2012-03-18 FCST: TX/OK/KS/NE

[Stan Rose]

Ordinarily I'd refrain from commenting on a highly conditional chase this far out, but hey, it's a weekend and I'm dying to hit the road.

Main headache is the evolution of the big trough and cutoff low out West; every run has come up with something different. Last night, both EC and GFS were bringing it through as a progressive open wave, now it's back in the 06Z GFS as a cut-off, but who knows where it's going to end up by early next week.

Nevertheless, there's enough consistency early on to be confident that the svr chances should pick up over the C and S Plains by Sunday. Upper low center should be moving into the Great Basin and 4 corners region by late Sunday. Ahead of the system, strong mid-level flow in the 50-70 kt range will overspread the TX and OK panhandles and W KS. The dryline will mix EWD pretty quickly in the afternoon, but moisture E of the dryline should be adequate; 50s and possibly 60s. A low level jet of 30-40 Kts will develop into the evening and will keep moisture advection going through the night.

My current thinking is that convergence along and E of the dryline will allow for initiation by late afternoon to early evening over the Ern panhandles, in the relatively narrow tongue of modest CAPE of around ~1500 J/Kg that will be present. Shear will be more than adequate for rotating updrafts; my main concern is the strong meridional flow, which could organize things in a linear fashion especially by evening. Hopefully there will be some pockets of good helicity near the dryline.

So, a bit IFFY, but could be a good early season chase, a good chance to shake off the rust and prepare for April-June!

 

[Rocky Rascovich]

This could be quite an interesting event if it all comes together. Cyclogenesis occurring across the central high plains across SW. Neb. and a stout mid level speed max blasting into the TX. PH, that coupled with rich low level moisture,bulk shear values approaching 50kts, not to mention helicity values averaging around 300 m/s across the TX. PH on up to southern NE. ..
well, you get the picture. This could be the first significant severe weather episode for the season for the aforementioned areas. Looks as though the timing with peak afternoon heating and the approach of the initial mid level wave, interacting with near 2000 g/kg. cape, adequate EHI values and sufficiently low LCL's could mean some robust updrafts commencing by mid afternoon from roughly HLC to AMA and possibly further north into NE. Initially, I can see cells staying somewhat discrete until early evening then congealing into clusters, then a squall line by late night across Cntrl. OK. and due to an increasing low level jet, storms should have plenty of juice to work with through the night. TOR's, 2-3 inch diameter hail and damaging straight line wind gusts will be likely. Unfortunately, I won't be able to chase, so I'm sure that's all the more reason it could be a big day.

 

[Gabe Garfield]

Sunday could be a very good chase day, indeed. At this range from the event, the consensus forecast of a negatively-tilted mid/upper tropospheric trough impinging on the west Texas dryline around peak heating portends the tipping of the atmospheric hand -- which appears to feature a few aces. In general, I like the forecast of adequate (40 - 60 kts) deep-layer shear over the southern High Plains and the associated (chaseable) storm motion (20-30 kts). Also, given dewpoints in the upper 50s - lower 60s and the presence of steep lapse rates, moderate instability (CAPE ~ 2500 J/g) should develop over the eastern Texas Panhandle. Finally, the formation of a strong low-level jet (40+ kts) will result in significant low-level shear (0 - 1 km SRH > 200 m2/s2) which -- along with the aforementioned parameters -- should yield a significant tornado threat, given an isolated supercell.

Some limiting factors include relatively-weak, backing winds in the upper-troposphere, less-than-ideal moisture (though, not bad), a cirrus shield, and potential storm mode problems (i.e.,squall line). Even with these cons, it certainly looks like the pros will be worth the long drive (I'll be starting in Houston).

 

[Brett Roberts]

I'm surprised at the lack of discussion for this system the past few days. The setup for Sunday, while imperfect, looks quite potent for mid-March. Even if today were May 15 instead of March 15, it would probably catch my attention and I'd plan to chase it.

The model trend over the past 24-48 hours has been generally toward a broader, more progressive trough, which should help the case for significant severe weather on Sunday. Prolonged eastern ridging has already allowed rich low-level moisture to overspread much of the central U.S., so we certainly won't be waiting on last-minute return flow. The only caveat is that the ridge is forecast to retrograde and amplify over the Midwest/Southeast between now and the weekend, which might mean slightly less favorable low-level trajectories come Sunday and Monday (i.e., slightly more northeasterly flow into the Gulf instead of easterly). If the models continue their current trend of beating down that ridge a bit more each run, the moisture situation could improve further.

Current NAM/GFS dew point forecasts show upper 50s immediately ahead of the dryline at 00z Mon. If the dryline sets up along the OK/TX border, as suggested by the GFS, this might be somewhat marginal and present problems with LCLs. However, given the trough axis is still hanging back in AZ by Sunday evening, I suspect the dryline will actually be located along the Caprock, if not farther west. If this is the case, surface dew points of 57-61 F should be more than sufficient for a tornado threat, given the elevation.

