9/14/05 FCST: Southern Plains

[Jeff Snyder]

There appears to be a pretty good chance of significant severe tomorrow across portions of northern TX and Oklahoma as an active southern stream noses into the area. The synoptic cold front should press into at least central OK by afternoon, though it may be aided by outflow from post-frontal convection. It appears right now that areas on the immediate "cool" side of the front / OFB in the TX panhandle and into western OK may warm enough to support strong instability by afternoon. Easterly/southeasterly surface flow north of the boundary underlying 35-50kt southwesterly 500mb flow and 50-90kt WSW 250mb flow should yield 45-55kt 0-6km deep-layer shear, while strong low-level veering will result in signficant low-level shear. Depending upon the degree of moisture pooling along the boundary (NAM shows low 70s between Red River and I40 in OK), low LCLs may characterize the near-boundary environment as well. CAPE progs from the 12z NAM aren't extreme, but 2000-3000 CAPE appears likely in areas that see insolation.

Right now, it appears to me that the key will for the coverage and intensity of tomorrow's convection in the southern plains will be the location and motion of the cold front / OFB. Without a strong push forward, forcing along the front shouldn't be too terribly strong, especially in the absense of strong 850-700mb flow (resulting in only weak isentropic lift above the frontal boundary). With easterly storm motions progged by the 12z NAM, there appears the definite possibility of a couple of significant supercells latching onto the boundary and producing tornadoes.

 

[Chris C Sanner]

Well, I couldn't agree more. Too bad I'm in Norman now and now Velma, I'd be getting ready to ride that frontal boundary tommorow, then again, looks like that may be the plan anyways. Low Level Shear is gonna be pretty dadgum strong across OK as is the shear 55kt in places at least. IF, and I do mean IF, coverage of storms is not as great in the morning early afternoon and the storms today lay down an E-W outflow boundary somewhere in OK, I can see supercells along both the front and OFB producing tornadoes tommorow if they both stall somewhat. Looks like a chase day for me!

 

[Amos Magliocco]

Agree with Jeff and Chris on tomorrow. I think a supercell or two is likely and, invof the boundary, a tornado can't be ruled out considering the forecast low level shear and LCLs in the presence of the front and/or outflow boudaries from prior convection. If the precip will stop early enough to several hours insolation on the front or boundaries, things could work out nicely for storms firing later in the day.

Concerns now are the duration of discrete supercells given a sharp boundary, the chance for storms to rain into their own easterly inflow, and the same aforementioned weakness in low level wind fields as a hinderance to sustained updrafts. Oh and the chance for the MCS to generate a cold pool driven, too-dynamic OB that undercuts sfc convection. I guess there are even more now that I think about it, but I don't want to discourage myself.

I don't think these storms (or any associated tornadoes) will be very pretty or long-lasting, but it's September. I'm not complaining.

I hope to be on the road no later than noon racing somewhere between Altus, Elk City, and Shamrock.

 

[Mickey Ptak]

Pretty nice setup for tomorrow across Oklahoma. Deep layer shear is more than sufficient for supercells and as mentioned by Mr. Snyder, the low level wind and estimated storm motion could help produce a few tornadoes.
One concern is the possibility of to much cloud cover during the late morning/ early afternoon. This is being picked up with the 12z NAM but GFS tells another story and limits the various level cloud decks. That will only be determined tomorrow with close eyeballs on the visible satellite imagery. So I will take that as a grain of salt and keep my hopes up for a possible backyard chase.
The second concern is how long cells will stay isolated or if they will be linear from the get go. If have forgotten how to distinguish the elements that produce squalines. Time to review...

Trying to round of the girls I chase with to see if they are in. Unfortunately we are having an audit here at work but there is a good chance I can still get out a few hours before initiation.

Mick

 

[nickgrillo]

Unmodified CSM (Clinton, OK) forecast soundings at 21Z show a parcel of 83F/65F which yeilds 2456j/kg of muCAPE. I just saved the sounding:

http://midwestchase.com/csm_sounding2.jpg

Nice!

Rather favorable veering profiles should support a few sustained/strong supercells. Given the very favorable low-level shear and presence of OFBs and low LCLs (800~m) - these storms could easily drop a few tornadoes. I pretty much agree with Amos' target region of Elk City/Altus area for tomorrow...

