Low-level lapse rates are important and often overlooked in severe weather setups.The other factor to note is that non-supercell processes were at play in Jarrell and the May 19, 2012 Rago, Kansas event. In those two examples, deep-layer shear was on the weak side, but the vertical stretching on the boundary in an environment with strong 0-1km CAPE/low-level lapse rates was the main mechanism for tornadogenesis rather than the standard supercell/RFD process.
Jon Davies has a good writeup on this here:
Yesterday (May 19) in south-central Kansas was a great reminder that potentially strong tornadoes (see images above near Rago KS in t...davieswx.blogspot.com
I wouldn't say Jarrell "created its own shear". Jarrell moved extremely deviant to the 0-6km shear vector due to boundary interaction, which resulted in more effective shear than for a storm following the mean wind. Looking at modified FWD soundings for that day, there was actually a decent bit of effective shear already in place in the order of about 45kts or so which is plenty for Supercells, especially with such extreme instability in place. What was lacking was low level shear, which the deviant motion to the Southwest along the boundary likely made up for.
Be careful with the term "storm-relative shear" or "effective shear due to deviant motion". Technically, the only shear property that is dependent on storm motion is storm relative helicity, which is not the same thing as bulk shear. But I agree with the broader message that the deviant motion allowed for more significant vertical vorticity generation.Agree with Devin - we have to consider the storm-relative flow, and the effects of boundaries and cold-pool generation can have the updraught moving at a highly deviant motion, which can result in quite large storm-relative shear.
On days like May 24th, 2016 ('Dodge Day') the effect of a storm 'tacking-on' to a boundary helped with what was, overall, not 'great' background shear to create the tornadofest. Similarly on the next day around Chapman, KS.
Big CAPE helps, of course, just down to the higher thermodynamically-induced vertical accelerations.
Thank you! I agree very strongly and despise the use of the term "mesoscale accident". To me it's like attributing some other mysterious occurrence as an "act of god". No...it's just something you (or the entirety of humanity) has yet to figure out or explain.Shear is one of the mechanisms for sustaining supercell updrafts and good turning with height often makes tornadoes likely, but it is good to remember we are still working on tornado formation. Focusing on shear, or CAPE or combinations of ingredients doesn't always tell us precisely how they come together in dynamic environments to create a tornado. Some people call theseJarrell type surprise storms mesoscale accidents- I attribute it with things we just don't know yet, or cannot observe due to resolution of our tools. Some of the supercomputer simulations by Leigh Orf have accurately modeled real storms and shown very interesting features and mechanisms at 30 down to 10 meter resolution, much more than forecast tools provide.