4/29/10 FCST: TX, OK, KS, NE

[Mikey Gribble]

It's late April. You can't expect 65 to 70 degree dewpoints. I can't believe how few of people are on board with Thursday being a good setup. Look at the hodograph ahead of the dryline. CAPE > than 1500J/kg is fine. My biggest concern is LCL heights, especially if dewpoints don't break 60, but that is a limiting factor, not a deal breaker for tornado potential. Besides, the models have done a great job forecasting moisture return so far this year and the NAM actually increased dewpoints in the latest run. The makes me a lot more confident in the us getting decent moisture quality over the target area. I hate just in time moisture, but I'm not going to complain too much about it.

You can't over look wind shear like the models are showing for Thursday. When you get 50kts out of the south during daylight hours that should be a big red flag.
If we do get dewpoints greater than 60 and CAPE AOA 1500, then I think a tornado outbreak is a pretty good bet. I hate being the overly optimistic guy in the room, but I just don't see what there is to complain about. You are never going to get a picture perfect tornado day.

 

[Mikey Gribble]

Just finished taking a quick peek at the 0Z GFS, and I'm not overly impressed ATM. With unidirectional upper winds and slight backing at the surface, I would say we're looking at yet another squall line situation. Moisture is marginal, shear is weak. There appears to be a bit of a CAPE spike just south of the OKC metro at 0Z that could yield something of interest, but it's too soon to get too excited.

Shear vectors are pretty much normal to the boundary along the dryline, so I don't know why you would get a squall line off the dryline. Discrete storms should be the favored mode of convection.

Shear is weak??? Seroiusly? There is 50kts out of the south at 850mb and 60kts out of the SW at 500mb. Unless you are launching balloons in the great red spot on Jupiter that is strong wind shear. I'm not trying to be a smart ass here, but come on. 0-1km SRH was over 300m2/s2 with the OUN forecast sounding. What more can you want.

 

[Derek Weston]

It's late April. You can't expect 65 to 70 degree dewpoints. I can't believe how few of people are on board with Thursday being a good setup. Look at the hodograph ahead of the dryline. CAPE > than 1500J/kg is fine. My biggest concern is LCL heights, especially if dewpoints don't break 60, but that is a limiting factor, not a deal breaker for tornado potential. Besides, the models have done a great job forecasting moisture return so far this year and the NAM actually increased dewpoints in the latest run. The makes me a lot more confident in the us getting decent moisture quality over the target area. I hate just in time moisture, but I'm not going to complain too much about it.

You can't over look wind shear like the models are showing for Thursday. When you get 50kts out of the south during daylight hours that should be a big red flag.
If we do get dewpoints greater than 60 and CAPE AOA 1500, then I think a tornado outbreak is a pretty good bet. I hate being the overly optimistic guy in the room, but I just don't see what there is to complain about. You are never going to get a picture perfect tornado day.

I agree that the shear looks good, especially across the southern end of things. There is enough turning with height there -- evidenced by some impressive hodos -- and certainly vigorous enough. Should manage to avoid linear forcing down there.

For this area my biggest concern remains moisture. Agree that 1500 j/kg ought to do with that sort of shear.. however, the width of the area, and consistency of moisture still have me concerned. Quite a few gradients (in CAPE values) in a geographically small area of land. From watching the models for a few years now... this sort of return of moisture makes me wonder. I don't recall too many nice setups with this sort of 'skinny' CAPE 'struggling' CAPE. . . (again, just a pattern recognition sort of deal with models here)

For this reason, at least, I'm not thinking outbreak... though, would not be surprised to see tornadoes. (or close to)

That said, 0z is better than 12z NAM... CAPE looking a little better. Upper level flow more favorably oriented.

I am on the fence, honestly.

