5/1/10 DISC: LA/AR/MS/TN/KY/MO/IL/IN

[Jeff Snyder]

I'll start a DISC thread since I'm addressing the setup a bit more from a hindsight standpoint than from a nowcast standpoint...

A couple of problems I'm noticing with this setup (although quite late in the game now). The storms are moving with the mean flow with very little deviant motion. Without any right turning supercells the storm relative shear is not being enhanced. Also the surface winds are rather light and mediocre. With storms moving away from the these lights winds, the relative inflow is probably very weak. Also, low level lapse rates are terrible. Sure Cape is up to 3000 but without those lapse rates and weak inflow we're not seeing that explosive updraft development and low level structure with fast rising motion. Yes, the low level jet is ramping up tonight but these storms have been down trending for the past hour. This looks like the biggest bust of '10 so far.

I agree with Skip's assessment for the most part. However, looking at the LZK hodograph (SEE HERE), the low-level shear would be WEAKER if the surface winds were stronger. Remember, length and shape of the hodograph are important. In this case, if we increase the surface winds, the length of the hodograph in the low-levels decreases, the low-level shear decreases, and the storm-relative low-level winds correspondingly weaken. In essence, the shear and low-level storm-relative winds were strong as a result of the weaker surface winds.

The LZK and SHV hodographs from 00z sounding data are rather straight, without a considerable amount of low-level curvature. Despite this, the 0-1km SRH is still quite high -- ~300 m2/s2 at LZK and ~200 m2/s2 at SHV. Though there's quite a bit of crosswise vorticity since the hodographs aren't particularly curved, the length of the hodo in the low-levels with some curvature has enough streamwise vorticity to yield impressive 0-1km SRH. Interestingly, there is almost no SRH between 1-3 km (i.e. 0-1km SRH = 0-3km SRH) at LZK, so perhaps, despite the strong near-surface SRH, the high-helicity air isn't very deep. In fact, the LZK sounding has negative SRH between 1-2 km AGL, as a result of a shallow weakness in the flow near 2km. Certainly the measures of MLCAPE, SBCAPE, MLCINH, 0-1km SRH, LCLs, etc., are all favorable for sustained tornadic supercells. The LZK sounding doesn't have very good near-surface lapse rates, but it was also just north of a surface front, and the temperature to the immediate south of the front were 5-10 F warmer, which implies that the low-level should have been a little steeper for those cell on the south side of the front.

I certainly expected more from today, that's for sure. Perhaps there were issues with storm interaction?

 

[Matthew Piechota]

Could it have been due to a warm layer aloft? The lapse rates were just not that great. There was quite of bit of supercells at the onset with a good number of left movers that were interacting with other storms.

As Skip said, the storms were moving with the mean wind most of the time but when that storm west of Wynne, AR made a right turn the LL rotation substantially increased with a good meso from 3,000ft to 13,000ft and then started putting down tornadoes.

The storm further south looked pretty good on radar but just could never get that LL rotation even when the LLJ picked up later in the evening. And with pertains to both storms, I would of thought the further north storm would of had problems with inflow and rotation due to the southern storm but it backwards with the northern storm being the dominate one.

Im pretty shocked with the lack of tornado reports with this setup given the storms were able to stay discrete. Would of been nice to have V2 on these storms to see what the near storm environments were like.

 

[Matt Chatelain]

(This is Daniel Betten not Matt, I don't have a name and I don't want to pay to post once a year.)

