10/17/07 DISC: KS, OK, MO, AR

[Mike Peregrine]

Nice shot of the tornado near Mt. Vernon (the cell that became tor-warned south of Joplin) on CNN here.

Let's talk about yesterday. Wondering if everyone feels their fears concerning shear and instability came to fruition and these were the flies in the ointment - or if there were other factors at play to make for a less-than-expected result (tornado-wise).

One thing I noticed on this end of the boundary was the ridiculous amounts of moisture available in this system. As the various rounds moved through this area, clouds were condensating everywhere out of thin air. At one point we noticed an enormous batch of fog that condensated over the mid-town KC area and then lift into the cloud base, which was pretty interesting to watch under the base of a strong (to become severe) storm.

Also interesting to me yesterday was the rogue tornado in Indiana ... was this along a kink in the warm front somewhere? Wondering what provoked the lift and localized SRH values to make that one happen?

 

[Brian Stertz]

The SW Missouri tornadoes kind of followed the historical pattern of LLJ interactions with the Ozark-Boston Mtns. I know this has been studied often by the NWS Springfield and tornadoes (some bad ones) have occurred in the past. I am wondering what boundary had focused that particular tornadic supercell? I know that first wave had robbed alot of the deeper moisture and shifted the main LLJ axis off to the east of OK yesterday. I feel this probably kept things from going too nuts there....that and the major subsidence as Rocky and others have mentioned.

 

[Jeff Snyder]

I found it interesting that many of the storms that developed in C and NE OK seemed to produce killer outflow that spread well outward from the updraft bases. This was readily apparent as a fineline on radar, and it was equally apparent in the field. Here are a couple of my thoughts, developed after some post-chase discussion on the ride back to OUN:

1. Although it appeared that deep-layer shear was quite strong, I'm not convinced that the shear "experience" by the bulk of the depth of the updrafts was... If we assume that the OUN sounding from 1km and above is likely at least 0th-order representative of the environment immediately ahead of the dryline in central OK (i.e. the environment of the storms from OKC to Cushing, and the later storms that developed E of OKC near Stroud), then I contend that the magnitude of the 0-6km shear is quite misleading in terms of the effect of that shear on the storms. The 00z OUN sounding shows 45kts from the SW just off the surface, with 55-65kts at 6km. Sure, the winds at the surface ahead of the dryline were southerly or SSWrly at ~10-15kts, but it's likely prudent to average the sfc wind to formulate a mean lower-layer averaged wind (similar to using 75 mb ML CAPE as opposed to purely SB CAPE). Doing so, a rough estimate of the near-surface wind could be 30kts from the SSW or SW. This means that, though the sfc-6km shear may have been 55-65 kts, the shear from 500m-6km or 1km-6km was only 20-30kts. Though we had pretty strong shear in the low-levels, the shear from just above cloud base to 6km was actually quite weak, much much suggestive of multi-cell mode than supercell mode. In addition, the EL was likely quite low (not surprising -- most were expecting relatively low-topped convection), so the 800m-6km layer encompassed the vast majority of the updrafts. Visually, we saw something similar to a more typical multicell mode, with new updrafts that would develop upshear / to the south-southwest of ongoing convection... These new updrafts would intensify quickly, but then a new "puff" of convection would develop to its south or southwest while the previous updraft weakened. Intermittent supercell structures were observed, but these features were much more transient than, certainly, I had expected.

2. The veered flow off the surface also reduced the depth of the moisture along and a little ways ahead of the dryline. I haven't chased too many supercells in an environment of low-level moisture characterized by a quickly decreasing mixing ratio. Farther east, the 850mb flow hadn't yet ushered in the drier air from western OK/TX, and the 850mb flow a little less veered, so moisture depth better and deep-layer shear was stronger. In addition, with very strong near-surface shear, shallow moisture depth may have allowed for a considerably lower MEAN "updraft parcel" RH (i.e. a 75mb mixed-layer mixing ratio or specific humidity considerably lower than the surface observations suggested). My first observation of the cell on the SW side of OKC was "Hmm, that cloud base looks higher than I thought it would given surface dewpoint deficits of ~10-13 F... In addition, drier air just off the sfc may have allowed for stronger cold pool generation, which may have lead to outflow containment problems I saw through the afternoon.

Just a couple of things to ponder, I suppose.

 

[Kevin Crawmer]

Here is a good link to some pictures from the tornado SW of Springfield, MO.

http://www.ktts.com/tabid/1666/xmid/15614/Default.aspx

It was amazing how isolated this cell was from the time it was on the OK/AR border until it fizzled out in central MO. As someone mentioned in the NOW thread that day, there was literally nothing within 100 miles of it for a long time.

 

[Patrick Marsh]

1. Although it appeared that deep-layer shear was quite strong, I'm not convinced that the shear "experience" by the bulk of the depth of the updrafts was... If we assume that the OUN sounding from 1km and above is likely at least 0th-order representative of the environment immediately ahead of the dryline in central OK (i.e. the environment of the storms from OKC to Cushing, and the later storms that developed E of OKC near Stroud), then I contend that the magnitude of the 0-6km shear is quite misleading in terms of the effect of that shear on the storms. The 00z OUN sounding shows 45kts from the SW just off the surface, with 55-65kts at 6km. Sure, the winds at the surface ahead of the dryline were southerly or SSWrly at ~10-15kts, but it's likely prudent to average the sfc wind to formulate a mean lower-layer averaged wind (similar to using 75 mb ML CAPE as opposed to purely SB CAPE). Doing so, a rough estimate of the near-surface wind could be 30kts from the SSW or SW. This means that, though the sfc-6km shear may have been 55-65 kts, the shear from 500m-6km or 1km-6km was only 20-30kts. Though we had pretty strong shear in the low-levels, the shear from just above cloud base to 6km was actually quite weak, much much suggestive of multi-cell mode than supercell mode. In addition, the EL was likely quite low (not surprising -- most were expecting relatively low-topped convection), so the 800m-6km layer encompassed the vast majority of the updrafts. Visually, we saw something similar to a more typical multicell mode, with new updrafts that would develop upshear / to the south-southwest of ongoing convection... These new updrafts would intensify quickly, but then a new "puff" of convection would develop to its south or southwest while the previous updraft weakened. Intermittent supercell structures were observed, but these features were much more transient than, certainly, I had expected.

While watching the storms on radar that day I was thinking to myself how weird it was that none of them were "splitting" with such a straight line hodograph. "Mesocyclones" - if you can call them that - were extremely transient and were frequently outlasted by "anti-mesocyclones" (which also were transient). At the time I was arguing to myself that the storms seemed to be rooted just above the strong shear (which as Jeff mentioned was primarily located just off the surface).

After talking to several chasers that day who reported those cells to be relatively high based, I'm fairly certain that was the case.