit destabilization during the afternoon. We've seen this numerous times this year already (4/8, 4/24, 5/8, 5/9, etc.), and it looks like we'll face this problem Saturday. Low-level flow is fine, and moisture should be OK pending issues with outflow from previous convection. Predictability is low, to state the obvious.

60-80 kts @ 500 mb = fast storm motions. Vroom vroom everyone!

The setup Friday looks to be just beyond my chasing range/domain for a week day, so I'm hoping I can see a pretty supercell somewhere Saturday...

 

[Jeff Snyder]

Since I can't edit my previous post any more, I wanted to add that the 00z ECMWF initiates convection by 12z in the TX panhandle and moves it eastward through the day. The 21z forecast 2 m temperatures are 55-65 F across most of western OK and adjacent portions of the TX panhandle and southern KS with 700-1500 j/kg SBCAPE across most of that area (for mid May, this is pretty unimpressive!). It does destabilize behind all of the early junk by 0z similar to what happened last Saturday, but the results may be the same (including perhaps needing to get S of the early stuff -- the ECMWF leaves open the possibility of somewhere S of Seymour being OK). Otherwise, the 00z Euro does not look favorable for siggy supercells anywhere within a 4-5 hr drive of OKC.

 

[Jeff Duda]

Since I can't edit my previous post any more, I wanted to add that the 00z ECMWF initiates convection by 12z in the TX panhandle and moves it eastward through the day. The 21z forecast 2 m temperatures are 55-65 F across most of western OK and adjacent portions of the TX panhandle and southern KS with 700-1500 j/kg SBCAPE across most of that area (for mid May, this is pretty unimpressive!). It does destabilize behind all of the early junk by 0z similar to what happened last Saturday, but the results may be the same (including perhaps needing to get S of the early stuff -- the ECMWF leaves open the possibility of somewhere S of Seymour being OK). Otherwise, the 00z Euro does not look favorable for siggy supercells anywhere within a 4-5 hr drive of OKC.

Interesting. The 00Z MPAS forecasts a scenario that makes me a bit nervous, especially for the OKC metro. It doesn't prog nearly as much precip over the region early in the day and sets up the DL just west of I-35 with some pretty rich moisture and temps reaching the low 80s as scattered supercells pop up and move off into the warm sector throughout the afternoon and evening. In fact, it shows a good three rounds of initiation across OK from the mid-afternoon through late evening (although the late evening round is down by the Red River). There are many high-UH tracks associated with these storms, including one rather big one that tracks very close to the OKC metro area just after 00Z.

Additionally, the 12Z 4 km NAM portrays a similar scenario except 100 some miles west.

So it looks like predictability is actually pretty low given the multi-model spread. Synoptic scale predictability for this event seems to be rather high, but it's all those mesoscale details that will really determine how the day goes.

22Z hourly max UH from the 00Z MPAS:

 

[Andy Berrington]

Looks like some of those other tracks are awfully close to ICT as well. I'm getting the suspicion that the Euro is going to end up being low with instability in the areas not stabilized by morning convection, given recent performance (although obviously the recent upgrade might throw a hitch in that logic). Also, the lack of low level wind response on the 12z GFS makes no sense given the location of the secondary surface low/etc.

@Jeff Duda How well did the MPAS do at that range for the previous event? Was it getting more consistent closer in or was it good even from >60 hrs out?

 

[Jeff Duda]

@Jeff Duda How well did the MPAS do at that range for the previous event? Was it getting more consistent closer in or was it good even from >60 hrs out?

In my subjective opinion it turned out to be pretty close on the mesoscale details. I don't think anyone should just throw out the MPAS because of its infancy and lack of performance history.

 

[Andy Berrington]

In my subjective opinion it turned out to be pretty close on the mesoscale details. I don't think anyone should just throw out the MPAS because of its infancy and lack of performance history.

Understood, I was asking since that is quite a high end looking solution there (especially if there's more to come afterwards).

 

[Brett Roberts]

@Jeff Duda: are those MPAS images public? If so, what site can they be found on?

In both an NWC seminar last week and hearing anecdotally from those who have been viewing its output this spring, it sounds like the current configuration of MPAS may have a notable eastward bias in dryline/convection placement, at least for Plains events. In the seminar, its 2-3 day forecasts for 20 May 2013 had convection mainly in eastern OK, for example. Just something to keep in mind, for those of you viewing the image above who haven't heard of the model before.

The forecast for Saturday is pretty tough, and not necessarily getting easier as lead time decreases. The NAM continues to show significant potential across OK/TX, whereas the ECMWF (as Jeff Snyder mentioned) and GFS simply smother the entire warm sector with convection all morning into the afternoon, probably decimating another potential big day in the style we saw repeatedly last week. Given it's still a 60+ hour forecast and letting persistence be the tiebreaker, I'm leaning toward the GFS/ECMWF scenario for now. The MPAS depiction is interesting, though.

 

[Jeff Duda]

@Jeff Duda: are those MPAS images public? If so, what site can they be found on?

STAFF NOTE: since I'm getting many requests from users about where to find MPAS output, I have posted a new thread here: https://stormtrack.org/threads/new-nwp-model-model-for-prediction-across-scales-mpas.28425/
However, I will post the link here since it is relevant to the thread: http://wrf-model.org/plots/realtime_mpasconv.php