My thoughts
I posted this earlier during the event... somehow it wasn't as talked about as the dozens of supercellular tornadoes...... wonder why!?!
It's most interesting that the main storms in Texas and Kansas are developing well east of the primary dryline. Surface obs put the dryline near EHA to AMA to GNC. That's matched by the westernmost cu field west of Amarillo which is just starting to see initiaion, and on up along the KS/CO border.
So I've been wondering what the other storms are developing on. Originally the arguemment could've been made that the initial cell developed due to a very fine shortwave or such moving through... but now the additional development up in the far eastern OK panhandle has led me to think dual-dryline. The SPC has had such feature in their MD, but have had both drylines well west.
SPC mesoanalysis doesn't show much to convict of this... but sfc data shows a local moist axis maybe through HHF, SNK, and BPG (though those stations may have been anomalously high on Td all day). There is also a faint hint of sfc convergence along the same axis (winds 130 in MCS vs 160 in PPA).
It actually looks like winds are starting to back significantly east of the main dryline the panhandle in response to it starting to move east, which may be beginning to smear out this other finer feature.
It could also just be favored HCRs, and in N Ks the largest storms are developing on a band between the dryline and where I'd place this second feature (which also had one little cell pop near Edmond, KS, even under cirrus still).
I remember both dual drylines and HCRs were of extreme interest on May 3, 99 (http://www.srh.noaa.gov/fwd/soopaper/20thSLS.html for one).
I'm interested for other's thoughts on the issue. Yes the environment was more favorable east... but you still need some focus to generate isolated storms. What was that feature?
I think the timing and evolution of the somewhat uncommon southern stream feature played a key role in how this played out. WAA started healthily in the region like last week... usually the WAA is tied to a strong polar-connected system which will drive through and wipe things out. Instead it was a southern branch disturbance that came through, the air mass had no polar connection whatsoever, and so the juice advection continued at a good clip for most of a week helping set up a better airmass than we typically see over such a large geographic area (especially early season).
Then the weakening low aloft was slow to come out. Last night's 500 map at 0Z still showed the trof axis from C KS through SE LA.
Therefore you had NVA continuing into the early morning, and the LLJ also didn't have the typical thermal gradient of a rapidly developing system because the air mass was already in place... and so there wasn't any overnight convection, and very sparse cloudiness.
***EDIT: I thought the cloudiness was generally cirrus, but see that wasn't the case. Guess the LLJ was still influential at low-levels, but all the cloud layers appeared to be pretty shallow, and further west the cap held them down entirely, I guess?
***
And then there was the healthy cap that advected in from the mountains and further helped keep preliminary grunge from mucking things up... and also helped limit weaker cells.
There was even still some weak backing of winds with height / CAA showing up at times this afternoon, as evidenced a bit in
http://moe.met.fsu.edu/~syoung/Viciprofiler.jpg
Though by this time and based upon the heights, that's more indicative of the capping... but I saw other images throughout the day that I think may've shown this better.
I was wondering for a little while what effect this could have on things today... but it quickly became apparent any slight effect on the low-mid level mesocyclone structure was being smothered by storm-scale vorticity processes.
I was also impressed how much the air mass cooled up around 500 mb overnight. That was the real requirement for real juicy CAPE as the sfc setup was already top quality. And it came in very well.
Then it was a matter of getting the boundaries. The feature Jim and I both alluded to was really the big beast today I believe, as parts of the dryline really struggled to get going, probably in part due to the cap and the slight modification of inflow air, as well as maybe some low-level diffluence that is requirement between two convergent boundaries.
There was at least one other feature with storms up in NW KS/SW NE that appeared to be between the two "drylines" we noticed (I would consider that the primary dryline was maybe a bit further east up there and the cold front the further west feature, but obs from same time frame in NE CO don't seem to support), and the first cell of the day actually developed further east than our 2nd feature down in the Tx Panhandle.
But all you need do to see the dual boundary nature of the event is look at SPC storm reports. Heck even NCEP analyzes a features in their current stuff, so I'm glad I wasn't selectively seeing things.
SPC indicated a second dryline feature on an early md, but I don't think it was the feature we are looking at:
http://www.spc.noaa.gov/products/md/md0361.html
Also mentioned the feature in a few other mds without totally coming up with a reason for why it was there, specifically:
http://www.spc.noaa.gov/products/md/md0358.html
Here is satellite along with my rough guess as to focus boundaries based upon tcu and sfc data:
http://moe.met.fsu.edu/~syoung/vissat032807_23Z.jpg
I definitely considered the theory that it was a differential temperature boundary due to the cloudcover. There is also maybe a bit of evidence to support this, with slightly higher temperatures over the general region by late afternoon... but I really don't see evidence of any T gradient whatsoever until 21Z at the earliest.
Also considered was orography. The first major storms of the day were right invof the Caprock. I've seen enough storms take off there... and I have no doubt it contributed a lot to the initial development, and where it occurred. But as the development started to fill into other regions without any connection the cold pools of the initial storms, I think that idea fizzled.
Also, as I mentioned earlier, the fallback explanation is always that it was just a particularly healthy HCR. But why this one was favored so much? Did it just get lucky, have extra convergence early on which helped to magnify its circulation and set it off?
I also find it interesting the feature held on even well after winds shifted much more easterly through the region in response to pressure falls to the west as upper dynamics encroached.
But definitely this boundary helped things be a show today. Double the locations to focus storms. And even moreso, it was embedded in more SSE flow, meaning more favorable helicity generically, and flow was further backed east of it due to convergence/lift along both boundaries, so definitely a superb SRH enhancer.
I was really not confident in storm mode last night, as, indeed, shear vectors were quite parallel to the boundaries. But I think the slightly more SSW to NNE layout of the boundaries helped, and then the flow over the southern Plains was so much more backed than I think almost anyone imagined. I guess the PVA aloft was stronger than considered?
One last thing... does anyone else remember setups that had as much moisture through the column as there was, especially last night before drier air came in aloft associated with the upper flow. PWATs were as high as 1.2" in the southern plains... but I guess the better indicator is just the high RH values in mid and upper levels (PWAT skews towards low level moisture amount). Meanwhile freezing levels were very low (below 600 mb at all 4 18Z soundings... even beneath 700 mb up in RAP). SPC did a great job forecasting the higher hail threat and lower wind threat (which continue into today). Just goes to show it's not just a broadbrush for the wind/hail threat as it sometimes may seem like, considering how similar the numbers are typically. But have there been seen such moist overall profiles in anyone's recent memory?
Anyways, always up for discussion and to be proven wrong. There's a lot of data out there, I'm hoping something will pin this down with some certainty.
It sure would be nice if they had an actual profiler in the N Tx Panhandle/S KS... and to have a project focusing on HCR/finer features in severe setups beyond the common OFBs, as these features have been the crowning feature in tornado outbreaks like today and May 3.
Thanks for the discussion!
Shane