 

[nickgrillo]

Unmodified CDS forecast sounding at 21Z WED shows a parcel of 83F/67F which yields 3009j/kg of CAPE. Overall speed shear and instability is conductive for severe thunderstorms... With very distinct veering in the 700-900MB layer, I would have to say supercells are still pretty likely. Surface winds are pretty much easterly... However, the CDS sounding shows winds more southeasterly at the near-surface layer. Moderate directional shear and decent speed shear (much stronger in the 500MB-250MB layer) will result in sufficiant deep-layer shear of >40kts across most of the eastern TX panhandle and southwest OK along the frontal boundary which will be sufficiant for supercells. Low-level shear is pretty strong too... With SRH progged to be a widespread >300m2/s2 across the majority of the threat area (eastern TX panhandle/western OK).

I would have to say supercells and a couple tornadoes are likely. My target is pretty much towards Childress, TX at the moment (and probably points slightly W-SWwrd).

 

[afischer]

Not having yet considered the effects pre-event (elevated) convection may or may not have on the set-up.... it's at least nice to see the target shift southwest a bit, nearer the incoming mid/upper jet axes. One thing that jumps out at me about the shear environment though is the near-zero directional shear between the surface and 1km... and that models keep the speeds in that layer at 10-15kts. This wouldn't seem to bode especially well for low-level mesocyclones.

Maybe some panhandle magic will occur anyway. 8)

 

[Shane Adams]

CF (per OKmesonet obs) is currently draped along a NE/SW axis roughly from Bartlesville to the I-40 corridor along the OK/TX border. Along and just north of the boundary, wonderfully-backed (though relatively weak) surface flow exists, which will be the focal point of any tornado chances today. Windfields will remian favorable (but not ideal) for supercell development, per a morning glance at profilers in the region. According to RUC, development begins at the southern egde of the CF near the Quanah, TX area aoa 21Z. This makes sense as the CF will be located in that general area at that time. Right now the plan is to monitor satellite and surface obs through midday, then sculpt a target down by 17Z. Current thinking puts us in the Altus area by 20Z at the latest.

EDIT: With storm motions expected near 270, there seems a good chance of any isolated storms sustaining themselves easily with the NE/SW orientation of the CF. Even if a storm riding the boundary turns right and leaves it, it will remain in the juicy air and ingest winds that will still be somewhat backed to around 160-170. It's September, so this is a very alluring set-up IMO.

 

[Jay McCoy]

Mostly agree with Shane on this one so wont rehash same parameters. I am a bit worried about ongoing convection near Altus and subsidence in its wake and may decide to play the outflow from it a bit furthur south along hwy287 in North Texas but will still be in same general area. will decide (generally) and be on the road before noon. Either way I will stop in Childress for a data update and make final choice there.

 

[Jason Branz]

Agree with the above assessments. The 15Z obs are encouraging, with backed flow becoming more apparent along and north of the Red River, although the surface flow could be a bit stronger, as Shane mentioned. I'm liking the Vernon-Quanah-Childress corridor, since they should see the most heating. Quanah is my target as of now, since the farther west you go the closer you'll be to the stronger 700mb flow. I'll be out the door by 17:30 UTC.

JB

 

[Shane Adams]

CF seems content to stall further north than originally thought, at least the eastern half (through NC OK). I'm liking the Altus area more and more, though I do share Jay's subsidence concerns. However, winds are turning more easterly in this area, there's a nice clear area coming in behind the convection in SW OK, and the moisture is great. It seems like systems this time of year always show a list of problems but then produce the storms of the day behind the area of concern (ongoing convection) in the clear air. I feel today will be no exception. The new ETA (if you believe it) shows the h7 level a lot more love than the RUC, with 35kts max throughout a large chunk of OK (as opposed to the 15-20kts advertised per RUC). Current profilers support the RUC solution unfortunately, but there is a heavy speed max coming from the west (as evidenced on the TCUN profiler).

Still thinking Altus area, but still have a few more hours to analyze and decide.

EDIT: There's an area of enhanced LL winds in western OK, currently showing around 20kts. That's more than sufficient to aid in LL circulations. These winds are currently straddling the front and just north of it, into the drier air (upper 50s dews). However the southern end of these higher winds do cross into the juicier air just south of the front, and right now that is what I'm bullseying.

