2012-04-02 FCST: TX/OK/AR/KS

[J Trueblood]

I could be wrong, and I probably am, but would you guys say the current SPC outlook for day 5 is going to be adjusted by the next forecast period? It looks like the GFS initially sped the system up as of 18z wednesday, progging the cold front to reach the Ozarks by 12z on Monday. Since then, each initialization has slowed the movement more and more. As of the 12z run this morning, the GFS is showing the cold front over central Kansas on 12z Monday.

Again, I'm pretty new at this, but isn't this a more favorable setup for the Kansas/OK/Texas plains than what was previously thought?

By 18z a dewpoints will reach the upper 60's across E TX/OK/KS. CAPE is pretty modest in the same area, with a maximum in SE texas at about 3000 J/kg. As mentioned before, the surface low looks to be sitting on the KS/OK border by midday Monday, so that should help with lift, as indicated by the good LI indices. Low to midlevel shear actually exists in that SE portion of Texas, with 20kt at the sfc-850 level.

Model certainly shows some precipitation by 0z Tuesday in that region, so I feel like something is possible.
The question for me is the cap. I'm not confident in my analysis of it, but it certainly doesn't look as strong as it did on previous runs.

This is the first forecast I've ever attempted to post on Stormtrack, so hopefully I'm not completely way out in left field on this one.

 

[J Trueblood]

As of the 18z GFS run, I'm still pretty interested in the setup.

At 18z a shortwave trough is sitting over W TX/E NM with embedded jet streak of around 80 kt. The upper level divergence is creating a surface low to the SE in SW OK of 1000 mb. A cold front extends South and West into the TX pandhandle with a fairly distinct dryline stretching due south in southern TX. I'm not experienced at dryline forecatsing, but it looks like this might be a good setup for a dryline type event.

Winds are backing the dry side at 15-25 kt. The GFS looks like it's trying to show a dryline bulge with this associated surface wind pattern around Abilene. Given the divergence and bulging of the dry line, this looks like a pretty nice area.

Instability is not through the roof, but definitely looks like it's modest enough to support some type of activity, with values around 1700 J/kg in the target area. The cap will obviously be a problem, but there does look to be some erosion taking place by 00z Tuesday. This again is indicated by GFS predicting precipitation along the dryline. Low level shear is pretty meager, but 30kt seem to be possible in the mid levels. I guess we'll see.

Thoughts? I still think this is quite a bit further west than what the SPC outlook is, but they are the professionals, not me.

edit : It also looks like into the night, the cold front will catch up with the retreating dryline leading to a squall line.

 

[ChristianTerry]

J, Im in agreement with you. This looks like a Linear setup along the dryline with a squall line forming with embedded supercells. Those storms also look like they will be the LP types with the amount of moisture flowing up and the fact that they will be right on the dryline. I honestly think a line from Wichita Falls southward to near Abilene looks quite good if this model verifies. With me living in Southeast Texas, this is kind of a bummer because I have school still and can only go out after 3:30 but I will watch these things, still 4 days out.

 

[Stan Rose]

Looks like SPC dropped the day 4 outlook as too unpredictable. I'd have to agree with that in terms of the delineation, although there will likely be some severe out there with that impressive upper low and strong cold front.
Not to pick on you, J, but a couple corrections to your analysis: the upper level 'divergence' (you probably meant diffluence) is not 'creating' a surface low (it's actually filling at the time you mention). Also, the Lifted Indices don't indicate anything about 'helping with lift', rather I assume you mean to say that higher LIs show the potential for stronger updrafts. Keep up the forecasts, and always express strong confidence in your analysis; we've all been wrong numerous times so there's no need to invite folks to doubt you.

 

[J Trueblood]

Thanks for the constructive criticism, Stan. I appreciate any feedback I can get. Just to clarify: diffluence is when divergence and convergence happen at the same time, right? Looking at the trough, I thought as air exited a jetstreak on the lower right quadrant, divergence occurred aloft. Additionally, as air moves up a ridge, more divergence occurs, no? Both of those lead to an area of low pressure and convergence at the surface. That is what I thought was going on. Can you explain where I went wrong in that analysis?

