What is this?

[Tim Nendick]

mods, please move this if it is in the wrong place -- I'm still learning.

I was chasing the tornado producing storm near Oxford, NE.

At ~6:13 PM we noticed this right on the ground. Normally something like this would get my undivided attention, but there was a very active wall cloud directly in front of me, and the RFD was really cutting down. Indeed, the storm dropped a rope about 4 minutes later. Going through pictures, I'm a little perplexed by this feature.

A minute later, it had developed a little more

A different view, more telephoto:

Not a minute later, the funnel was descending to the south.

The thing is, in the field, it looked basically stationary. It wasn't persistent, but then stuff way really fleeting in general.

My friend took a brief cell phone video of it

Watch video >


Radar at the time (my position marked)

Finally, this was north of the wall cloud, significantly. It is pretty much in the RFD area.

This is where I'd expect anticyclonic motion if the storm was splitting.

Any ideas? I have a few, but I'd like to hear what you think

 

[Jeff Duda]

At ~6:13 PM we noticed this right on the ground. Normally something like this would get my undivided attention, but there was a very active wall cloud directly in front of me, and the RFD was really cutting down. Indeed, the storm dropped a rope about 4 minutes later. Going through pictures, I'm a little perplexed by this feature.

Not a minute later, the funnel was descending to the south.

The thing is, in the field, it looked basically stationary. It wasn't persistent, but then stuff way really fleeting in general.

Finally, this was north of the wall cloud, significantly. It is pretty much in the RFD area.

This is where I'd expect anticyclonic motion if the storm was splitting.

Any ideas? I have a few, but I'd like to hear what you think

Given the radar images you showed and your position, this looks to have been a cyclonic supercell, so some of your statements seem backwards. You would expect to see anticyclonic rotation to the far SE of a NE moving supercell, not to the north of the wall cloud. Second, the velocity image looks pretty tame, and I don't see any organized rotation on that tilt. There are hints of an RFD, but the velocity pattern suggests that it's weak at best.

My first impression upon seeing that feature is that it is scud. You're likely post-precipitation in there, so the environment is dominated by rain-cooled, nearly saturated air there. It wouldn't take much additional cooling or moistening to get local saturation, which is probably what's going on. That tendril sticking up out of the scud looks sort of ominous, and is difficult to explain other than possibly being a very weak tornado, but I'm having trouble finding any connection to the cloud base, and I see no rotation in the video (too much shaking and the video is too short). So if it is a tornado, it's of the very weakest possible kind.

 

[Skip Talbot]

I agree with Jeff, this is most definitely a chunk of scud. Ground hugging cumulus is common in the wake of thunderstorms where the air is nearly saturated and there is a lot of turbulent motion. The turbulent cloud base above the feature really highlights this as an area of outflow, while there is a prominent updraft base/RFD evident to the right at the start of that video. The vertical component might be caused by a small updraft stretching this convection. I've seen this a few times in the wake of storms. Here's one from behind the July 13, 2004 Roanoke, IL supercell/derecho:

 

[Jeremy Perez]

I was chasing a storm near Oberlin, Kansas last year that gusted out and put on a excellent scud show. There were an entertaining variety of shear vortices spindling along the interface of the outflow boundary. The one you caught looks like a nice example.

This time lapse video starting at 03:27 shows some of them forming and dissipating--although not as persistent as the one you witnessed: Time Lapse Video

 

[Tim Nendick]

I was talking through it with one of my professors, and the formation of the cloud through saturated mixing is in line with what we thought. That makes sense to me, and is pretty much what I came up in the field and on my own. The explanations and examples help a lot, thanks!

Still, I am left with the question -- why would there be an updraft in an area dominated by the RFD? I know that turbulent air is highly complex and trying to simply draw a schematic representation isn't enough to understand details like this. That said, this is an area of very dominant downward motion and so it is the tendril that interests me. The scud on the ground I can wrap my head around. The tendril, a little less so.

I'm not great at envisioning what is happening, but I think the RFD probably acts like an oar in water -- right? A local area of high density air rushing forward drives the more buoyant ambient air out of its way in all directions. Ignoring storm motion and the wind environment, wouldn't that effectively work like this?

Put that in the real environment, and one leg of the "split" will be favored. Am I on the right track there?

If I squint, I imagine that happening on radar -- the rotation is definitely chaotic at the time, especially considering the scans directly before and after it:

Radar to the due east.

If it is helpful, this is what was happening in front of us simultaneously (at 8x speed) -- the feature would have existed out of frame to the north.

Watch video >

So in that ramble, I guess my questions are two fold:

1.) How would an updraft exist in the RFD environment?
2.) Is rotation in the area a result of the RFD intrusion?

And yeah, the documentation is terrible. We were focused on the rapidly intensifying wall cloud/tornado in front of us so all of this was effectively afterthought, a "huh, not seen that" moment.

 

[Mike Peregrine]

I've noticed areas of heavy scud to the west and south of RFD on many storms, particularly the vigorous ones with a history of producing tornadoes. Sometimes these 'scudnadoes' can be really convincing too. The telling evidence is in rotation, upward motion and physical location in the storm. Most of them have some turbulant motion, but not true rotation. And they are far removed from the actual wall cloud.

I'm totally guessing on the mechanics involved, but possibly condensation is occurring after RFD moves through because the now-saturated ground (which had previously been under the storm's warm sector) is now coming in contact with the much cooler air mixing in from behind the supercell. It's this mixture that factors into tornadogenesis closer to the updraft. I'd venture to say that I notice heavy scud coming up behind the rear flank of most of the strong storms I can name in the past. There is some sort of subsidence or sinking action in the heavy, moist and quickly cooling air back there.

 

[Jeff Duda]

The RFD region of a supercell generally lends itself to a lot of chaotic, turbulent motions. Some of these involve upward motion, especially along the leading edge, which can result in rapid condensation by lifting and the appearance of tornadic development. Some are quite rotational, and can briefly appear to be vortices associated with a mesocyclonic tornado. However, the most you would ever get with these are gustnadoes. Given the degree of turbulence, it's difficult to know exactly why a narrow updraft like that may form. It's probably just a function of the chaotic motion in there.

By the way, the sequence of velocity images doesn't really illustrate the chaotic nature of the air flow in an RFD. The sampling volume is just too large to do that. However, the images do show that the storm-scale rotation was increasing in time.

 

[Paul Knightley]

Scud is the first thing that comes to my mind too, as others have said above. Was there hail associated with this part of the storm? Hail fog can be lofted by gusty winds and can look interesting, even when there is nothing of much significance there.