I was noticing the same things that have been mentioned so far. One thing I would like to throw out there is a possible consequence of weaker flow in the 1-3km layer. It was mentioned before that the storms went with the mean wind for the most part today with a couple exceptions. This might be due to a lack of convergence on the eastern and southern flanks of the gust front because the storm motion was faster than the mean inflow layer flow, this would lead to stronger updrafts on the northern flank and thus storm propagation will move in that direction.
The storms had persistent hooks but it did not seem like they wrapped up much, so the gust fronts had a hard time occluding, another symptom that the low-level updrafts were stronger on the northern flank which in this case would be downwind of the mean storm motion. The few storms that did deviate were farther east and south were the 850mb winds were stronger. I wasn't out there so this is all speculation, and without seeing the actual 3D wind field I wouldn't be able to verify this. But I'm working on dual-Doppler analysis of the May 29th 2004 storm which was very cyclic in nature and had a persistent updraft that focused on the southeast and southern flanks. Just throwing it out there.

 

[Jeff Snyder]

Matt (err... Daniel) -- that seems possible, but note that the 00z LZK sounding indicates that the storm-relative winds were from the east in the 1-3km layer (not from the north). As such, if you're speaking purely of inflow into a storm, the 1-3 km layer was characterized by easterly (storm-relative) winds. For those storms that moved slower than 40 kts and/or had a component of motion that was ENE (were there many? I wasn't able to watch radar much this evening), the storm-relative 1-3 km winds would have been southeasterly. If the storm motion was faster, the low-level SR winds would have been from the ENE or NE, which means there might have been a tendency to advect into the updraft cooler air from the FFD. I don't know the answer as to why we didn't see more tornadic supercells today, so you may be right for all I know.

The motion that was more along the mean wind vector typically results from a straight-line hodograph (before a supercell splits), wherein the storm-relative wind does not veer appreciably with height [it is the veering with height of the storm-relative wind, after all, that results in the dislocation (i.e. rotation) of perturbation pressure minima/maxima with height that results preferentially in updraft enhancement on one flank of the updraft and updraft "discouragement" on the opposite flank). The LZK hodograph is pretty much straight-line between 1 km and 7-8 km, which may not be particularly favorable for storm motion that deviates from some fraction of the mean tropospheric wind. I'd have to look through other hodographs associated with strong tornadic supercells to see if there's much curvature of the hodograph above 1 km.

Now, let's suppose that "most" of the air comprising the updraft was located near 1 km, instead of a surface-parcel. In fact,there's some evidence to suggest this may have been the case, since the maximum CAPE (i.e. MUCAPE) is for a parcel at 925 mb (at LZK) instead of at the surface. At any rate, if we only look at the wind profile from 1 km to the troposphere, we see strong but straight-line shear, despite winds that veer from southerly at 1 km AGL to southwesterly at 3 km (and above). Again, though, I'm not sure this is applicable in those areas just south of the front/boundary, where temperatures were 5-10 F warmer than at LZK IIRC, and where MUCAPE should be for parcels very near the surface (as seen in the SHV sounding). In addition, I'm not sure how much we know about the depth of the inflow layer in environments like this one (where a sfc-parcel has significant CAPE, yet doesn't have the MOST CAPE compared to parcels at higher levels).

One final thought... We know that water loading can significantly affect updraft intensity. Despite the strong shear in place, I wonder if the significant water loading that must have been occurring within the updraft (with sfc tds of 70-75 F and PWs >1.6" on LZK and SHV soundings) may have affected updraft intensity. Then again, we CAPE isn't a terribly good (define that however you desire) predictor of tornado development, so the weakened updraft intensity (resulting from high water loading) may only be analogous to being in an environment with lower CAPE, which (since CAPE, at least assuming it's high enough to support a vigorous updraft, doesn't predict tornado development well) shouldn't have affected tornado development appreciably. ?