The other concern I have is with the lack of veering above H5. Forecast soundings show 200-300 mb flow that is slightly more backed than the mid-levels, resulting in hodographs that are beautiful below 3-4 km AGL, but rather messy above. In my experience, it's difficult to discern in advance how much of a role this will play in mitigating classic supercell structures (and tornado potential, by extension). I've seen days where it seemed to be a big problem (26 April 2009), and others where it hasn't (20 June 2011, 22 May 2008). This is another concern that could be alleviated by the trend toward a broader trough and less blocking downstream, if said trend continues. If not, then the tornado potential will be a fairly low-confidence forecast even as we head out the door Sunday morning, from my point of view.

My initial area of interest from three days out is roughly Perryton to Clarendon. Low-level shear is likely to impress in this area, barring drastic changes, and instability should be moderate to high for March. Deep-layer shear will be large in quantity, but the quality (directional shear above 500 mb) is still a big question mark. Initiation does not look like too big a worry with a sharp dryline, minimal CINH, and the trend toward faster arrival of upper support. At minimum, we should see a couple hours of intense supercells producing giant hail; at most, this could be an awfully impressive March chase day, IMO.

 

[Skip Talbot]

As the NAM and GFS are currently plotting this event, I don't see it as a big tornado producer. Isolated tornadoes are certainly possible on one or two discrete storms, however, so this means we could still have a major chase event. The NAM is showing favorable instability peaking above 2000 J/Kg across portions of the TX panhandle by late afternoon. It drops off significantly before, 0z which is troublesome. The GFS is far less bullish on the instability. It has a tendency to under forecast instability, but seeing barely 500 J/Kg across the panhandle is troublesome. There seems to be quite a bit of difference between the two models in magnitude of instability and cap strength.

My main issues with this event, as the NAM is currently plotting it, is that the instability and directional shear combinations are just not there. The NAM is plotting 2000 J/Kg over a pretty narrow sliver of the panhandle at 21z. Effective SRH at this hour is practically 0. At 0z, there are pockets of increased SRH showing up, but the instability drops off markedly. I'm guessing this is a combination of convective feedback and the surface temperatures dropping off quickly as the sun lowers due to the high dewpoint spreads. Even at 0z, I see a pocket of 100 SRH behind where the model is moving a line of storms. That's not knocking my socks off. Severe and tornado parameters are pretty paltry as a result of these combinations. If that surface low were parked down in SE CO instead of Montana, and that trough were a few hours earlier on its arrival, I think this would be a different story.

LCL's are approaching almost 2000 where the NAM has storms initiating, due to dewpoint spreads close to 20 degrees. This improves a bit with time as the boundary layer cools some by 0z. The trick is getting the initial high based activity to stay discrete long enough before it all goes into an outflowy MCS.

The GFS's solution isn't even chaseable in my opinion due to insufficient instability and a stouter looking cap. I'm more inclined to side with the NAM at this range, however.

Modest to moderate instability, a weakening afternoon cap, more than adequate deep layer shear, and lift along the caprock should yield a few supercells across the Texas panhandle. However, lackluster 1km SRH and rather high LCL's may diminish the tornado potential. If a storm can realize some enhanced directional shear from a subtle surface boundary, or upper level disturbance, while surviving long enough to overcome the high LCL's and dewpoint spreads of the afternoon heating, I think we could see a couple of tornadoes here. This looks like a fairly conditional event at this point though, especially given model inconsistency. I'd be out for sure if I were in the TX PH or even Norman. A marathon run from anywhere else looks like a bit of gamble, so I'm not sold on making the trip quite yet unless I see the models converge and a more favorable trend with some of these negative points.

 

[Chip Redmond]

Watching the northern Central Plains as a trip to TX maybe a bit much for classes the next day. Initiation is a bigger issue this far north. Neglecting the nasty and disheartening GFS run at 12z, I am staying optimistic with the 12z NAM. Projected helicity is fairly high around 300 (m/s)^2 and with dewpoints just ahead of the dryline near 60F, there is easily 1500J/kg of CAPE. The negative tilting the trough has is much more appreciable than the neutral high amplitude that was showing the past few days. I really wish the 500mb trough would be a bit farther to the north and give a more westerly direction, but there is still a considerable 850-500mb crossover that will aid in low level rotation.

One concern I really have is if a storm does get going, upper level winds are fairly weak and maybe storms topple over themselves and become more cold pool driven. I wouldn't mind seeing that trough de-amplify a bit and drifting farther to the north.

