5/23/06 DISC: SD / NE / KS

[Jeff Snyder]

I had this in my REPORTS post, but it's not really suited for a REPORTS thread, so I moved it here...


So some brief post-chase notes (my 2 cents)... I think that the small size of many of the updrafts and 'storms' in northcentral KS today may have been influenced/affected by the combination of respectable low-level shear and the deeply-mixed boundary layer / low mean boundary layer RH / high LFCs / high LCLs. With ~30F dewpoint depressions, LCLs and LFCs were quite. In addition, with 30-40+ kt 850mb flow, there is a possibility that plumes that tried to get going were facing strong boundary layer turbulence. This turbulence may have helped to "eat away" at the edges of the plume / inflow going into the updraft, yielding a relatively small updraft. Now, this doesn't entirely explain things obviously, since the LFC was quite high over much of the area (~3km); so, with a deeply-mixed boundary layer, the updraft may not have been significantly affected (in terms of turbulence in and above the boundary layer and entrainment above the boundary layer) by the strong low-level flow and decent shear (mesoanlysis indicated ~20kts 0-1km shear by 0z), given that the updraft base was largely above that level. Regardless, I think this (strong turbulent mixing at the base of the updraft / inflow of the updaft leading to relatively-small updraft sizes) may have been the case with the storms in nc KS. By 1z, storms that developed south of I70 were congealing into small lines, but the cells between I70 and the KS/NE border seemed to be discrete for much of the late afternoon and early evening. If we had seen 70F Tds, much stronger CAPE would have been present, with much lower LFC and LCL heights as well, and I think we would have seen much more significant supercells in KS. In addition, stronger CAPE may have yielded stronger cloud-base / sub-cloud convergence (per mass continuity), thereby possibly offsetting any potential turbulent mixing effects influencing the lower-part of the updraft and inflow. This is pure speculation on my part, however. Props to the 4.5km WRF run last night, however, as it correctly depicted the squall-line evolution in NE, and also picked up on the discrete-turned-linear storm mode evolution in KS. It wasn't perfect, but not too shabby.

 

[Jack Beven]

Jeff and all,

Since this was the only chase I've made during my chase vacation, I thought I'd add a few observations and questions about the May 23 events.

First, I wasn't very impressed with the updrafts I saw yesterday. Most of the towers in Nebraska looked pretty mushy, and most of the time I didn't see a lot of lightning with the storms. Indeed, the squall line in northern Kansas was much more electrified than any of the storms I saw in the Kearney-Grand Island-Hastings area.

Were these storms actually tapping the surface-based CAPE, or were they feeding on some higher- elevation air? The only time I thought the cells in my area were becoming surface-based was when the developing squall line was approaching Wood River. At that time an elonagted lowering formed, just before or as the entire system started to gust out.

Second, has anyone looked at the hodographs at 00Z 24 May for the area? I was just ahead of the squall line and just west of a cell that passed over Gibbon, Nebraska, and it sure looked like the Gibbon cell split with twin elevated-based updrafts. Another related question: Would the elevated-based updraft have a different storm-relative helicity as compared to a surface-based updraft?

Third, Jim Leonard and company had told me earlier in the day that the upper-level flow (300 mb) was forecast to die off rather dramatically south of the Kansas border. I think this probably happened based on 1. Storms I saw forming south of Hastings where the upper portion of the updraft seemed to be vertical, not tilted, and 2. The tremendous amount of anvil cloud and mammatus I saw *west* of the squall line in northern Kansas. In the latter case, it looked to me that the cell outflow wasn't being affected much by upper-level winds. I wonder how much that messed up potential supercell development.

Finally, one interesting event was the strong *southerly* winds I encountered behind the squall line near Hastings and again northwest of Concordia. Was this this line outflow, the low-level jet, or a combination of the two?

Jack Beven

I had this in my REPORTS post, but it's not really suited for a REPORTS thread, so I moved it here...
So some brief post-chase notes (my 2 cents)... I think that the small size of many of the updrafts and 'storms' in northcentral KS today may have been influenced/affected by the combination of respectable low-level shear and the deeply-mixed boundary layer / low mean boundary layer RH / high LFCs / high LCLs. With ~30F dewpoint depressions, LCLs and LFCs were quite. In addition, with 30-40+ kt 850mb flow, there is a possibility that plumes that tried to get going were facing strong boundary layer turbulence. This turbulence may have helped to "eat away" at the edges of the plume / inflow going into the updraft, yielding a relatively small updraft. Now, this doesn't entirely explain things obviously, since the LFC was quite high over much of the area (~3km); so, with a deeply-mixed boundary layer, the updraft may not have been significantly affected (in terms of turbulence in and above the boundary layer and entrainment above the boundary layer) by the strong low-level flow and decent shear (mesoanlysis indicated ~20kts 0-1km shear by 0z), given that the updraft base was largely above that level. Regardless, I think this (strong turbulent mixing at the base of the updraft / inflow of the updaft leading to relatively-small updraft sizes) may have been the case with the storms in nc KS. By 1z, storms that developed south of I70 were congealing into small lines, but the cells between I70 and the KS/NE border seemed to be discrete for much of the late afternoon and early evening. If we had seen 70F Tds, much stronger CAPE would have been present, with much lower LFC and LCL heights as well, and I think we would have seen much more significant supercells in KS. In addition, stronger CAPE may have yielded stronger cloud-base / sub-cloud convergence (per mass continuity), thereby possibly offsetting any potential turbulent mixing effects influencing the lower-part of the updraft and inflow. This is pure speculation on my part, however. Props to the 4.5km WRF run last night, however, as it correctly depicted the squall-line evolution in NE, and also picked up on the discrete-turned-linear storm mode evolution in KS. It wasn't perfect, but not too shabby.
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[Blake Michaleski]

I think Jeff really makes a great analysis of the day Tuesday.

high T-Td spreads and resultant high LCLs, high LFCs and the deep mixing of the boundary layer are just a few of the things that I was cursing yesterday.

As we sat in Beloit, KS at the super8 (wifi) and watched the nice cumulus field develop to our west and overhead, I watched in near horror as the dewpoints all across the region dropped. I had worried about the marginal at best Tds early on and then became really concerned when the mixing started up along with the intense heating.

It was the only day I was able or will be able to chase this year aside from anything local to me.