 

[Drew.Gardonia]

i believe, just as the case was last week, that the lack of diurnal heating (east of the Mississippi) due to thick cloud cover prevented this system from reaching the potential severe levels that the NWS had predicted. The surface temps weren't warm enough to promote intense long-tracked supercells, even though the dew points where nominal, as was CAPE and windshear. Despite the lack of diurnal heating, the Nashville area still had several tornado warned cells in which Doppler radar indicated rotation within the cells, with one confirmed funnel spotted in Maury County, TN at 0319 zulu (5/2/10). and one at 1945 zulu (5/1/10) a funnel cloud was confirmed and photographed here (shown on Channel 2 news, but i can't seem to locate the photograph on their website) in Murfreesboro, TN (Rutherford County) but never touched down.

at 1715 zulu when a cell in Maury County, TN went tornado warned and headed towards Rutherford County, I made a brief attempt at an intercept in Murfreesboro but came up empty handed. I did however encounter intense straight line winds (not sure on wind speeds since I have nothing to base the speeds off of) torrential down pour and frequent lightning. Rotation was visible within the low hanging cloud base. On the return to the house, roads were nearly impassable due to extreme flooding.
(images are larger than 900 pixels, so i've just posted the links)

http://i590.photobucket.com/albums/ss347/mtsumustang/IMG_1571.jpg

http://i590.photobucket.com/albums/ss347/mtsumustang/IMG_1585.jpg

http://i590.photobucket.com/albums/ss347/mtsumustang/IMG_1586.jpg

I arrived home to a rather large lake in the backyard

http://i590.photobucket.com/albums/ss347/mtsumustang/IMG_1587.jpg

http://i590.photobucket.com/albums/ss347/mtsumustang/IMG_1588.jpg

all in all very disappointing, considering it looked like a nominal setup for a major outbreak.

 

[Darrin Rasberry]

Playing off what Andrew just wrote, I noticed that temps aloft were VERY warm - they were 10+C at 700mb at around 6 PM, for instance. I thought the setup was going to blue-sky for a bit due to this, since for a long while there was nothing at all but a couple of persistent underachieving popcorn storms. Didn't think the heat at the surface was enough to really overcome the sort of temps aloft that we saw.

I'm pretty new at forecasting, so I'd love to see some other thoughts down this line. I seem to learn a lot more from busts of this magnitude (think 6/5/08) than anything else specifically.

 

[L.B. LaForce]

A couple of problems I'm noticing with this setup (although quite late in the game now). The storms are moving with the mean flow with very little deviant motion. Without any right turning supercells the storm relative shear is not being enhanced. Also the surface winds are rather light and mediocre. With storms moving away from the these lights winds, the relative inflow is probably very weak. Also, low level lapse rates are terrible. Sure Cape is up to 3000 but without those lapse rates and weak inflow we're not seeing that explosive updraft development and low level structure with fast rising motion. Yes, the low level jet is ramping up tonight but these storms have been down trending for the past hour. This looks like the biggest bust of '10 so far.

Yes I agree to some extent on your analysis. The low level lapse rates were pretty aweful, at about 5.5 C/km, but the relative inflow did not seem that bad. Looking back at the mesoanalysis site shows the SR sfc to 2km inflow winds at around 25-30 kts, which is fairly decent. I'm not exactly sure though went wrong last night, and im awaiting a case study

 

[Brian Stertz]

One observation I had from the field was that the strong mid level jet seemed to overpower the updrafts and lean them over. This stretched out the mesos so that they could not get a solid vertical vorticity column organized and produce. Could also make a very strong argument that lapse rates were also sucky (as mentioned above). The towers never did get that explosive crisp look. Fuzzy and sheared entities. Thirdly cap was also certainly a factor. Had it been 6 degrees C at 700mb as opposed to 9-11...things may have progressed in a more violent High Risk manner. Jeff and I chased the Pine Bluff area first and then committed to the strong supercell that tracked all the way from the LA border southeast of El Dorado AR northeast to south of Memphis TN. Radar indicated it maintained strong rotation and t-warnings the whole way. Visually it often looked on the HP side...but we could certainly see very large bowl shaped wall clouds in the occasional lightning. Probably saw what was the early stages of the De Witt AR tornado too. All in all while radar made Jeff and I drool at times, bottomline was the supercells were sloppy Mississippi Valley type. Lots o rain around...and sometimes quarter sized hail when we'd find the core.