 

[Mickey Ptak]

I like the clearing that is taking place in far SW OK. I have not seen the front move to much in from the 7am hour so as long it stays in place Snyder OK ,Altus OK, Qunnah TX does look like a good area to focus on IMO especially with the extreme time crunch I am facing. Surface winds have me turning more easterly in that past few hours in that area as well and if you believe the RUC precip starts to brake out around 5-6pm.

Well, I plenty of time to mind screw myself on a target between now and 3pm when I can leave work. :evil: IT always works this way. :roll:

Edit: Well I see taht Shane posted everything I was think. lol. My bad for to same info. I guess it's nice to know we are on the same page. lol

Nick - Can you just set up a link for that old forecast image so we don't have to scroll to the right to read each line ? Thanks bro...


Mick

 

[Jeff Snyder]

Well, there are certainly some good developments (e.g. 12z NAM forecasts of 3000+ MLCAPE) and some not so good developments (stratus sinking southward in sw OK). It should be noted that (thanks to Gabe Garfield for bringing this to my attn) that the 12 NAM underinitialized 500mb and 700mb winds (to a lesser extent) across the southern plains. In some places, the 500mb flow is 10-15kts stronger (per area soundings/RAOBs) than what the NAM initialization shows. This will likely result in a little stronger deeplayer shear than forecast, which isn't that great since it was already forecast to be quite good (45-50kts). Unfortunately, sfc-700mb winds are still pretty weak across the area, particularly along and just south of the front/boundary. The lack of speed shear in the <3km layer should be compensated by significant directional shear in the same layer. I'm also a little concerned about the orientation of the front (SW-NE), as surface winds to the south / east of the front (such as in sc OK) are largely light/variable. However, I suppose the good thing is that, with a deep-layer shear vector from the WNW, we may see a better chance of discrete activity given the more normal orientation of the shear vector to the convergence boundary. I'll probably head out to extreme SW OK in a little bit. In addition a nearby OFB should provide enhanced low-level baroclinically-generated vorticity to add to the mix.

I'd like to stay farther north owing to stronger mid and upper-level flow, which decreases to the south. In addition, boundary-layer directional shear is reduced south of the Red River. However, max destabilization will likely occur farther south (into western north TX).

 

[Glen Romine]

Originally posted by Jeff Snyder
I'm also a little concerned about the orientation of the front (SW-NE), as surface winds to the south / east of the front (such as in sc OK) are largely light/variable.... In addition a nearby OFB should provide enhanced low-level baroclinically-generated vorticity to add to the mix.

My thought would be to avoid plans to chase along the front unless you are getting all the way into the southern panhandle of TX. Instead, I'd focus in on the convergent mesolow just west of FDR. An outflow boundary extends ESE from there, just south of the Red River, and that boundary has the potential to be favorably oriented for tornadoes later this afternoon. Subsidence should start to pinch off ongoing activity soon, and RUC has a modest LLJ perpindicular to the outflow boundary by early evening that could offer some help in the shear department if storms have not already merged into a convective cluster by then. Should see rapid upscale development though so discrete window could be small. Gorilla hail reports I expect to be the main news story, but a tornado or two certainly looks possible.

Glen

 

[Bill Schintler]

Sept. 14 FCST

Perhaps a BUST today...? At least during daylight hours. In the absence of subsidence, it looks like current convective temperatures range from the low 90’s in the OKC and LBF areas, to around 90 in AMA. I don’t see much in the way of favorable dynamics, with the (CDS) target area remaining in the convergent right-exit region of the H5 max through 00Z. The WV loop does hint at a weak wave entering the WRN TX panhandle, while guidance keeps the stronger vorticity max. well to the north in KS. Guidance suggests that a compact mid-level wave will approach the CDS to PAD area well after 00Z. As has already been mentioned, any outflow boundary from the ongoing elevated convection around the Red River will have to be closely watched, as this will likely be the only location where convection during the daylight hours might happen. The FDR radar loop (0.5 deg. tilt) shows an E/W-oriented boundary near CDS. If I were out today, I'd target an area between Paducah to Crowell.

- bill