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

[Derrick Herndon]

NAM continues trend of everything coming together right around sunset. Not a fan. CAPE is down some on this 00Z run. I am also a little concerned about warm sector cirrus with this jet. Something that has not been mentioned. Surface heating is forecast to be just barely enough to break the cap but too much cirrus and we may not get there. That leaves the dryline, which is forecast to retreat West in KS as height falls and u/l divergence force synoptic low to redevelop near KS/CO/NE border. Should that verify the backed low level flow will make for some nice low level shear profiles. But will convection fire before dark? Hard to say with certainty right now. This system needed another 12-24 hours of moisture return.

 

[Jeff Snyder]

The current dewpoint in Houston is 40, with nearby obs in the low 40s. Two buoys/platforms in the northwestern GoM (S of the TX/LA border, E of CRP) are currently reporting 50 and 52 F dewpoints. The nearest 60F Td is at BRO.

The CAPE forecast is down from past runs because the mid-level trough is trending a bit farther north and west. Comparing 500 mb temps Thursday evening from this morning's run vs. this evening's run is pretty telling. There are some areas like in northwestern Oklahoma where the 00z NAM has 500 mb temps 3-4 C warmer than the 12z NAM had. Looking at the 0-30 mb mean Td, it looks like primarily 57-59F in Kansas and maybe 60-62F in OK. With temps in the upper-70s to mid-80s, Td depressions look to be widespread 20-30 F. Fortunately, the 21z SREF makes me a bit more hopeful in terms of low-level moisture, with the ensemble mean 60F isodrosotherm from near Arkansas City, KS, to Lawton, OK (and the Td gradient may be greater than the SREF is forecasting owing to the resolution of the SREF). Not coincidentally, the 21z SREF also shows a non-zero prob of convective initiation (primarily N of the OK/KS border) by 0z.

The hodos still look good, but they've always looked good for Thursday. The hodo doesn't matter if we don't get initiation, and, if the mid-level warming trends from the 12z to 00z NAM hold, it may be tough to get initiation. Of course, we still have the well-known problems with LCLs and low boundary layer RH. Again, I'll probably be chasing anyway, but the trends aren't good, IMO. If only we had another 12-24 hours of moisture return, since the good juice is moving into central TX by Thursday evening.

 

[John MacKay]

It's a bit early with all the uncertainty, but there are some features concerning me in the 00z GFS and NAM runs.

The wind fields the last few forecast periods on the NAM and GFS don't give me the confidence in the moisture return we need. The winds get off track a couple of times from Thu/00z to Thu/18z (i.e, southwesterly at 850mb in OK and KS) leading me to worry about the quality of the moisture. Also, the system in the eastern US will cut off quality moisture return from the northern Gulf for the next 24 hrs or so. With a cap as strong as it is along the dryline, I have serious doubts on initiation if dewpoints do not get to at least the 62-64 area.

There is no doubt the rest of the setup is great for tornado potential. If I lived close to the dryline, I would go out just because (as Mikey has said earlier) you don't get winds this strong that often, and you won't forgive yourself if you miss it. I'd try to setup somewhere in between Newton (KS) and Blackwell/Tonkawa (OK) at the moment, setting up near an east-west highway (50, 54, 160, 166, 60). This will likely change in the next 24-36 hrs. once the moisture return starts.

 

[Phil Moyer]

Looking at the 12Z NAM run, I'm getting discouraged. At first I was excited by the potential moisture advection with that 850mb trough, but that doesn't seem like it's going to be enough. Projected dewpoints don't look all that promising, and the cap is strengthening. Alongside that, CAPE is weakening. I'm not sure there's enough energy there for initiation.

It will be interesting to see how it all plays out, since looking at actual events and comparing them our thoughts 24 to 48 hours earlier is always educational, and not infrequently amusing.

(BTW, I learn more from this site than almost anywhere on the Net, regardless of the discussion domain. Many thanks.)

