2013-09-27 FCST: OK/TX

[James Gustina]

The GFS and Euro have both been hinting at a pretty big pattern shift with high-amplitude longwave coming onshore here Tuesday. The main jet core has been sped up a bit on the last few 12/00Z runs with a secondary jet streak on the tail end of the wave ejecting through the southern Plains on Friday afternoon/evening. The GFS has gotten more onboard with giving a more favorable thermodynamic environment along the dryline the past couple of runs with MLCAPE in the range of 1500 j/kg (taken with a grain of salt of course). If this next cold front doesn't absolutely obliterate the moisture thats been pooled on the southern plains the past few days then there's a legitimate shot at decent storms in western Oklahoma/eastern Texas panhandle with decent turning in the lowest levels and bulk shear aoa 40 kts. Definitely bears watching as it gets in range of the SREF and NAM.

 

[Rocky Rascovich]

Been following the evolution of this upcoming system on the GFS for about a week now.. finally seeing the whites of it's eyes 24 hours off.. GFS wasn't all that far off on this from a week ago.. by only 36 hours. Skill wise, close to adequate.

I looked mostly at the NAM and SREF with the GFS some as well, which seemed like as far as surface features, had the best potential area of SVR criteria storms near and north of the AMA area by 0Z. I think that storms should be well under way then, but with the upward forcing projected to occur and the general progression of the storm system in general, I think that initial discrete cells will go up along the I-27 corridor west of PVW to AMA to maybe as far west as Boys Ranch and north to the Lake Meredith area by about 21-22Z.. this should be ahead of a broken line of convection to the northwest which will likely be along the cold front. As area of strongest vertical velocities occur by the aforementioned times, I expect cells to only have a window of opportunity of about 2 hours, if that.. before congealing into a squall line or at least a QLCS. The environment ahead of this is definitely conducive to raise eyebrows in watching any discrete's that pop up. Up to 2500 j/kg of SBCAPE, EHI's in the 3-4 range, Bulk shears up to 45 knots in areas around the PPA area on down to I-40 .. SRH values up to 200-225 m/s is progged just east of AMA by about 23Z per 12Z NAM data off of Twister Data. The SREF had a similar set up expected. What I find impressive are the expected PWATS which is up to 2" near the CDS area. I would think storms that tap this rich moisture supply will harbour some excessive rainfall rates.. maybe 1-2" within an hour period for the SE TX PH in particular and up to 1" up towards the I-40 corridor.

It is just north of I-40 that I think based on the NAM, GFS and SREF will likely give the highest tornado possibilities for tomorrow and this as I said will occur in about a 2-3 hour window before everything gradually builds into a surging, large squall line. I would think that a TOR warning or two will be issued for sure with perhaps one or two verified sightings in my outlook area. I would be paying special attention to forward flank spin ups if a bonified QLCS gets going due to locally backed surface winds juxtaposed to the 7h-5h flow. Lapse rates look about average, nothing spectacular based on forecasted soundings off the NAM.. maybe up to 2"hail in discrete cells that become supercelluar. Wind gusts to 75mph not out of the question in the most intense storms.

I may just jump on this if the models continue to hint at least a marginal threat for discrete cells. Storm motions while not excessively quick, will keep one on the move (35kts). Further south as the front slows, places like CDS on south and even southwest could be looking at a 2-3 hour period of training cells, with expected PWATS.. impressive rainfall totals from there into SW OK. is possible. As of now.. I would pick the PPA area, maybe on toward AMA if I do this. I will try to post a brief update tomorrow before I leave which will be around 1730Z at the latest. All the best and be safe.

 

[James Gustina]

I think its also interesting to note the moisture fetch thats already occurring in central Texas at the moment. With dews in the 60s around and south of San Angelo and in the mid 50s pooled east of the caprock, I'm not so sure that the low-level moisture problems that I've seen people point out will be as substantial as previously though. With the amount of localized enhancement from the caprock along and a bit north of Groom, theres a definite tornado risk if one cell gets going before the front gets near (this brings October 12th of last year to mind, supercell that went up out of nowhere and spat out some dusk tubes). The 4KM NAM in particular was giving some extremely good 0-1 SRH values on the 12Z this morning and 00Z last night around 250 m^2/s^2 aligned right with that narrow instability axis just east of I-27 around 23-01Z.

My only concern is that if this shifts a bit further north into the Canadian River Valley. Its not awful but the roads northwest/west of Pampa are questionable to say the least.

 

[Tim Supinie]

Watching the observations this afternoon. Part of the problem for the TX panhandle is that there is no clear surface boundary to focus convective development. The dryline is way the heck out in northeast NM and southeast CO, so that's not really of use. We might be relying on differential heating boundary from the showers over the western half of the panhandle. An additional problem for the TX panhandle is that the majority of the upper-level forcing is going to be to the north, so I think we'll be relying on surface forcing for convection. In the absence of a clear surface boundary, this might be hard to come by. Farther north, say DDC, dewpoints have come up a bit since this morning, and they'll be closer to the upper-level lift, so I'm a bit more optimistic about intense convection developing in this area. Of course, farther north, we'll probably have problems with the upper-level wind being too close to the orientation of the boundary, and that cold front is approaching from the north.