What happened on 2/23?

[MikeD]

Okay. So I live in the Memphis district area. And I decided to chase.

Rant starts: Sure, there was good CAPE. Sure, plenty of directional wind shear. Sure, great helicity values. But what went wrong? There were only a few tornado warnings dropped. But moderate risk? I was expecting five times the amount of warnings.

Rant ends: What went wrong today? What happened to some of the cells?

More specifically, why didn’t the Collierville cell produce even though it passed through the MDT risk area at 4:30 PM? There was rotation on radar. Even a wall cloud lowered. I just don’t understand.

 

[Quincy Vagell]

Up in western Tennessee, I have to wonder if some of the early convection that passed from AR into TN around 18-19z was just enough to slightly overturn the atmosphere. Yes, if you look to the south, there was an unseasonably unstable air-mass in place across much of Mississippi, but some of its northward advancement was shunted a bit.

Typically in the spring, you'd expect the southern flank of such convection to be a focus for storm development, especially near the instability gradient, but given the time of the year, there wasn't quite enough time for much recovery.

I understand that storm relatively helicity (SRH) was still fairly large in the wake of this initial convection, but mesoanalysis (and MEM obs) showed that low-level winds veered somewhat to SSW. Through the column, there was much more speed shear than directional shear. High shear/low CAPE setups can usually work with this, but usually it's easier if upper level winds are more westerly. It looks like in this case, winds were SW at 500mb and SSW near the surface.

Based on short-term forecast soundings, I noticed some backing of winds around 700mb (creating a bit of a VBV profile), which may have thrown a wrench in updraft organization.

Severe setups are fickle, especially when it's early in the season and you don't have a completely uncontaminated environment. Such environments are rare anyway, as you'd hope for a strong EML to maintain a capping inversion (not as common in Dixie as in the Plains) to prevent "premature" convection from getting in the way. Again, while surface winds weren't awful, higher-end events tend to have more strongly backed surface winds, such as SSE/SE, rather than SSW, when upper level winds are southwesterly.

Convection allowing model solutions weren't that bad. Most of them showed mixed storm modes with a broken line of storms, rather than a more classic outbreak setup with distinct, discrete supercells. Today, we had storms along a cold front, a prefrontal trough, another band farther east (that outpaced better forcing/instability), as well as mixed in throughout the warm sector. It was more messy than you'd like for a supercell tornado event.

 

[Bob Schafer]

I want to share a little anecdote.

At the Denver convention in 2003 I attended Tim Vasquez's forecasting class. I had only been chasing a couple years and was pretty much a noob. Tim did a case study of the 2002 Happy Texas tornado. I had chased that day, but I had glommed onto a developing storm that passed to the north of Amarillo, which did not "produce". I asked Tim what had gone wrong with "my" storm that day.

The answer was very simple. My storm was in an environment with surface winds that were veered to the SSW. Ever since then I have strongly avoided setups with veered surface winds, and I see those situations "busting" time and again. (There are the odd 1% exceptions.)

 

[Ethan Schisler]

I think as Bob said, it has a lot to do with the veering of the surface winds. The crossovers were better further east. I noticed this on surface obs while sitting in Batesville as the pre-frontal convection moved overhead. However I wanted to play the better terrain....(which ended up not really mattering as I got stuck in it). 500mb down to 850mb were mostly southwesterly....we needed due southeast winds at the surface to get good low level rotation (at least my thoughts). Where I was, winds were starting to turn further southerly instead southeast as they were in the morning. Further east, the winds were more southeasterly allowing for better low level rotation in any supercell that could sustain itself. So I think even despite the "messy" nature of the storms further east, there were embedded supercell structures that managed to do their job because of this....I might be wrong....but those are just my thoughts.

 

[Dan Robinson]

One thing I noticed in model guidance is that the low-level shear and surface-based CAPE values (and thus the corresponding STPs) were shown peaking around 1-3PM, then dropping off thereafter. Convection in the open warm sector didn't seem to get fully established until after that time.

 

[Warren Faidley]

I want to share a little anecdote.

At the Denver convention in 2003 I attended Tim Vasquez's forecasting class. I had only been chasing a couple years and was pretty much a noob. Tim did a case study of the 2002 Happy Texas tornado. I had chased that day, but I had glommed onto a developing storm that passed to the north of Amarillo, which did not "produce". I asked Tim what had gone wrong with "my" storm that day.

The answer was very simple. My storm was in an environment with surface winds that were veered to the SSW. Ever since then I have strongly avoided setups with veered surface winds, and I see those situations "busting" time and again. (There are the odd 1% exceptions.)

Bob, I was also chasing that day. I believe the northern storm was encountering SSW surface winds generated by storm outflows from the southern storms, but I could be wrong.

 

[Jeff House]

Surface wind discussion is spot on! I try to chase boundary intersections. Winds are more likely to be backed near an outflow boundary OFB or the warm front WF. Then target the intersection with another trough. In this case it was the pre-frontal trough east of the quasi dry line.

Dixie pre-frontal troughs can go nuts. In the Plains the dry line DL is typically reliable. I often favor the OFB intersection with DL or WF intersection with the DL. Because winds are typically more backed near the OFB or WF, it does not leave as much to chance. Final pick (OFB or WF) is based on conditions near each.

 

[David Williams]

Surface wind discussion is spot on! I try to chase boundary intersections. Winds are more likely to be backed near an outflow boundary OFB or the warm front WF. Then target the intersection with another trough. In this case it was the pre-frontal trough east of the quasi dry line.

Dixie pre-frontal troughs can go nuts. In the Plains the dry line DL is typically reliable. I often favor the OFB intersection with DL or WF intersection with the DL. Because winds are typically more backed near the OFB or WF, it does not leave as much to chance. Final pick (OFB or WF) is based on conditions near each.

You guys can correct me if I'm wrong but, did the Burnsville tornado form while the storm was over the warm front? I did not end up chasing on this day because I didn't think the environment and the terrain were very favorable for a fun chase, but my virtual Target was to go to Clarksdale, MS and move NE with storms as the WF moved north. If I remember correctly, CAMs settled the warm front on the MS/TN border around midafternoon (roughly the time of the Burnsville tornado). Does anyone know if it was interacting with the warm front when it produced?

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