Question on Initiation - Harper, Kansas 5/19/12

[Peter Wharton]

I've been reasonably successful in targeting so far this season (3 weeks), but on the 19th I ended up initially well north of the storms that initiated between Great Bend & St John, Kansas, and ultimately went on to produce the tornadoes near Harper, Kansas -we did make it down in time to see the white tornado that damaged the wind farm near Rago, but would have been better to have been there a couple of hours earlier.

My question is - does anyone have some informed insight as to why these specific storms initiated where they did, and so powerfully - when elsewhere on the line, it was much later and weaker? I had targeted south-central Nebraska and north central Kansas, but after observing conditions there, decided to move south - still was taken by surprise by these storms so much farther south.

What were the critical factors - and did any of you forsee them? Especially those that were in the immediate vicinity - always good to learn from specific situations.

 

[Andy Jackson]

I've been reasonably successful in targeting so far this season (3 weeks), but on the 19th I ended up initially well north of the storms that initiated between Great Bend & St John, Kansas, and ultimately went on to produce the tornadoes near Harper, Kansas -we did make it down in time to see the white tornado that damaged the wind farm near Rago, but would have been better to have been there a couple of hours earlier.

My question is - does anyone have some informed insight as to why these specific storms initiated where they did, and so powerfully - when elsewhere on the line, it was much later and weaker? I had targeted south-central Nebraska and north central Kansas, but after observing conditions there, decided to move south - still was taken by surprise by these storms so much farther south.

What were the critical factors - and did any of you forsee them? Especially those that were in the immediate vicinity - always good to learn from specific situations.

Try this blog:
Jon Davies Blog

Steep low level lapse rates usually are what you need for powerful initiation. I don't think I saw any wording of landspout outbreak in S. Central KS in any of the forecasts I read, yet people targeted that area. I targeted NE and pretty much got what I expected.

 

[Peter Wharton]

Thanks, Andy - good read on the mechanics of why tornadoes once the storms initiated - still want to know - why there?

 

[Jesse Risley]

Thanks, Andy - good read on the mechanics of why tornadoes once the storms initiated - still want to know - why there?

I didn't chase Saturday, but I'd imagine it did have something to do with the southwestward moving OFB that Jon noted in his excellent analysis. I do recall seeing other storms further north and south when I glanced at radar, though if I were to guess why they didn't produce the same results, I would hypothesize that it had more to do with enhanced shear owed to the OFB, coupled with the fact that the S KS storms seemed to form in vicinity of the norther periphery of the steepest low-level lapse rates and the deepest moisture gradient (Td values > 60 degrees F), which coincided with some of the better vorticity values along this seemingly innocuous shear line (abrupt velocity change).

 

[Jeff Duda]

Convective initiation is a very sensitive process. Likely, the true reason convection initiated where it did and didn't initiate where it didn't will forever be lost in the small scale details (e.g., HCRs in the PBL) that are left unresolved in the RUC mesoanalyses and the surface METAR network or that were not directly observed (e.g., vertical motion in and above the top of the PBL).

When it really comes down to it, all you need for initiation is for a parcel to reach its LFC and then not have the resulting updraft killed off by entrainment of dry/cold air. For individual air parcels this is simply not feasible to measure. The best that meteorologists can do is determine average or representative values on a larger scale. However, the PBL is especially inhomogeneous and each variable (e.g., temperature, pressure, and wind) has significant spatial and temporal variability that can make the difference between an unstable parcel that creates a thunderstorm and a stable parcel that just doesn't quite reach its LFC.

When you have strong forcing in an environment that is approximately thermodynamically homogeneous on the larger scales (i.e., similar CAPE and CIN over some region larger than a fraction of a county), usually the area preferential for initiation will be that with the strongest forcing. Another way to look at it is that when you have a similar amount of forcing in a region, the area that is most unstable (i.e., least CIN) will be preferential for initiation. As a chaser without your own personal 10-m resolution network of sensors, the best you can do is pay attention to boundaries via satellite, radar, or surface obs that lie within a synoptically favorable environment. Sometimes you'll hit it, sometimes you won't. Such is what makes chasing still a matter of luck.

 

[Peter Wharton]

Thanks for the thoughtful responses

 

[chrisbray]

Try this blog:
Jon Davies Blog

Steep low level lapse rates usually are what you need for powerful initiation. I don't think I saw any wording of landspout outbreak in S. Central KS in any of the forecasts I read, yet people targeted that area. I targeted NE and pretty much got what I expected.

Thanks for that link...that was indeed a great read and helped shed some light on weather questions for a relative newbie like me. I'll have to read some more of that blog!

 

[Dave Kaplow]

There was a very pertinent post about this day over at the American Weather forums which touches directly on the “why there?“ question. The posted map is from the SPC mesoanalysis page, and is a product I didn’t know much about prior to seeing it in that thread. After familiarizing myself with it as much as possible (the math is definitely over my head) I can see how useful this parameter might be. Basically, if I understand things correctly, the HVGTM (it’s awkward even as an acronym) highlights boundaries, areas where shear and convergence are maximized and which we know can sometimes be conducive to tornado formation. As you can see on the map, the 5/19 tornadic storms all fired in the pronounced HVGTM bullseye. As Jon Davies mentions in his blog entry, the non-supercell tornado parameter was no help at all that day, but clearly the HVGTM was, if you knew enough to look at it. This is a product I will definitely be using in the future, and I thought it worth mentioning here because of its accuracy on the day in question.

Note: The AW thread where David Reimer originally posted this image can be found here, but since non-members can’t see images on that board I’ve deep-linked directly to the Texas Storm Chasers site hosting the image. All credit goes to Dave for knowing the usefulness of this product and posting about it on 5/19.

 

[chrisbray]

Thanks for that, I found that on the mesoanylsis page, but I seem unable to find anything explaining basically what this chart is displaying...would anyone be so kind?

 

[Mike Johnston]

Thanks for that, I found that on the mesoanylsis page, but I seem unable to find anything explaining basically what this chart is displaying...would anyone be so kind?

When you're selecting the parameter from the drop down menu at the top (this one is under "kinematics"), there is an adjacent blue question mark button. Click on that button, and it will give you a brief explanation of the parameter. Frankly, I don't understand all the terms referenced, but at least it can be further researched from there.

 

[chrisbray]

When you're selecting the parameter from the drop down menu at the top (this one is under "kinematics"), there is an adjacent blue question mark button. Click on that button, and it will give you a brief explanation of the parameter. Frankly, I don't understand all the terms referenced, but at least it can be further researched from there.

grr, sorry for the post. i know they have that question mark, but I swore they didn't have that as an option for that parameter. Sorry about that

 

[Jeff Duda]

Basically it's the sum of all the possible kinematic manipulations involving the first partial derivatives of the horizontal winds. Since you have a u and v component in the x and y directions, you can make the following derivatives:

du/dx, du/dy, dv/dx, and dv/dy.

Now make all possible combinations of pairs of these that involve both the u and v components (with some sign differences). They are:

du/dx + dv/dy (divergence)
dv/dx - du/dy (vorticity)
du/dx - dv/dy (one type of deformation)
dv/dx + du/dy (another type of deformation)

Square each of those components and sum them. Then take the square root. That is the value of the product shown. Each of those terms represents some type of forcing related to frontogenesis, so their combination gives an idea of the total forcing present.

Deformation in particular tends to create and tighten gradients (e.g., temperature), while divergence is associated with vertical motion. Vorticity, as you know, is associated with rotation. Therefore, it seems natural to think that s**t may be most likely to go down in areas with the largest value of that product.

 

[chrisbray]

Wow, thanks for that explanation!