4/29/09 DISC: TX/OK/KS

[Jeff Snyder]

The 00z soundings for AMA and DDC have pretty good thermodynamics, but the kinematics are very marginal. Though there is a veering wind profile with height, the flow aloft is weak. For example, on the AMA sounding, the anvil-level flow is anyway from nearly CALM to 25 kts; on the DDC sounding, 500 mb winds are only 25 kts, yielding only 36 kt 0-6 km "shear". The one sign of hope in those soundings is the 0-1km SRH, which is likely larger than calculated one those soundings as a result of the observed southeasterly motion of the GCK supercell.

Other than that, I think is another reminder of the importance of low-level boundaries (e.g. OFBs) on weak-flow days.

 

[Michael Carlson]

I love these days the best cause not many people expect tornadoes and then all of a sudden the OFB that you forecasted turns into tornadofest and your the only one out there. I posted some maps on my blog about the atmosphere around the Dodge City area when my dad was seeing the tornadoes/gustnadoes. Mike O'Keeffe mentioned that there was an OBF in that area a few days ago but when I looked at it before this afternoon It was hard to pick out until the cell formed.

 

[Gabe Garfield]

What is strange to me is that the usual ">40 kts of 0-6 km shear" wasn't required in this case. Even with the storm motion on the boundary, you aren't going to do much to improve that hodograph (the winds above ~3 km are atrocious). This leads me to believe that strong thermodynamic forcing (i.e., large CAPE) can actually augment existing vorticity (i.e., along outflow boundaries) to support supercells and tornadoes.

I guess that might seem obvious, but I can't remember a day that so clearly lacked strong deep-layer shear and yet produced several photogenic supercells and tornadoes. Also, the RUC mesoanalysis showed < 100 m2/s2 of 0-1 km SRH today over the TX panhandle. This is another example of why it's good to chase days with a non-obvious tornado threat.

 

[afischer]

In comparison to the 00Z AMA low-level hodograph... the Jayton TX profiler showed an impressive low-level evolution in the 23-00Z timeframe (around the time of the primary tornado photographed by chasers in the Plainview area?) that is worth checking out. In fact, hodographs plotted from the 00Z and 01Z data show 0-1 km SRH of 280-300 m2/s2 for right-movers (lining up reasonably well with the motion of the "Plainview cell") as the 500-1000m flow accelerated and backed on the profiler. This corresponded pretty well with an apparent surface response just east of the dryline during that same time frame (assumedly either isallobaric in nature, or due to effects of modifying/dissipating outflow air on the far western edge of where the Red River MCS tracked?) I am just now digging into the data (watched TV during most of the event), and will look at it more tomorrow... regardless, the low-level shear was far more impressive via the Jayton profiler than I would have expected... though it is inconclusive until I do more analysis whether this profiler represents the Plainview cell any better than the AMA RAOB does. Also looks like the Planview area storm seemed to track just to the cool side of a boundary via radar imagery. Panhandle magic does it again.

 

[Jeff Snyder]

I guess that might seem obvious, but I can't remember a day that so clearly lacked strong deep-layer shear and yet produced several photogenic supercells and tornadoes. Also, the RUC mesoanalysis showed < 100 m2/s2 of 0-1 km SRH today over the TX panhandle. This is another example of why it's good to chase days with a non-obvious tornado threat.

Gabe, it kind of reminds me of the Orienta, OK, day on 6/13/07 (Severe Thunderstorm Events entry), a day on which we saw a myriad of tornadoes from a nearly-stationary supercell (for those who aren't familiar with that day). Like today, 6/13/07 featured an OFB, relatively high CAPE, and relatively low LCLs; likewise, it featured very marginal deep-layer shear owing to only ~20-25 kt 500 mb flow. Actually, the deep-layer wind field from that day isn't too disimiliar from that from today in the Texas Panhandle and southwestern KS. As much weight as we tend to place on deep-layer shear necessary for a supercell, both of these featured cyclic tornadic supercells in quite marginal deep-layer shear. Alas, good CAPE + good moisture + good surface boundary = good time to chase? Too bad I have so much to do in the next 10 days before VORTEX II starts, since I actually enjoy the marginal chases from time to time, primarily when we have strong instability (CAPE >3000 j/kg). Unfortunately, I've chased on other days that featured similar parameters (good CAPE, weak deep-layer shear, good surface boundary, etc) that resulted in little other than multicell trash. *shrug* Props to the SPC today, though, for recognizing the threat with a 10% tor prob and tornado watch.