Having actually never chased in central Nebraska, I am not familiar with the road network in this area. Hoping this is a good chance at checking it out and perhaps even getting a supercell in the meantime. Just need some lightning! Heck, even clouds! I'm almost getting tired of this sunny weather in SD...

edit: Think I overlooked the mere fact of convergence along the dryline that far south. Southerly winds behind the dryline could be a big issue... 18z NAM isn't changed much, although 500mb winds have improved slightly.


Chip

 

[Gabe Garfield]

As the NAM and GFS are currently plotting this event, I don't see it as a big tornado producer. Isolated tornadoes are certainly possible on one or two discrete storms, however, so this means we could still have a major chase event. The NAM is showing favorable instability peaking above 2000 J/Kg across portions of the TX panhandle by late afternoon. It drops off significantly before, 0z which is troublesome. The GFS is far less bullish on the instability. It has a tendency to under forecast instability, but seeing barely 500 J/Kg across the panhandle is troublesome. There seems to be quite a bit of difference between the two models in magnitude of instability and cap strength...

The dropoff in CAPE from 21 to 00Z in the NAM is due to the convective parameterization in the model: anytime you generate convection in the model, it reduces the instability over a broad area. In reality, though, I believe we can expect CAPE values similar to the preconvective environment. The GFS, as you mentioned, has a low bias. Additionally, the GFS CAPE seems to be reduced by surface cooling from a cirrus canopy associated with a subtle shortwave trough moving through west Texas early on Sunday. This shortwave trough is not as strong in the NAM.

My main issues with this event, as the NAM is currently plotting it, is that the instability and directional shear combinations are just not there. The NAM is plotting 2000 J/Kg over a pretty narrow sliver of the panhandle at 21z. Effective SRH at this hour is practically 0. At 0z, there are pockets of increased SRH showing up, but the instability drops off markedly. I'm guessing this is a combination of convective feedback and the surface temperatures dropping off quickly as the sun lowers due to the high dewpoint spreads. Even at 0z, I see a pocket of 100 SRH behind where the model is moving a line of storms. That's not knocking my socks off. Severe and tornado parameters are pretty paltry as a result of these combinations. If that surface low were parked down in SE CO instead of Montana, and that trough were a few hours earlier on its arrival, I think this would be a different story...

At first glance, the low-level shear doesn't look that great on the model pages. But, if you look at a forecast hodograph, it's not hard to imagine that -- with a little tweaking of the surface winds and a deviant storm motion -- very strong low-level shear could be realized. Physically, this makes sense. It's hard not to develop significant low-level shear when the low-level jet is > 40 kts.

LCL's are approaching almost 2000 where the NAM has storms initiating, due to dewpoint spreads close to 20 degrees. This improves a bit with time as the boundary layer cools some by 0z. The trick is getting the initial high based activity to stay discrete long enough before it all goes into an outflowy MCS...

This may be the biggest issue. I'd like to see dewpoints in the low 60s, but we may not be able to realize that. But, as Brett Roberts pointed out, there is still hope that we might eek out a little more moisture. Even so, the NAM forecasts LCLs < 1500 m, which isn't bad for tornadoes.

 

[Jeff Duda]

I've decided to hold off on analyzing details until the event comes into better focus in the short range models. Well it is now.

This definitely looks pretty decent for a March setup in the SGP, but there isn't anything that makes it "sparkle" in my eyes. Deep layer and low level shear look to be decent (around 40-50 kts), but not spectacular. The NAM has shear vectors more west-east than the GFS, so the storm mode may still be in question. I'd like to see a tad more instability, too. The NAM progs up to 2500 by 21Z across the eastern TX PH, W OK, and SW KS, but the GFS is a bit more stingy, only in the 1000-2000 range. The GFS is continuing its trend to progress the trough further east before cutting it off. I'd really like to see the trough axis farther east than it is. The GFS has had it pretty consistenly over the NV/UT border by 00Z, with some of the better flow just barely making it over the warm sector by then. That probably explains why the GFS holds off initiation until after 00Z. If the dryline could end up farther west, then this would look a bit better. Unfortunately , it looks like 850 mb winds will continue to remain SSWly, thus holding back any westward progression of the dryline over the next few days.

In summary, I think there's some phasing issues to worry about, and I'd like to see better instability. However, for March it still looks decent at this time.

 

[Chip Redmond]

Very disappointing 12z runs for GFS and NAM. Best moisture is off and east of the dryline. The vertical profiles aren't nearly as curved as they were last night. Yes upper-level winds have increased but overall deep layer directional shear isn't nearly as significant as it was before. Southerly winds post dryline will significantly hurt convergence.

Let the rollercoaster continue... it has plenty of time to change! Still hoping for a decent target in south-central Nebraska.

Chip

 

[Jeff Duda]

Very disappointing 12z runs for GFS and NAM. Best moisture is off and east of the dryline. The vertical profiles aren't nearly as curved as they were last night. Yes upper-level winds have increased but overall deep layer directional shear isn't nearly as significant as it was before. Southerly winds post dryline will significantly hurt convergence.

