2015-04-08 EVENT: KS/OK/TX/MO/AR

[Scott Hammel]

because the lone "triple point storm" that's consistently been advertised by the NAM and its 4 km nest rides right along the stationary/warm front with low 50s F surface temperatures immediately to its N. I think we're going to need something to fire at least a little S of the true triple point to have a good chase day,

Without taking the NAM too literally, this was almost verbatim what I said earlier when discussing this forecast with my chase partner. Again, I realize the NAM isn't the best model to take stock in to when it comes to simulated reflectivity, but it is concerning to see it trend the triple point further south and east yet keep firing convection in the same spot north of the front. That storm could very well wander off into the low temps and be toast. Further down the dry line the overall parameters seem better for tornadoes but the cap is mighty strong. Like a bunch of people have mentioned already, perhaps the models will resolve better (particularly the GFS still being a hundreds of miles off outlier) once this system makes land fall tomorrow.

 

[Greg Blumberg]

Wow. I'm catching up to this thread and everyone is attempting to squeeze out every little detail from numerical models and observations! I'm really pleased to see such a good discussion. I have not looked very much at the numerical weather prediction models, but I wanted to help provide some constraints on the scenario that I'm seeing unfolding. My points below are also a ranking of the availability/certainty of severe weather ingredients in this setup.

1.) I do not believe that moisture will be an issue with this set up. I am normally quite critical about the moisture availability of severe weather setups, but this one does not have any significant signs of being an issue, despite the moderately shallow (1 km) moisture depth. I should add that the MIMIC-TPW graphic Kelton posted earlier has many caveats to it. The first is the distribution of the moisture in the atmosphere with height. This can significantly impact what could be called the fractional PWV with height, and in some instances the moisture aloft can contribute significantly to the total PWV. I did some tests today with this and found out that the 12 UTC CRP sounding had a little less than 50% of the available PWV above the surface moist layer. Secondly, the PWV retrieval (this is from polar orbiting satellites) this product is using is a non-linear equation model developed in 1990 that does have some small random errors and biases that are dependent upon latitude. It is not a retrieval based on physical radiative transfer models, which are substantially more accurate than a statistical retrieval such as the one being used here. In fact, there have been many updates to our knowledge of radiative transfer since 1990, and some of the assumptions in this product ought to be revisited. Lastly, this product assumes that PWV is a passive scalar (meaning that nothing modifies it with time) and that the advection is occurring by the surface to 600 mb mean wind. This means that this product should really only be used in environments where you have winds throughout a deep layer that are consistent with direction. Typically this is true in the tropical Atlantic, which is what this product was originally designed for. While I agree that the drying in retrieved PWV is interesting, it is probably not a concern given how small of an area it is, and how it may be tied to what looks to be a upper level (300 mb and above) cyclone that was moving east last I saw of it in the water vapor imagery. This may be differential advection problem that breaks the mean wind assumption of the product.

2.) I believe Andrew Lyons touched on this, but one of the things I noticed first with respect to today's sounding was the opportunity for the EML to strengthen in depth and in potential temperature. I often think that if you want to be a good severe weather forecaster, you need to master nailing the high temperature forecast. In this case, the 12 UTC El Paso sounding indicated a secondary inversion on top of the residual layer/EML. Looking at that sounding, and knowing how fast the nocturnal inversion would get mixed out given how shallow it was, I expected that the atmosphere would then begin to mix out the secondary inversion. This evening's 00 UTC soundings validated this forecast, putting a high around 83 F. This is pretty warm, and generally once you get a deep and strong EML layer, you've got to get rid of it through convection. This has me unsure about what the capping to moisture ratio will look like come Wednesday morning, as it looks like AMA will undergo something similar to what I saw in El Paso today. This brings me to my next point.

3.) Lift. If in fact we have a stronger than predicted EML, we'll really need this on our side. I've read many posts discussing the timing and quality of an upper level source of ascent, and the discussion is solely based on numerical weather prediction models. I've seen many cases like this where the timing is very questionable this far out and the solution is to just wait until the wave gets sampled adequately by the radiosonde network tomorrow evening. Usually issues like this is due to some substantial uncertainty upstream of the trough we are expecting. Your best bet will be to utilize ensembles here...SREF, NAEFS, etc. I have other things to do than to dissect those, especially given the uncertainty and my other work. If any one wants to look at those, have at it. It's not going to change what I see in the 12 UTC observations on Wednesday, unless mouse clicking is the butterfly flapping its wings needed to give us beautiful storms.