 

[Niccolò Ubalducci]

Hello everyone,

As for the setup tomorrow, after seeing the latest issue of WRF are less pessimistic than I was this early morning. The dewpoint at NorthEast of Salina KS are discrete around 62 degrees with 1500 J/kg CAPE. The thing that strikes me is the dry buldge expected on Salina KS with a good convergence to the ground. This convergence will definitely be the trigger for the erosion of the CAP that although this will not be too high to prevent the starting of the Convention. The cells may become immediately supercellular, this could be favorable for the tornadogenesis, which will have a couple of hours to take place before the system is linear.

Regarding the vertical profile of winds, we will have a significant SpeedShear unidirectional, not the maximum for a durable Tornadogenesis, but a breaf Tornado will be possible in the first hour when the cells can be isolated yet.

So, I am moderately optimistic about the setup tomorrow, I think a pair of Tornado between KS / NE / MO are possible late afternoon.

Starting target: Salina, KS, with option to go a little to East.

 

[Niccolò Ubalducci]

The last update of Spc confirm my idea about tomorrow. Salina would say that at this point is already located in the west too, I think a great preliminary target is located halfway between Salina and Topeka, to chase the tail end of a probable squall line late afternoon.

 

[Jeff Duda]

Well it looks like at least the 12Z GFS this morning took a squat on the northern target areas (NE/IA), and even some of the southern areas. It looks to me like there will be no daytime initiation away from the front from MN through SE NE. Any initiation that does occur is going to have to be forced by the front -> linear storm mode. I don't see any triggering mechanisms out ahead of it.

I'm still surprised by the amount/speed of progged moisture return. I think the GFS's dewpoint progs (not quite reaching 60 through most of KS/NE/IA) is more realistic, especially given the current dewpoints. It looks like the gulf is getting set to open up in the next few hours here, a wee bit sooner at the low levels (925 and 850 mb) than at the surface, but boy, that is a lot of moisture to advect in a short amount of time. I think the instability will suffer unless that kind of advection really happens.

The 12Z WRF-NMM doesn't look as bad as the 12Z GFS, but it has been progging less shear overall, and more straight-lined hodographs oriented with the boundary in the northern area. Again that seems to favor a linear storm mode. I've been back and forth on going out Thursday for the past few days, but after looking at this morning's runs, I'm leaning towards saving my money. Hell, if the system slows down enough, I might get lucky and be able to do some local spotting/chasing of a squall line in IA for Friday.

 

[Mikey Gribble]

Very tough forecast for tomorrow. If you are going to play the dryline in the KS/OK area picking a target is pretty easy IMO, but determining whether or not we'll get a storm and what they'll be capable of if we do is not so easy.

The NAM backed off a little on the quality of low level shear with the latest run. It is still more than adequate for a good tornado threat, but it isn't the 50kts at 850mb it was showing before.

I think we have about a 50/50 shot of getting a storm off the dryline in the southern Kansas to northern Oklahoma area. The models have shown convection breaking out near the OK/KS border with several runs over the last few days. Today the GFS doesn't have anything going along the dryline, but the NAM does show precip along the border. This is also where the NAM shows the thermal axis intersecting the dryline.

What has me down about this setup is that there are so many eventualities the models have hinted at that could wipe the tornado potential out tomorrow. The most obvious one is the cap, but in addition to that veering low level winds and a wide moisture gradient could also be killers. Having several things that HAVE to go your way doesn't lend a lot of confidence with this setup.

It looked like the NAM had decent moisture arriving on scene a little earlier than it had in previous runs, which is nice. Regardless, waiting on moisture and winds to back while you're in the field doesn't work out very often for me. I do think we'll get 60 degrees at the border or very close to it. The models have done a great job with moisture so far this year IMO. I have no reason to doubt them now (may tomorrow though lol). I'm more worried about low level winds not backing in the afternoon. The GFS has been the primary backer of that idea and typically I favor the NAM inside of 72 hours, but it still has me worried.