Afischer (Andrew? Andy? Aaron?) -- good catch! I was watching the Jayton profiler earlier in the afternoon, but forgot to look at it when I was looking at the RAOB data from this evening. In addition, the West Texas Mesonet folks launched a sounding from Reese Center (Lubbock) at 19z, and it recorded some slightly better winds aloft with a pretty good "loaded gun" thermo profile.

 

[afischer]

Yeah, and similar to the Reese Center sounding you posted, the 22-01Z Jayton profiler data showed no major weaknesses in the flow aloft... with 7-10 km winds solidly in the 45-60 kt range. Given the extremely light and semi-erratic nature of the winds above 8 km agl on the AMA sounding, I wonder if the convection immediately north of AMA was causing problems to the balloon (wherever it was by that point in the atmosphere). Though I see no evidence of saturation/anvil level clouds in that same layer, the balloon may have been "munched" by convection when the data stopped above 200mb.

All in all it looks like this was a surprisingly favorable environment for tornadic supercells on the mesoscale starting shortly after 23Z, in the area of maximized low-level mass response just east of the dryline and near the OFB(s)... with upper 60s sfc dewpoints streaming into the area, no less (the PWATs from the 00Z OUN and DFW RAOBs were unseasonably high). To go along with the strong low-level shear and at least moderate deep layer shear... modified AMA soundings adjusted for slightly lower elevations east of Plainview show a solid 3000 J/kg MLCAPE, with a very large amount of it below 3 km agl (~150 J/kg). Very impressive!

Andy

 

[Rich Thompson]

I haven't seen any mention of the terrain in this discussion. If I've got my locations straight, the tornadic storm occurred right on the edge of the Caprock, while the next storm to the east was just off the terrain break at a lower elevation. I'm guessing that the rain-cooled air mass was a little deeper just off the Caprock in Motley County, and the low-level lapse rates were greater on the Caprock in Floyd County near the wrn edge of the old outflow.

The radar presentation of the Floyd County storm still puzzles me a bit - it had a very pronounced rear-flank gust front/fine line, and it was moving rather quickly to the east compared to some other storms. I'd normally take that to mean it's being driven more strongly by cold outflow, and that the tornado threat would be limited somewhat.

Did anyone actually leave the Motley County storm to go look at the Floyd storm, or did everyone with a closer view of the Floyd tornadoes just happen to already be looking at that storm when it cranked up? I ask because I doubt I would normally leave the Motley storm for the Floyd storm, unless the surface obs suggested a large difference in environments (e.g., much higher Tds and somewhat warmer T compared to the Motley County environment near the outflow boundary).

Rich T.

 

[Wesley Luginbyhl]

I was giving up on the Plainview storm when it hit the boundary. I was dropping south (207) and then was going to go east (97) when I noticed pretty strong rotation and a small funnel along what I thought was outflow from the storm. I went east a few miles to get in front of the storm and thats when I noticed what looked like a large funnel coming down in front of the main updraft behind it (too bad contrast to see it well). You could actually see the condensation of the updraft getting pulled around the funnel. Moments after the funnel had pulled the updraft around it, the tornado was extending down from the cloud base. I was wondering if anyone had see or heard of something like it this? The large funnel was extending down from much higher out of the storm and the lower part of the updraft got pulled around it, and quickly produced the large tornado. I have never seen anything like it, thats for sure. I might have the whole process on video if the contrast is not too bad. Did anyone else have a better view of this?

Here is a pic of the funnel brfore the updraft wrapped around.

 

[mikegeukes]

Tornado Causes Damaged in Southern Finney County Kansas
http://www.crh.noaa.gov/news/display_cmsstory.php?wfo=ddc&storyid=26507&source=0

NWS Lubbock has information about the tornadoes.
http://www.srh.noaa.gov/lub/?n=events-2009-20090429_storms

Mike