 

[Dean Baron]

I dont get why people are pessimistic about the setup. While it may not necessarily be an ideal setup, it still looks good.
Anyways, 00z WRF is definitely interesting. The new run brings in better moisture, CAPE, and Theta-E for NE/KS compared to the last run. Inhibition is broken down much better on the new run, H5 winds have veered more, and it looks like there is almost a new low in southeast Nebraska. Haven't taken a long look at the new run and gonna go back and compare it to the last run, but after a quick glance I'm feeling more confident in the northern play of this setup since getting to the southern target is out of the question.

 

[Scott Weberpal]

Sure, the lowest 2-3km of the hodographs looks nice, but above that, they are straight and long which will favor line segments later especially as the CF surges. Every convective model thus far has indicated a quick transition to linear mode, but hopefully the slower trending NAM is the verified solution.

I have no doubt that if the NAM verifies there will be supercells, but LCLs are a different story.

The joys of early season chasing.

 

[afischer]

The last few runs of the GFS have been trending gradually downward w.r.t. moisture return ahead of this system. The new 00Z run shows lowest 100mb ML dews in the 56-60F range ahead of the dryline by 00Z Fri. The last few runs of the ECMWF have been more consistent in showing ML dews in the mid 50s F ahead of the dryline by the same time. This certainly isn't optimal for tornado potential with a late April/bordering-on-early-May southern Plains system (in the absence of higher terrain and/or cooler-than-typical low- thru mid-level thermal profiles), so I agree high LCLs may be one of the bigger concerns. Haven't really looked at the last few runs of the NAM, but a quick glance at the 00Z NAM shows very similar BL moisture quality by 00Z Fri as the 00Z GFS run does.

If the background/synoptic pattern and model progs looked more favorable w.r.t. initiating and maintaining deep moist convection along the dryline south of the T.P. (where fcst hodos and deep shear vector orientations look quite nice for discrete tornadic supercells), as opposed to initiation being most likely along the cold front, I'd be less pessimistic about the overall chaseability of the set-up.

 

[Chris Carter]

Unless the GFS slows to more of a Euro solution with a slower system allowing for more a moisture return for the central plains. I think I will be spending my Thursday in Lincoln watching a movie.

 

[Kevin Rolfs]

I'm rather new at this so please correct me if I'm wrong, but I'm not seeing how the convection will go linear shortly after initiation. When I think of convection going linear, I imagine a cold front slamming into an unstable environment forcing convection to go up all at once along the boundary. As far as I can see, the cold front at 0Z Friday extends WSW from the low along the central KS/NE border into east central Colorado, while the dry line extends south from the low into far W OK and on south into W TX. This to me indicates discrete storms will be likely until the CF catches up the dry line sometime during the night when storms would form into a QLCS. This would give a fairly large window time for tornadic supercells to work their magic before it goes linear. I should clarify that all of this is per the 0Z NAM, but that is the model I am putting my stock into as of now because of its slower progression of the trough, and because it nearly always outperforms the GFS wrt to timing of systems.

The GFS does show the cold front catching up to dry line much faster resulting in greater forcing which spells linear storm mode earlier in storm evolution. BUT based on the NAM, I'm not sure why everyone is so worried about linear storm mode at this stage of the game. I give the GFS 24 hours before it comes around to the slower, more appealing solution the NAM portrays.

 

[Melanie Metz]

I think many tend to look at the least favorable setup to expect the worst. And to determine if it's worth the time and money if that model verifies. If GFS and NAM agree and both look good, it's easier to justify the chase.

But I also prefer to be the optimist and go with the better looking model run!

And if LCLs are high, maybe we can squeeze out a beautiful, higher based supercell with a long, high contrast tube! Positive thinking. It's rare that all parameters are perfect so you gotta try when most are in place.



Melanie

 

[Mikey Gribble]

I'm rather new at this so please correct me if I'm wrong, but I'm not seeing how the convection will go linear shortly after initiation. When I think of convection going linear, I imagine a cold front slamming into an unstable environment forcing convection to go up all at once along the boundary. As far as I can see, the cold front at 0Z Friday extends WSW from the low along the central KS/NE border into east central Colorado, while the dry line extends south from the low into far W OK and on south into W TX. This to me indicates discrete storms will be likely until the CF catches up the dry line sometime during the night when storms would form into a QLCS. This would give a fairly large window time for tornadic supercells to work their magic before it goes linear. I should clarify that all of this is per the 0Z NAM, but that is the model I am putting my stock into as of now because of its slower progression of the trough, and because it nearly always outperforms the GFS wrt to timing of systems.