Yeah, that disconnect between the wind shift and the dewpoint gradient is baffling to me. I don't see any meteorological reason that should happen unless there's no true dryline there and it's some sort of messed up warm front instead. However, the moisture is already in place across the SGP. In fact, 60s Tds are already in place across much of W TX and the TX PH, and there won't be any major westward wind shifts to severely deplete the moisture supply before the event. S to SSE sfc flow will remain across S TX pretty consistently leading up to the event, so again, I have some problem believing the dryline won't end up a tad west of where it is progged. I guess only time will tell, though.

 

[Stan Rose]

Look at a sounding and you wont be baffled, Jeff. The moisture is super shallow, it's just mixing out. Bummer... But i havent given up hope yet.

 

[Jeff Duda]

Look at a sounding and you wont be baffled, Jeff. The moisture is super shallow, it's just mixing out. Bummer... But i havent given up hope yet.

That's logical, but my gut is telling me that's not the only thing causing the disconnect, especially given the Sly 850s that are progged to hold solid throughout the day.

 

[Chip Redmond]

Look at a sounding and you wont be baffled, Jeff. The moisture is super shallow, it's just mixing out. Bummer... But i havent given up hope yet.

You don't necessarily want a very deep layer of moisture. Dry upper-levels are key. I wonder even if the layer is shallow, that more dry air will mix down faster leading to a quicker advancing dryline that would develop a stronger wind shift? (edit: I suppose if you are mixing down winds as well, the southerly winds at 850mb even would mix down therefore still no wind shift...) Just a thought. As Jeff mentioned, moisture is already there and honestly models just may not have a good handle on it. Morning obs will be all telling in this situation, although it would boost confidence if the models would just say what we want to see!

The 500mb jetstreak has the exit region over SD/ND, so the best divergence aloft will be east of it in that region. I wonder if the subsidence in the entrance region already rounding the trough has anything to do it... though the strongest part of the jetstreak is still situated in east CO by 0z. Perhaps this convergence at the surface behind the dryline has anything to do with the issue as well.

Chip

 

[cdcollura]

Good day all,

Some of the later models (18z) were showing a bit more backing (more SSE instead of S) from SFC to 925 MB by 0z Monday (late Sunday afternoon). Meanwhile, the upper exit region's best part (above 500 MB) is still far to the NW of the surface dynamics (the best surface / backed winds are over North Central KS to S Central NE with the upper support trying to catch up still over NE CO / WY. Moisture should NOT be a problem with this setup. The only problem is the backing of the surface winds and required verring of the high-altitude flow. Right now, it appears to be mainly a squall line with very large hail and damaging winds as major threat.

Looking at the later models, I am seeing a bit more surface backing, and more SW flow at 300 MB, instead of due south or SSW (meridonal). It's a very similar to a case I saw in March 2007 (I believe the 28th?) and even similar to May 22, 2008 (less the CAPE) which both had fast northerly storm motions but both produced tornadoes as well.

I also found out that I cannot use my free tickets when I tried to book a flight from FL to MCI (Kansas City) from Sat to Monday - Thanks to Spring Break ... So this will be a sitout for me as well ;-(

 

[Jason Persoff]

Work kept me effectively away from weather surfing until today. Suddenly I
have time off for the next 40 hours and have a lot to consider...but I don't
have to be back at the hospital until Monday at 9am, so...

I'm sort of hamstrung with a wide swath of possibilities and a very narrow
window to get from DEN out east and then back. That definitely colors my
forecast quite a bit. The narrow region of the OK PH into
SC KS strikes me as a very good balance of shear, CAPE, and LCLs. I certainly
would like to see the next forecast runs before committing to an 8 hour car ride
each way.

Ultimately, my initial target is probably Liberal KS to Goodwell OK along US
54. Aside from geographic proximity to home, the things that just leap off the
page to me from the models include a wickedly demarkated dryline based on the
18z NAM with estimated LCLs dropping from the stratosphere down to just around 1
kft on either side of the dryline; an impressive, strengthening LLJ (slightly
displaced east of my target, but close enough to pump up the atmospheric
moisture); right exit region of the H5 jetstream with progged SSW winds AOA 50kt
(June-speed ); and the GFS (more so than the NAM) with precip forecast in the
area by 00z.

The negatives include lackluster rotational sheer (<90 deg rotation with
height from sfc to 500mb), mild (though seasonably impressive) CAPE, and the
potential for rapid linear development. Another big negative for me is the
potential for pre-frontal isolated storms firing further south and further
east--outside my comfortable driving range for this haul-a-palooza.

I impatiently await the 00z model runs...oh and to tell my wife that my only
day off in a few weeks may *not* necessarily be doing honey-dos .