4.) I have no comments on the shear other than the aforementioned S-hodographs. Again, this will be primarily consequence of the trough structure, which is still quite uncertain.

 

[Kelton Halbert]

I'm going to go along a tangent that Greg brought up and talk a little about the SREF ensemble forecast for a few locations...

I've uploaded 21Z SREF soundings for 2 locations: Ponca City, OK, and Wichita, KS. If you would like to look at other locations, I would be more than happy to generate and upload them for further discussion.

For starters, I think this excellently displays just how much uncertainty there is in this forecast, and that it is vital to utilize every tool available to interrogate what this system is actually doing. This is also why I am/will be paying very close attention to the observations over the next few days, and supplement that information with subsequent model runs. Side rant: we are within the range of predictability based on observations, it's time to start using them!

While this discussion is primarily based on the KPNC sounding (my current target), these thoughts still apply for the other profile. Some important things that jump out at me:

1) There is considerably STRONG agreement on the mid/upper level lapse rates for this event. Nearly all ensemble members follow the same trace with only slight displacements to the left/right of the mean SREF member that is displayed. This is good - mid level lapse rates play a crucial part in parcel buoyancy in addition to the fact that steep mid level lapse rates allow for a more robust mass response at the surface when speaking from a QG/Synoptic lift perspective. Seeing such good agreement on this part is encouraging.

2) Low level lapse rates start to take on a little more ensemble spread. It's a little hard to see because they get so clustered, but there's essentially a clustering of models that mix out to the high 70s, and another clustering in the lower 70s. This could be due to a number of factors, including, but not limited to differences in EML strength at 850mb, and early convective initiation/outflow contamination. The NAM 4km has consistently blown up convection around 22-23Z, so at least a few of these may be convectively contaminated. I don't have the time to interrogate every SREF member, but it would be a good homework assignment if someone is interested in investigating the spread in the surface temperature forecast .

3) Dewpoints... HOLY SPREAD! As I'm sure this does not come as a surprise to anyone, the moisture profile spread in the low levels is rather large. We have all the way from 57 up to mid/upper 60s. This is likely a function of spread in 1) dryline placement, intensity of mixing, and depth of moisture. I tend to ignore the moisture spreads in the higher levels, but it should be noted that there is a lot of spread in the depth of moisture as well. Convective contamination from some members may be part of the problem here too. Again - I have not interrogated every member. I am, however, encouraged by the clustering of ensemble members that have higher than/near the mean DPs. Usually these clusterings are ensemble members with the same dynamical core (i.e. NMM/ARW), so it would be interesting to see which ones are clustering on the lower/higher ends.

4) Hodograph... HOLY FREAKING SPREAD BATMAN! I think this is an excellent example of how small uncertainties/changes in the wind profile can make huge differences. I mean, you have everything in there from beautiful sickles to ugly S shaped curves to N curves, etc. However, there is a trend in the mean to a sort of veer-back-veer profile as seen in the colored trace... not super thrilled about that. It's relatively high up compared to the effective inflow layer, going back to what Jeff Snyder said about ingesting negative vs. positive streamwise vorticity in the updraft when talking about the dynamic pressure perturbation equation (something that I've only learned about in the last few months from class... awesome stuff if you ever get the chance to learn it), but is still not something I would like to see when looking at this forecast.

Something else to note about the wind profile is the clustering of members that have a backed surface wind (looking at the wind barbs on the skewT). This is an interesting trend that could be worth keeping an eye on, but based on the hodograph, the clustering of those members is relatively small compared to those with nearly straight southerly winds at the surface. Something else Jeff Dudda brought up that I didn't think about was the storm relative wind and the venting of precipitation. Looking at the SRWind v. Height plot below the hodograph (which uses the ensemble mean data), it's definitely suboptimal.