To adress the newest model run: A surface low still sits over the TX panhandle at 18z on Monday. The dryline extends south from the surface low from OK through the TX panhandle. CAPE still looks nice, with large areas of around 200 j/kg in much of mid TX. The cap looks moderate at 18z but certainly looks to erode by 00z Tuesday.

I think the big problem is the lack of shear at a majority of the levels. With nearly zero low level shear, and only spots of 30kt from sfc-750, the environment doesn't look great for sustaining any convection that occurs. I assume this is due to the weakness of the jet streak?

 

[Tom Dulong]

To adress the newest model run: A surface low still sits over the TX panhandle at 18z on Monday. The dryline extends south from the surface low from OK through the TX panhandle. CAPE still looks nice, with large areas of around 200 j/kg in much of mid TX. The cap looks moderate at 18z but certainly looks to erode by 00z Tuesday.

I think the big problem is the lack of shear at a majority of the levels. With nearly zero low level shear, and only spots of 30kt from sfc-750, the environment doesn't look great for sustaining any convection that occurs. I assume this is due to the weakness of the jet streak?

Assuming the models are correct, this should be played as a "cold core low" event. Jon Davies has an excellent write-up(http://www.jondavies.net/tornado_fcsting/500mbcoldcorelow_tors.htm) on these type of systems, which do occasionally produce photogenic tornadoes.

Lows of this type tend to have less vertical shear than their open wave counterparts, since the system is more vertically-stacked. Surface dewpoints in the 50s can be sufficient to produce an isolated tornado or even a small family of them. Based on the latest
NAM, I would place an initial target area in central KS.

That being said, this morning's early run of the Short Range Ensemble Forecast has a large spead in surface low placement (OK-NM-Srn CO) by its forecast members for 18Z Monday. I would wait another couple of model run cycles before pinning down an actual target location/time.

 

[J Trueblood]

Thanks for the great article. I'm trying to forecast the situation with that in mind. I am looking at the NAM now and a bit confused. I was under te impression the surface low was always to the SE of the upper-level low. However, at 12z, NAM puts the surface low in northern Kansas with the upper level low in Colorado/NM. Why is that happening?

I'm looking for the triple point as indicated in Davie's writeup, but having trouble. Obviously I see the lowin central kansas, and a dryline south of it, with a cold front to the west of that (correct?) But I'm not seeing the shift from South to East winds indicative of a warm front. http://www.twisterdata.com/data/mod.../30/06/NAM_221_2012033006_F84_TMPC_925_MB.png

If I were going purely based on what that Davie's writeup shows, I'd be in S/Central KS.

That said, can you explain some of those discrepancies I highlighted?

 

[Adam Simone]

J I'm fairly new to this as well but the 3rd part of the triple point is a surface front stretching from the low in central Kansas northeastward into Iowa. Someone with more experience could correct me on that but that's what I'm seeing. And I agree I'd target near the low but I don't have big hopes for this system as of now tornado wise. Looking at the wind profiles you basically just see southern winds at all levels basically throughout the threat area. That's why I think this will mostly be a linear event. The best chance of tornadoes will be near the low and along the warm front into Iowa. The NAM is showing 1km Helicity values over 100 in that area but the rest of the threat area has very low helicity values.

If I'm correct with my triple point area hopefully the image below will help you.

 

[Tom Dulong]

My reply will be short, since this is a forecast thread. Conceptual models, such as the one presented by Mr. Davies, depicts mean locations for features associated with this type of system (a.k.a., an occluding frontal system). It is not unusual for the warm front-cold front intersection to wrap around to the northeast of the surface low center with occlusions. For the NAM 84hr 925 mb fcst valid 18z UTC 4/2...I would place the occluded surface front from the low in southeast CO to the warmfront/coldfront intersection somewhere s of Colby, KS (note shift from sse-ene winds here). I would not normally read such detail in a prog this far out...but
that is where "X" would mark the spot.

 

[Stan Rose]

This is shaping up to be a very interesting storm! I'm more intertested in the heavy snow potential for the Southern Sangres and Raton/Palmer Divides (cause i'm sick of this 85 degree absurdity) but both the GFS and NAM are now pointing towards C-Central KS as a a triple point. I couls see some spin-ups coming out this event.
J--I'll adress your questions via PM since I don't want to derail the forecast discussion. The only thing I'll say here is that conceptual models should be used cautiously since the atmosphere can get pretty complicated

 

[Trey Thee]

There are a lot of timing issues as of now but this system looks like it could pack one helluva punch. Could be the first significant severe weather event for my area. The instability on Monday is going to be quite high. If the timing of the various upper air dynamics fall into place Monday is going to be very interesting indeed.