Assuming that we do get 60 degree dewpoints up here, 850mb winds back as forecast by the NAM and we do get a storm to develop along the dryline (lot of assumptions there), then I think there will still be some decent tornado potential. The biggest obstacle to tornado potential is the LCL heights, which has already been discussed at length. The NAM has MLLCL heights around 1500m at 00Z ahead of the dryline in southern Kansas and drops them about 300m by 03Z. The shear profile isn't that great until late afternoon/evening. At 00Z the Wichita hodograph is supportive of tornadic storms, but in enlarges significantly from 00Z to 03Z, with 0-1km SRH around 400m2/s2 by 03Z. The bottom half of the 03Z Wichita hodograph is pretty impressive.
If you believe the NAM there could be a window after 00Z when the tornado potential could spike. LCL heights will drop through the evening (they will still be high, but a little more manageable) and low level shear will increase dramatically. Forecast soundings don't show the surface layer stabilizing until after 03Z. If that is the case, there will be a window from 00Z until at least 03Z when we have moderate instability, LCL heights have lowered and the low level shear is very favorable for tornadoes. I am a tad overly optimistic with this setup, but if you take a snap shot of the 00Z-04Z time frame I think it looks pretty good. Even though moisture is a little inadequate, I don't think you can rule out a strong tornado after dark with any storm coming off the dryline in the OK/KS area.
Basically I think if the NAM verifies and we get a dryline storm I think theres a good chance it will be tornadic. I will probably stick around Wichita for most of the day, but if it looks like the cap is going to break I'll probably head out to Kingman for my initial target.

 

[Rob H]

Well it looks like at least the 12Z GFS this morning took a squat on the northern target areas (NE/IA), and even some of the southern areas. It looks to me like there will be no daytime initiation away from the front from MN through SE NE. Any initiation that does occur is going to have to be forced by the front -> linear storm mode. I don't see any triggering mechanisms out ahead of it.

I'll preface this by saying I might have some starting location bias, and that I've never tried forecasting something like the NE/IA play - but am I missing something?

This looks like a 700mb low, boundary intersection setup to me. The 700mb low 200-something miles to the west, upper air divergence, warm air/moisture advection... sure there isn't a dryline or an OFB, but couldn't that be enough to spin something off just ahead of the front? It would be a shame to rule out this setup and waste the shear and instability axis in IA. There are some very good recent examples of large tornadoes that come from similar setups: 05/01/08, 05/22/08, and 06/06/08 (from a Jon Davies case study).

 

[Derrick Herndon]

The NAM has now backed off it's idea of an early crashing cold front into the Omaha area. Front is now 200 miles West of the 12Z run. I still do not see 60 dews reaching very far into KS when they are just now at the coast. 55-60 more likely for a 20-25 dew/temp spread. Might make for some pretty structure shots if a storm can fire. Bases could lower around sunset as boundary layer cools. If the storms can stay discreet it may get interesting. Shear continues to look quite good, especially near dark. RUC shows cold air advection over the Western part of the warm sector as early as 18Z so that seems good. Plan right now is to chase pretty local here in Omaha. Should have a better idea where cap will erode first tomorrow morning.

 

[Mikey Gribble]

I only briefly glanced over the 00Z NAM, but north central Kansas looks like the better bet in the latest run. I'm not going to change any plans just yet. Luckily I'm close to the target area on this one so I have the luxury of time. The NAM has a surface low kind of centered there and it puts a kink in the dryline where it wraps moisture back around in north central Kansas. It is just south of the boundary intersection, which I don't like much, but it is still very interesting. It shows low level winds being backed only a tad more there, but it's enough to locally enhance SRH. Anyway, with the capping issues we are dealing with tomorrow if that looks like a more likely place for convection I will probably take that. The environment is just as favorable, if not more so than the area farther south along the dryline. Moisture may not be quite as good up there, but that will have to wait until tomorrow.
So as of now I have two potential targets. One is the dryline in southern Kansas and the other once is north central Kansas. Hopefully I'll get it figured out tomorrow morning. Good luck to anybody that is heading out.