And this is only breaking it down for one SREF run at a single time step with very little discussion about what recent observations have been trending towards, and if the SREF is an accurate representation of what we're seeing right now. And I also didn't even talk about a single parameter... who needs parameters anyway? Perhaps I'll save my observation discussion for tomorrow - I think I've written enough for one night

 

[Andy Berrington]

00z Euro continues with a more south-southwesterly/southerly LLJ, N-S oriented dryline and broader 500 mb trough on Wednesday as opposed to the rest of guidance. I will say that is has been very consistent in its solution over at least the last three runs, although the significant differences from the rest of the models is worth noting when considering its idea. This would have some rather large implications towards the low level convergence along the dryline and the possibility of initiation further south along it.

 

[Kevin Rider]

The 12z Nam run yesterday did not initialize the dewpoints well at all. It was a good 3-5 degrees higher than what was actually being reported which is interesting. Haven't had time to see how the 0z run initialized. Glancing at the 6z Nam real quick. Looks like the location of the triple point moved back northwest a little bit. Will be interesting to see if that new trend continues for the 12z run. I am not sure if we will see any storms very far south of the Kansas/Oklahoma border. Right now I plan on staying fairly close to Wichita to do any spotting for the city if something gets close.

 

[Lanny Dean]

..... I am not sure if we will see any storms very far south of the Kansas/Oklahoma border. Right now I plan on staying fairly close to Wichita to do any spotting for the city if something gets close.

Honestly, that might be the best play of the day.... Waiting game now as its basically just coming onshore

 

[Jared Orr]

I may not be properly speaking for Ben, but I think he may be talking about the storm relative flow in the upper troposphere. Look at the black star indicating the predicted right-mover motion. There's very little storm-relative flow between 300 and 200 mb, likely near anvil level. That kind of hodograph basically screams HP. Doesn't mean there won't be a supercell or even a tornadic supercell, but it will be HP/messy.

Okay, Jeff...Thanks for clarifying. Yes, it's starting to look more and more that way.

 

[Tim Marquis]

In regards to the moisture depth issue, I think what's been happening is that the models are having a hard time resolving the dry air that has been lingering in the gulf from the last cold frontal surge. Yes CRP and Brownsville this morning have very shallow moisture but if you look up river at DRT this morning, it is saturated up to 850 mb. The NAM suggests this dry air is even stronger expanding it to( DRT at 12z) than what's being observed. If you take a look at MIMIC's total precipitable water, you can see that this dry air is quickly moistening on the back side and decreasing in coverage. The 12z RAOB just happened to be launched in the strongest part of this dry air.

 

[Michael Snyder]

Just an FYI guys,

Real time Aircraft OBS that are coming in right now (compared to the 15Z forecast) match up very well with the 06Z GFS at both 200/500MB from the samples I took from just SW of KPHX and over the KSFO area.

Ill check some more OBS when the 12Z run comes in.

 

[Jeff Duda]

I'm going to go along a tangent that Greg brought up and talk a little about the SREF ensemble forecast for a few locations...

While I will always default to singing nothing but praises about ensembles over deterministic forecasts, I'd been having a hard time buying recent SREF forecasts for one reason, and one reason only: several runs have been progging precip breakout (and growth) starting as early as 15Z and certainly after 18Z. It develops precip over W/C OK and spreads it northeastward, effectively knocking down temps and CAPE across the area for the rest of the day.

However, other deterministic forecasts are starting to come in line with that. I see some WAA from the west across WC TX in the 12Z NAM that may correspond to this and it is putting out some light precipitation across the area (although this is one of the first NAM runs to show this). It is associated with a pulse of moisture at 850 mb, so it may not be bunk. Additionally, the 06Z 4 km NAM also shows scattered showers across OK throughout the day.

Finally, many have commented that we should wait until the full trough is within the rawinsonde network before getting too worried. Obviously I will continue to re-evaluate new forecasts as new data are gathered and used to force the models. However, given the insane amount of data that is assimilated by the NDAS and GDAS, which includes satellite radiances, satellite derived winds, ACARS obs and the like, I'm not sure how much the forecasts will change once the system moves inland. I certainly think the forecasts will shift, but it may be pretty minimal, and there's no guarantee that the shift will be in a favorable direction.