The I35 corridor from DFW up to KC could be a prime spot for chasing although some data suggest the action may fire further west by a pretty wide margin. Systems like this always have a lot of variables, some of which won't be known until the morning or even early afternoon of the event. Hopefully we'll have a lot more clarity tomorrow.

 

[Jeff Duda]

It looks like at this point, any enhanced severe storm threat will be confined almost entirely to TX and perhaps far S OK. The models have progressively slowed the trough and have cut it off starting Sunday night. Even by 12Z Tuesday, the 500 mb low is still sitting over NM, and there is almost no overlap between the mid-level jet and the better low-level flow. Any decent deep-layer shear is now located behind the dryline, and a 30 - 40 kt LLJ is about all that gets going (with the 30-40 kt flow focused over a very narrow region near the Red River). Storm mode looks to be linear north of the Red River, and generally multicellular (could be discrete or linear) into most of TX.

While this disturbance looks to provide a nice dumping of convective rain over some of the drought area of OK/TX (which is now dwindling steadily), that looks to be about all it has to offer.

 

[Tim Marshall]

TARGET: VERNON, TX - Later today I will head northwest toward Vernon, TX. Latest RUC shows precip breaking out along the dryline and growing upscale tonight. It has been interesting how RUC, NAM, GFS have differed in the timing, orientation, and location of the wind maxes of this closed low system. Exit regions at 500 mb range from I-20 around ABI to I-40 around Shamrock. Best low-level shear is along the CF with unidirectional flow through the eastern TX panhandle and western OK. So, we'll need a little help from the approaching low in the form of surface pressure falls and storms themselves to help back the surface winds. Above 500 mb, the winds also back making for compact hodographs. Also, noticed some cape robbers on forecast soundings in OK and TX. All-in-all a marginal chase day here but it is not far away.

 

[Jeff Snyder]

It's a "no go" for me today owing to a previously-scheduled commitment tonight that I cannot change (barring a big chase day). That said, I'm not sure how much I'd be up to chase today anyway. As others have noted, seasonably strong instability, largely the result of appreciable low-level moisture beneath steep mid-tropospheric lapse rates, is currently "developing" east of the dryline in the warm sector. The primary impediment to a more significant severe event is the suboptimal deep-layer wind profile across most of the areas E of the dryline in OK today. Current Vici profiler looks horrible, with S to SE winds from the sfc to ~14 km AGL. 12z NAM forecasts the lower troposphere part of the hodograph to lengthen by and shortly after 00z as the LLJ intensifies, but the deep-layer shear profile never looks very good. The Sfc-500 mb wind profiles on the fresh-off-the-presses 18z NAM look better in far southwestern OK and into western north Texas, again as others have noted, but there is also significant backing with height above ~500 mb (such that the upper-tropospheric flow is nearly entirely meridional). A weakness near 700 mb also imparts an S-shape to the hodograph down low as well.

I haven't looked at a whole lot today since I'm not available to chase, but the little I've examined would lead me to target western N TX, maybe starting W of SPS. I'd like to be in an area of veered mid-troposheric flow for hodograph shape purposes, but that just means the unfavorable upper-tropospheric flow may be more of an issue. The hodographs SW of SPS at 3z on the 18z NAM look pretty good, actually, if we ignore the winds above ~400 mb. The shape is much better and seems to support a tornadic supercell if such a mode can be sustained. See the forecast sounding near Seymour valid 3z as an example.

To mention briefly, the HRRR runs are massively unforecasting 2m Tds at this time. The 15z and 16z run (the two most recent that go out to 23z and beyond) are showing only 40-50 F Tds in western Oklahoma now, compared to the verification of widespread 60-62 F Tds. As a result, the HRRR CAPE forecasts are going to verify much too low. I'm not sure what's going on -- I've quadruple-checked the forecast dates/times, and I know the graphics are labeled as 2m Dewpoint, but there must be something else going on. As it is, the most recent HRRRs are underforecasting 2m Tds by 10-20 F across western OK compared to current observations.