 

[Bill Schintler]

Chase Target for Thursday, April 29

Chase target:
15 miles south of Belleville, KS.

Timing and storm mode:
Storms will develop after 7 PM CDT, with a high-based discrete storm or two likely early in storm evolution before a cold front surges southeast with a resulting transition to a linear storm mode.

Discussion:
The WRN CONUS long wave trough will undergo a slow amplification process over the next 18 hours as a couple of lead disturbances eject from it over the SRN Plains. A more substantial impulse will round the trough base and overspread OK, KS, and NEB late on Thursday. At the SFC, a CF of Pacific origin will track towards the SE. A diffuse DL will mix SLY towards the E, while being overtaken by the CF in KS. Initially veered SFC flow will begin to back during the early evening hours as low pressure organizes along the CF over WRN KS. Moisture return is a concern as owing to veered LLVL flow until early evening, when backing winds allow a narrow axis of 60-65F dewpoints to advect NW into CNTRL KS.

Storms should initiate near the triple point after 00Z, along the ERN periphery of a CI shield associated with the stronger assent. The ERN edge of these upper clouds will reach a Greensburg to Salina line in KS by 00Z. A few discrete storms are possible near the triple point and along the DL early in evolution before the CF and attendant linear forcing pushes EWD. Strong deep layer shear in excess of 60 kts couplied with MLCAPEs of 1000-1500J/kg will support supercell storms until the CF surges SEWD. Further S, strong capping should inhibit convection along the DL.

Bill
10:38 PM CDT, 04/28/10

 

[Chad Cowan]

The placement of the secondary sfc low is going to be pivotal in whether or not tomorrow is a tornado day. Most of the recent model runs (including the new GFS) have developed it further down the front, in the Panhandles, but the new 0z NAM puts it in wcntrl KS. If this actually happens, it would really help out with low-level convergence, and also (hopefully) slow the forward speed of the cold front which would extend the tornado window.

This initial shortwave over the nrn Plains will pull moisture rapidly northward throughout the day but will also act to slightly veer the LL winds in KS/NE. If the secondary low does develop further north, as the 0z NAM suggests, hodograph shapes in the lowest 1km will be much more favorable for tornadoes before dark from cntrl KS northeastward into wrn IA.

Marginal low-level moisture will be a limiting factor for tornadoes but, regardless, there is going to be some WICKED structure out there somewhere.

 

[Scott Olson]

Looking at the 12z NAM, I think the area from Superior to Hebron, Nebraska to an area 30 miles south is a very active possibility. The dynamics look rather strong later into the evening so I would expect rather vigrious development to take place. Not entirely certain if a 4pm intiation is possible given that the best lift and convergence sets up in the next 3 hours.

GFS is faster and thus would push target east by at least 90 miles. If the GFS SLN is the morning reality, I would probably tend to favor Iowa as a possibility given the moderate instability with 60kts of bulk shear on an approaching front.

I should probably throw out the NAM since the GFS solution is more consistent with previous runs. St. Joseph was my previous favorite target in that timing pattern.

 

[George Limpert]

A quick verification of the 00ZF06 NAM and GFS against the low-level mass and wind fields from the RUC suggests that the NAM solution should be favored. A quick look at the observations supports this conclusion as well.

Both the NAM and GFS seem to be too aggressive with moisture fluxes over the Plains tomorrow and I am skeptical of 60 degree Tds in NE or northern KS. RAOBs from 00Z show razor thin low-level moisture even as far south as CRP. It's possible, I suppose, but I am unconvinced.

High-based supercells will be possible when deep convection initiates, and I suspect initial storms will be discrete and develop late in the afternoon. However, I suspect the transition to linear storms will occur rather quickly, especially as the cold front overtakes the dryline after 00Z and surges eastward. The tornado threat should remain low, despite strong low-level shear, because of the limited moisture and the high LCLs.

While I may chase tomorrow, that is a reflection of the proximity of storms to LNK and not due to being impressed with the setup.