4/18/04: FCST: Central/Southern Plains

[Jeff Snyder]

First off, I'm starting a new topic since there appears to already be adequate conversation regarding the risk across the IA/IL/upper midwest/northern plains area. Tim, my apologies if this ends up being unnecessary.

Well, the severe weather potential seems to be increasing (well, the potential isn't, but the forecast environment is becomign more favorable) as the ETA gets a couple of runs under its belt for Sunday afternoon. Things are starting to fall into place for a potential signficant severe weather event by Sunday afternoon. The ETA continues to forecast the trough position farther and farther west, which is taking care of any timing issue that was a concern on previous ETA and GFS runs. Unlike prior runs, the ETA continues with +60 Tds across parts of KS and OK. While CAPE is progged to be moderate (2000-3000 j/kg) across central portions of said two states. However, there is excellent shear and awesome helicity that actually coincides/juxtaposes nicely with the highest CAPE. Strong winds at all levels, along with good instability, could very well spell a risk of signficant tornados across OK/KS Sunday afternoon, before forcing increases rather drastically and a probable squall line develops. Definately something to keep an eye on!

 

[Robert Dewey]

Jeff, it was probably a good idea to start a new thread, the others were getting too cluttered.

Looking at the latest 00Z ETA, it looks like a good threat exists between 21Z SUN - 03Z MON...With a decent target region somewhere between ICT and OKC. Good jet structure, decent helicities >350m2/s2, CAPE in excess of 2500J/KG, strong mid level vertical velocities, and weakening CAP all suggest that severe thunderstorms are likely, with supercells probable. The only potential problem I see, is that forcing may be a little too strong, and organized into a linear fashion, which could make a squall line situation more likely than individual supercell type storms...But this would be considered mesoscale, and hard to determine based on numerical guidance.

Robert

 

[Jeff Snyder]

Oh man... The fresh-off-the-presses 12Z ETA is not looking very good at all from a thermodynamic standpoint! Gone are the 2500-3000 CAPE and breakable cap from last nights 0z run, and in is the weak instability and nearly-impossible to break cap. The ETA has flip-flopped again with regard to surface/lowlevel moisture across OK/KS on Sunday. Instead of the nice >60-65 tds progged by the 0Z run, much of the moisture mixes out on strong 850mb winds. This, in addition to the trend of slowing the arrival of the upper trough, is bad news. Much less instability and much higher CINH (stronger cap) are the result... I really hope THIS doesn't happen, as it'd be a total waste of an awesome wind field/profile, especially considering how the favored storm mode will likely be discrete supercells, until strong UVVs force a linear system later in the evening Sunday.

 

[Tony Laubach]

And the day 2! Lets make a weekend out of it! Figure since I am already out there, maybe I can post back-to-back days in Kansas. SPC's running the risk along the same lines as Saturday, but shooting it further south. A cold front will play into things on this day as it moves into Kansas during the day. Likely won't spend a lot of time looking over this, for if I decide to hang around through Sunday, I'll likely just stick around and play it by year since tomorrow will have my attention focused mostly on what's happening now as opposed to forecasting the next day. When the day's through, I'll adjust myself accordingly. Still in the air as of now, but worth the mention. Nice weekend shaping up none-the-less!

 

[Jeff Snyder]

Well... it looks like capping is going to be a rather significant issue. Looking at point-forecast soundings from the 12z and 18z ETA, both show very strong capping across much of central and eastward portions of KS and OK. I have included three below that may be of interest...

21Z ICT

21Z END

23Z END

It still looks like shear will be fantastic (45-60kt 0-6km deep-layer shear), except for the fact that the ETA has veered the forecast sfc winds more towards SSW on Sunday afternoon. If the sfc winds can somehow back ahead of the dryline, it would increase convergence along the dryline and increase low-level shear.

By the way, the above three are the best 'weak cap' soundings I could find. Areas farther west are mixed more and have much higher LCLs. Many other areas have a VERY pronounced and unbreakable (in this scenario) cap... I'm preliminarily targeting west of ICT...

Jeff

 

[Kevin Scharfenberg]

The eta model is finally beginning to realize the moisture return we've been seeing has little quality. Accordingly the CAPE is going down and the CIN is going up with every run. Tomorrow forecast for the plains: Warm, windy, and mostly cloudy but mostly dry.

This pattern change is turning into a bust thanks to the cold, dry air recycled into the Gulf off the Southeast U.S. Maybe we'll be able to salvage something around Thursday when the main system closes off over the plains and temperatures aloft cool.

 

[Jeff Snyder]

Well, as we get closer and closer to tomorrow, it's becoming increasingly clear that the lack of moisture is going to be the nail in the coffin for this otherwise incredibly-favorable setup. Despite fantastic low-level shear (and deep-layer shear for that matter), I don't really see much happening tomorrow because of the lack of instability (caused by the lack of deep moisture). With the exception of a single 0Z ETA run from a few days ago (which had upper 60 tds along the ok/ks border and thus had 3000 CAPE there), the models have been very persistent and consistent in mixing out the available surface moisture over the area on Sunday. Despite the occasional surface ob reporting >65td in deep south Texas, the western gulf continues to struggle to get to even 65. I haven't seen a 70td ob anywhere north of areas along the Bay of Campache (sp) way down in Mexico. It's very frustrating to actually have a setup where strong low-level shear covers much of the warm sector east of the dryline. If it hadn't been for the front that made it completely through the Gulf of Mexico. We're still working on modified continental air, which is why, with several days of strong southerly flow, we still struggle to get in the 60 Tds. It seems that evapotranspiration is helping bump Tds up into the 60s during daylight hours, but those drop when the sun starts going down.

The good news is that that true tropical air will eventually regain control of the Gulf. You can see this by looping the GFS 0-30mb Td graphic (http://weather.cod.edu/forecast/loop.avnNA030td.html).

Do I think there may be a few storms in the panhandles and western KS tomorrow? Yes, but the cap will be much too strong across most of western OK and central KS for much. Additionally, the instability axis is very very narrow, being located east of the dryline and west of the cap edge. In addition, the meager surface dewpoints will result in, literally in some places, mile-high LCLs. So if we do get a decent supercell going, the chances of it tornado-ing, IMO, are near zero because of the very high LCLs.

So.... the good news is that instability is progged over OK and southern KS through at least Thursday. Looking at shear/instability positioning, however, nothing looks very favorable until Thursday. Sure, Tuesday may have good CAPE over northern OK and sc KS, but the best shear is east of there. Whatever the case, I'm now putting my hopes in Thursday. The latest GFS is indicating a very broad upper low cutting off near the four-corners by Wednesday night. This will induce a surface low near the panhandles region. Again, by this time, we may FINALLY have true tropical air (upper 60tds) in here to provide for good instability and relatively low LCLs. If it were to verify, which it probably wont' since it is 5 days out, the area should have good wind shear, both aloft and in the low-levels, to support some good severe and tornado chances. Given it is so far out, however, we really can't put too much faith in this solution.