The GFS does show the cold front catching up to dry line much faster resulting in greater forcing which spells linear storm mode earlier in storm evolution. BUT based on the NAM, I'm not sure why everyone is so worried about linear storm mode at this stage of the game. I give the GFS 24 hours before it comes around to the slower, more appealing solution the NAM portrays.

I think you are pretty much right. I haven't looked at shear vectors for the cold front, but along the dryline if the shear vectors stay as they are, discrete storms should be the favored mode of convection. I need to look more closely at SR winds, but elongated supercells may be possible, but (without looking) I don't think the upper level SR winds were that strong. I don't know much about how the SR winds tend to distort storms though.

I agree that LCL's and the cap are by far the two biggest issues with this setup. LCL's only mitigate the tornado threat, but the cap can kill it completely. LCL's should be low enough for a tornado threat, especially late in the evening if we get dewpoints of 60 (I think we will). The cap on the other hand is impossible to predict. Figuring out when and if storms will fire is probably the most difficult part of forecasting IMO. My policy when the cap could go either way is to go out there and hope for the best. I'd rather go out and have a blue sky bust than sit at home and watch other people chase a tornadic storm that I should be on.

There are a few things that are working against us with this setup. One is low level winds veering early in the day. First of all I hate depending on veering winds to back (just one more thing to worry about), but more importantly this tends to widen the moisture gradient and lower dewpoints ahead of the dryline. That is something we can't really afford with this setup.
Another much smaller consideration is the idea that has been portrayed in a few runs that the low level winds will veer in the afternoon and we'll get a more northeast to southwest oriented boundary. That would severely damage the chances for tornadic storms for several reasons. The models have been consistent enough with a north to south dryline and backing 850mb winds that I'm not too concerned about this potential outcome.

I haven't looked at this mornings runs yet, but the cap and LCL's are the two obstacles we'll have to over come with this setup. IMO the high LCL's will only inhibit tornado potential and I think any discrete storm along the dryline will still have good tornado potential, especially late in the evening. The cap could be a deal breaker, but it does seem like a couple discrete cells are going to form along the dryline. All it takes is one storm.

 

[Jonathan Whitehead]

Well the 12z runs have come in a little more bullish on moisture return. And sometimes "just in time" moisture isnt a bad thing as was the case 2/10/09. But also with just in time moisture, the depth of the moisture isnt what you need for low LCLs and as a result the moisture can get mixed out by late afternoon and that could happen Thursday, esp with 60 dews. We'll know a lot more on what kind of moisture we'll be dealing with by tomorrow night looking at sfc obs along the TX coast.

Both models have 850 winds veered at 18Z but back them to straight out of the south ahead of the dryline by 00Z, however, Im worried about convergence along the dryline due the main upper energy being removed to the north so Im somewhat concerned that well have to rely on sfc heating alone to initiate convection. I say that b/c Im not seeing any distinct bulging of the dryline based off the models, however, if a sfc low develops late in the afternoon in the TX Panhandle as some of the models suggest, then that may be enough to back the sfc winds late in the day to create a small bulge.


Overall its a middle of the road type setup, not great, but not horrible either. My biggest concern is cap/convergence along the dryline. I want there to be storms first before I have to worry about quality of moisture. I say that b/c like Kevin Rolfs said, theres going to be no issues whatsoever with linear convection across OK/TX, if something goes, it will be supercellular. So if this setup were happening today, Id have to pull the trigger on heading out (and Im conservative on that aspect too, esp being unemployed). My target would be somewhere in the box bounded by SPS-LAW-LTS-Vernon and thats based strictly off the models.

Stay tuned, things will change.

