Wednesday, June 15, 2011

A good chase Part 2

It is always very instructive to go back and learn something about the storms you chase. This is really the only way to chase for me, and now that I have access to vast amounts of data with actual data visualization capabilities, like zoom, without writing massive amounts of code (downloading, storing, processing, picture making, looping, then zooming) I looked at our storm from Saturday.

Turns out there wasn't just one merger but at least 3, in different parts of the storm right about when we stopped just before Follet, TX to the west of Shattuck, OK. I had assumed that the one storm merger I saw coming was the culprit for the crazy behavior we thought was happening. The data present a different picture of what I might call a weak storm.

The storm really was weak in terms of rotation from the lowest elevation angle as seen from Amarillo. Visually we recognized this fact, but the slow moving storm, was really trying to generate rotation that may just have been small enough to avoid radar detection at relatively long distances from the radar.

So around 7:25pm a new storm went up to the west southwest of our storm (achieving 40 dBz) and merges to the north by 7:43pm. A new storm forms on the southern side of our storm at this time (hitting 35 dBz) and merges to the southeast by 8:01pm. Another storm initiates by 8:10pm and merges 9 minutes later.

All of these mergers had a constructive effect on the storm, helping it achieve a substantial size (even squareness) that made it resemble an HP supercell. The result was a decent vortex at 8:15pm (after we had already seen a few attempts at tornado-genesis) and while we were in the second RFD surge. As we continued after this surge, basically following close behind the demarcation between rain driven RFD and decent conditions, we re-entered the RFD and then after a brief "lull" another RFD hit us. This is, I think, the effect of the older circulation RFD hit us, the mesocyclone moved east and then southeast, and a new meso formed back to the north moving southeast. So our two RFD surges on 15 east of Follet were from different circulations, and it was the second circulation which jumped further southeast that produced a tornado which hit 6 miles north of Shattuck.

Just as a reference for the environment I saved the Amarillo and Norman soundings:


The bulk shear in the 0-6km layer is at or above 40 knots, the instability is marginal at AMA but if we account for a solid 10F increase in dew point where our storm was, then the instability was probably in the 3200 ballpark. Arnett, OK dew points just to the south of Shattuck maintained a 60 dew point well into 7pm. The other difference at Amarillo is the wind direction in the lowest layers, which was southerly in the sounding, and southeast at 15 knots, probably lengthening the hodograph.