2013-04-01 MISC: TX

[Skip Talbot]

I wasn't paying much attention today's setup, and was surprised when I saw screen captures of a large cone tornado that David Drummond was streaming:

Very impressive considering that the report was just outside of SPC's 2% tornado line. More intrigued about what was going on down there, I started to look at some of the surface observations. Metars in the region were showing dews in the lower 40's and temps in the 80's. Now I'm baffled. That's a huge T/Td spread for a large, fully condensed cone tornado like that. I had to dig a little deeper to see what was really going on.

The afternoon HRRR did a really nice job with the setup a few hours before the event occurred. There's a pinpoint spot with a moisture convergence bullseye near Silverton, TX. The forecast dews there are in the mid 50's, not bad at all for that elevation. The HRRR also showed the east/west line of cells of firing along the boundary draped across the panhandle. A forecast sounding in the warm sector shows some really nasty temp/dewpoint spreads and sky high LCL's. In the wake of the initial storms, however, there is considerable cooling and moistening of these profiles, and that's exactly where the producing tracked: right on top of this convergence bullseye, and in the wake of the leading supercells where the pump had been primed. It seems there was just enough instability, huge curving hodographs, and perhaps some outflow boundaries at work, and a fat cone was the result. It pays to pay attention to the finer details when there is weather in your backyard!

 

[Jeff Snyder]

Nice discussion, Skip! I'll just put this up here for reference. It's from the 22 UTC RAP valid at 22 UTC in Silverton. Obviously, looking at the thermodynamic profile, the model has convection in the area (nearly saturated through the troposphere with nearly moist adiabatic lapse rates), but the hodograph speaks for itself. It's a good reminder that a beautiful hodograph can overcome very marginal CAPE if storm mode is favorable. I'm surprised that the cloud base looked as low as it did (based on pictures and video) -- I wonder if the storm was ingesting some higher-RH air from storms to the south and east. The air must have still had enough CAPE and weak enough CINH to be ingested into the supercell's updraft. Just speculating, though. The dewpoints were low enough and dewpoint depressions high enough for me not to give the day much thought. Oops.

 

[Hank Schyma]

Here's a radar image just after the tornado warning... The cell to the south was moving North into the tornado warned cell... the outflow perhaps Skip was discussing.

 

[Adam Lucio]

Those soundings are interesting, because I've seen recent posts on FB from people saying those "inverted V" soundings are bad for tornadoes and promote outflow dominant crud. Proof once again the weather doesn't care about supposed "rules."

I was following that storm on radar and noticed the moisture pooling on the surface obs. There was a pretty sharp bounday in the area where you had 80/42 to the south and then 63/58 to the north. That storm probably found the sweet spot and the pooled moisture helped the LCLs, along with good surface convergence and a little caprock magic. Definitely an odd case. Props to David Drummond though, he called it the night before, did his hand analysis in the morning, went out and got the tornado noone else did despite what the forecast said. THATS HOW YOU DO IT!

 

[Skip Talbot]

Those soundings are interesting, because I've seen recent posts on FB from people saying those "inverted V" soundings are bad for tornadoes and promote outflow dominant crud. Proof once again the weather doesn't care about supposed "rules."

They do produce high based storms with strong outflow, and I think that's what was going on with most of the lead storms. Those storms were moistening and cooling those profiles though. You can see the first sounding (out in the hot air ahead of the storms) shows the huge spread, while the second (in the wake of the lead storms) squeezes it down to a much more respectable looking T/Td depression and moisture depth in the low levels. In addition to the moisture convergence at the triple point, I think these lead cells' manipulation of the thermodynamics for the cells in their wake played a huge role in why that one cell produced. Of course you usually lose your instability in the wake of a bunch of storms, as the midlevels warm and the surface cools. Lapse rates were quite steep, however, and the big spreads at the surface allowed for a lot of moistening before saturation occurred. There must have been just enough instability left over in the wake for that storm to produce, probably further aided by the orographic lift on the Caprock. That storm quickly died once it got off the Caprock and started ingesting more of that stable outflow, while the lead cells persisted but were high based with a lot of outflow.

 

[David Drummond]

Thanks for the interesting analysis Skip and Jeff. It was definitely one of those situations that if you didn't pay attention to the finer details of what was going on in near real time, you could easily miss it. A little Caprock Magic helped too!

video:
Watch video >

 

[Jeff Duda]

I have reason to believe that the cause of the tornado in this storm was due to one of two things:
1) Local terrain influences (the edge of the caprock being very nearby...great way to create horizontal vorticity)
2) Local storm interaction. This is most likely the reason I saw a tornado on May 22, 2011 in central Iowa and on May 29, 2012 near Piedmont, Oklahoma.

I'd be willing to bet a lot of money that if there was only one isolated storm this day, it would not have produced a tornado.