 

[Jeff Snyder]

Man, look at how far the 60F isodrosotherm moves from 12z to 18z -- it starts S of the DFW metroplex at 12z, then jumps all the way to far southwestern IA by 18z. Now, perhaps there is some mixing down of higher-moisture air from just off the surface that is advected on the very strong LLJ overnight, but it's not like the LLJ is southerly through the night anyway (i.e. the LLJ is veered at southwesterly by 12z). Assuming the models are doing something in the surface-air interactions (evapotrans?) that is causing the moistening rapidly after sunrise, we'll have to wait and see if this materializes. Just ahead of the dryline, the 12z NAM currently has low-80s over near 60 (with dewpoint depressions in the 20-25+ range), which leads to very high LCLs and the potential for large boundary-layer evaporational cooling for convection that may initiate. Forecast soundings show a deep boundary layer through the afternoon and into the early evening.

The hodos, as we all know, look very good, though it seems as though the 12z NAM has a very more veering of the low-level flow at 12-18z. I'm worried that the veered winds are going to move the better moisture (marginal as it may be already) farther away from the DL.

Finally, we need to worry about initiation. Synoptically, the central/southern Plains area is on the warm side of the mid- and upper-level jet, and KS is in the right-exit region of the upper-level jet streak nosing into KS and NE. The transverse circulation associated with the jet streak suggests large-scale subsidence aloft over the KS part of the dryline, though the cyclonically-curved nature of the jet streak may mean some DPVA is possible to offset the downward motion otherwise expected in the right exit region of the jet streak. Unfortunately, the cooler mid-level air on the cool side of the mid- and upper-level jet streak stays back in eastern CO and far western KS in the dry air through 0z.

Don't get me wrong, I'll be chasing Thursday, and I'm not saying this is a bad setup. Rather, I'm pointing out my concerns that preclude me from forecasting a significant tornado day. In terms of targets, I'm inclined to bias myself northward into Kansas (PTT to HUT?) out of consideration of past events, but I'm also inclined to stay in OK to stay farther away from the right-exit region of the jet streak aloft and maybe closer to better moisture. I'm not going to chase N of I70 Thursday, so I haven't examined any of far northern KS or southern NE much. As such, any lack of discussion for those areas in this post should not be interpreted as my having a pessimistic view of those targets.

 

[Phil Moyer]

In terms of targets, I'm inclined to bias myself northward into Kansas (PTT to HUT?) out of consideration of past events, but I'm also inclined to stay in OK to stay farther away from the right-exit region of the jet streak aloft and maybe closer to better moisture. I'm not going to chase N of I70 Thursday, so I haven't examined any of far northern KS or southern NE much. As such, any lack of discussion for those areas in this post should not be interpreted as my having a pessimistic view of those targets.

Forgive my newbieness, but I'm looking at the 12Z NAM also, and it looks like a more southerly target would play better. Max CAPE and EHI are both along the dryline in central OK into Southern KS. What am I missing, please?

Phil

 

[Mickey Ptak]

Forgive my newbieness, but I'm looking at the 12Z NAM also, and it looks like a more southerly target would play better. Max CAPE and EHI are both along the dryline in central OK into Southern KS. What am I missing, please?

Phil

At this point, one concern further south along the DL into OK is the CAP thus the hindrance of initiation. EHI really means nothing if you don't have initiation.

If this was 12z on the morning of, my target would be in northern part of OK and I'll play Russian Roulette with the CAP in this case.

 

[Brian Stertz]

I am going to favor the Salina KS area as my initial target...can adjust north or south from there. Believe deeper convective chances will be higher south of I-70 (here a more westerly deep layer shear vector on this portion of dryline to get them off the boundary and into spin mode). Moisture depth & cap should keep things pretty isolated from looks of things. Looks like the DL tornadic supercell window may be pretty short...6-8pm from looks of things before things congeal into a heavy rain/hail line.

Plan on a late night update tonight on Vortex Times once I can get a good handle on 00z data pkg.