5/13/06 FCST: OK/TX/LA/MS

[Jeff Snyder]

There appears to be decent potential for a few supercells from the Red River region eastward today, as strong shear profiles and decent instability develop across the risk area. The 12z NAM, consistent with previous runs, is indicating 45-55+ kts of 0-6km shear in the area, and temps in the 80-92F range with dewpoints in the low to mid 60s should yield >2500-3500 j/kg CAPE as well. Again, Tds on the NAM appear overdone (70s? Not a chance), but low to mid 60s are currently streaming across southern and central Texas towards the area. With southeasterly sfc flow near the front veering to northwesterly aloft, nice hodographs should characterize the area by afternoon (Gainesville TX forecast soundings), with 250-350 0-3km SRH.

12z soundings indicate that the moisture is pretty shallow over SHV and FWD, but it's considerably better on CRP and BRO. The lack of deeper and better moisture is precluding a more significant tornado risk at this time, as resultant high LCLs should keep that threat at bay. Regardless, some nicely-structure high-based supercells are a good bet given the CAPE and shear present.

Edit: I should note that a big caveat on this setup is initiation. Capping and relatively weak vertical velocities may make it a little difficult to get convection. Of course, this also means that there's a good chance that activity will be isolated or widely scattered. Initation chances increase near sunset as the LLJ picks up, but no guaranteeing that those will be sfc-based.

 

[Amos Magliocco]

Ah, excellent heads up on this potential surprise May chase. Thanks Jeff. I agree that the shear and instability invof of a slow moving cool front is never to be ignored in May, especially when the Pattern of Doom is right around the corner. (isn't it funny how many chases pop up in the midst of the doom? lol).

What impressed me most about the setup is the potential for storms to move along the boundary if our orientation is favorable. Plenty of concerns with something like this: moisture, contaminated inflow, capping, lack of significant lifting. That's why you only do it when it's in the backyard.

So instead of gazing into the ensemble abyss, I'll go to SPS in an hour or two and hope for an elevated, striated LP. Would be nice to put some big hail dents in Eric's SUV too. B)

 

[Jeff Snyder]

It's apparent that the capping is winning out over the forcing right now. 00z SPC/RUC Mesonanalysis and available observations indicate a strongly sheared and moderately to strongly unstable environment is in place across much of northern Texas. With >3000 CAPE indicated from of I35, 0-3km SRH peaking over 550 m2/s2, and 40-45kts 0-6km shear, the environment is certainly capable of supporting strong supercells (which is also indicated in the Supercell Composite Parameter, which is locally >30). However, CINH has largely remained in the -50 to -100 range, too strong apparently for the relatively weak convergence on the front. In addition, marginal moisture and very warm surface temperatues characterizing the deeply mixed boundary layer is yielding very high LCLs. DYX radar has been indicating several storms ongoing west and south of ABI. However, as is not entirely uncommon for very dry boundary layers, reflectivities have been considerably higher on the higher tilts, indicating that significant evaporation of the falling precipitation is occurring under that convection.

Previous RUC runs I looked at indicated initiation in the 21-00z period across western north Texas. This, however, has no occurred. Last night's (0z) 4.5km explicit convection WRF run kept convection at bay until 5-6z, when convection built westward along the cold front into northeastern TX.

Man, it's great seeing southeast to east surface winds, southerly 850mb flow at 30-35kts, westerly 700mb at 25-40kts, northwesterly 500mb at 25-40kts, northwesterly 300mb at 55-60kts. Granted, none of that matters if the cap holds back all convection.

EDIT: 0z FWD sounding shows the cap still in place (obviously), with -130 j/kg CINH (and >300 0-3km SRH), while OUN indicates -230 j/kg CINH and ~500 0-3km SRH. Looking at the distribution of CINH (a relatively long, slender profile) tells me that it would have easily breakable had we had better low-level moisture (high Tds would have resulted in a lower LCL, meaning the release of latent heat lower closer to the surface... In other words, the parcel would have risen along a moist adiabat nearer the ground than is the case now). After second look, the CINH distribution isn't entirely a 'long-slender' profile, but it's not a thermonuclear cap, either).

 

[Jonathan Whitehead]

At my home here in Ft. Worth,
Well SPC went ahead with the WW per increasing LLJ w/n the last hour.
I think what you might see out of this is what Jeff already described, which is basically storms that have high bases and are somewhat elevated. I just dont see the a tornado coming out of this. A brief glance at SPC mesoanalysis over this region shows the highest area of 0-1km helicity to not be colocated with the threat area and while the tornado parameter has a bullseye of 3 over the Decatur vicinity, I believe that is a bit overdone and a result of feedback or error. And LCL heights per mesonanalysis appear to be right around 1600m AGL.
Having said all of this I would just like to at least mention April 17th, the day of the El Reno tornado. I mention this b/c we have somewhat similar conditions in place... decent CAPE and shear, but shallow mositure. But of course that occured during peak heating and here were 90min away from midnight local. But middle of May, you never can rule out anything ala May 9, 2006. I definately learned something that day about tornado formation in the lack of shear that I didnt know before.

EDIT: as I sit here monitering IR Sat over the region (RAP-UCAR ABI Sat) and my north facing window of my house...I also consider the fact that it is not completely outside the realm of possibility of a blue...black sky bust but i doubt it...too much forcing and i believe cap will be overcome