I'm hopeful (yet I remain a pessimist), but the nagging doubts in my mind about this system are becoming harder to overlook. I can only hope that future shifts in the forecasts are indeed in the favorable direction.

 

[Stan Rose]

Ensembles tend to share the biases of the underlying model--that's why you need an ensemble of ensembles for a situation like this, and that's basically what we are doing by nit-picking every deterministic run.
The 12Z NAM looks pretty darn good and has been holding steady since yesterday. The 06 GFS looks terrible--so the 12Z GFS will be the tie-breaker, lol!

 

[Gabe Garfield]

Those are good points, Jeff.

The major models are still in disagreement, so the big picture will undoubtedly change. The American model suite (NAM/GFS) continues to be bullish in veering the low-level winds during the day on Wednesday, creating a stout cap; convective initiation is limited to Kansas, where winds are more backed and the effects of the EML are not so prominent. The Euro on the other hand, continues to forecast more-backed southerly low-level winds; the effect, of course, is to reduce CIN and produce storms (which are forecast along the length of the dryline to the Red River).

Now, for some shameless model cherry-picking (4 km NAM - 12Z April 7):

:

 

[Zack Hargrove]

IMO, the 06z NAM seemed to take a small step forward and the 12z continued that trend. It still doesn't make the setup a slam dunk, but it is at least a bit encouraging. The dryline orientation still isn't ideal, but from OKC to ICT... it has taken on more of a north/south look. There may even be the slightest bulge around the same latitude as OKC. The 850s are still SW along the dryline, but the 925s are more southerly and even a bit southeasterly in some areas.

The triple point is still the obvious target, but I'm a little more optimistic at least one or two more storms will fire south near or just on the other side of the OK/KS border... especially after seeing the 12z 4km NAM. The ECMWF also still looks quite optimistic about the potential for tomorrow. My main concern remains with the weaker upper level flow and the HP tendencies.

 

[Jeff Snyder]

This morning's soundings from the southern Plains capture a well-developed EML with very steep lapse rates. Again, whether this ends up supporting or supressing convection tomorrow depends upon if surface-based convection can break the cap at the base of the EML. Mean mixing ratios (presumably the mean over the lowest 100 mb -- whatever the SPC soundings show) at CRP, LCH, and DRT are > 14 g/kg, which is healthy for this time of year.

Interpreting the 12z 4 km NAM is a bit difficult for tomorrow's forecast since it blows up convection near the front this evening and produces an MCS overnight. That complex lays down an OFB oriented SE/NW across northeastern OK into southern KS, and I'd expect this OFB to affect storm chances during peak heating tomorrow. If the MCS doesn't develop as forecast, I'd expect the 4km NAM to not verify particularly well. That said, it's certainly something to look for given the potential enhancement of sfc convergence that may accompany such an OFB tomorrow. 09z SREF shows some support for convective QPF near the OK/KS border this evening and overnight. However, it also shows >70% of members with >0.01" convective QPF by 18z across north-central OK tomorrow (that is, early initiation that moves northeastward into KS during the early and mid afternoon hours). That's a good 3+ hours early for my tastes given the capping and potentially for stabilization of the near-ground layer owing to cloud cover from that convection.

A quick glance at the 12z NAM shows a slightly more favorable setup tomorrow for a non-trivial event, IMO. The hodos look supportive of siggy tors with stronger 700 mb winds and no nasty S shapes for the most part (in fact, some hodographs look quite supportive!). I'm still uncertain as to how the cap will weaken during the day, but that's not an uncommon uncertainty on days like tomorrow. Again, these are details that will change between now and tomorrow afternoon, so I won't give them too much attention.

 

[Russ Conlon]

12Z GFS confirms 12Z NAM for the most part regarding surface features. Both are projecting no cap over NW and N Central Oklahoma and south central Kansas at 21Z. Dryline orientation and wind profiles look a heck of a lot better than they did on the previous 2 runs of both models. Any target between Enid to Blackwell, OK north to Wichita, KS looks good. Personally I think the models are just messing us all around