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RFD notches and tornadogenisis

Mike Hollingshead

I'm working on something and before I assume something that perhaps I shouldn't I figured I'd "poll" the crowd. I'm trying to recall a tornadic event I've seen in person that has not had a visible RFD notch(one where I couldn't find one). Is it safe to assume this to be the norm or typical? That an RFD/Clear notch is visible >90% of the time a tornado occurs? One must realize sometimes this "clear" notch is full of precip. It is still an RFD notch and I think in most cases it should be able to be noted. Even further east where structure isn't always so clear I imagine most of the time one can find the RFD notch. Thoughts? Safe to say it is typical to have one and see it before or during tornadogenisis around 90% of the time?

Thanks...
 
Yes, I'd agree that 90% of the time, some evidence of an RFD/clear slot is visible with a tornado. I've looked through my images and the exceptions I found are mostly situations where it's a line or a tropical type set-up, with high shear and little or no CAPE. Another exception might be a landspout type tornado.
 
In the very early stages of the Conway Springs tornado event on May 29th, there is very little, if no evidence of a clear slot until several minutes after the inital touchdown. See the following sequence of images below. The first two images, there is probably the slightest evidence of RFD clear slot developing, but you have to know what you are looking for.. it is far from obvious...until the last couple pics.

<img src=http://www.underthemeso.com/chase2004/may29/Dscn7730.jpg width=400>

<img src=http://www.underthemeso.com/chase2004/may29/Dscn7743.jpg width=400>

<img src=http://www.underthemeso.com/chase2004/may29/Dscn7747.jpg width=400>

<img src=http://www.underthemeso.com/chase2004/may29/Dscn7755.jpg width=400>

The wall cloud just prior to the Milan/Conway Springs tornado event was rotating extremely vigorously, looking northeast. There was no evidence of a RFD clear slot whatsoever from my perspective. It was just a big mass of rotating cloud! I wonder how often this occurs in significant cyclic supercells. The Conway Springs tornado event formed very fast at the occlusion point from the Freeport-Argonia long lived tornado event, which was actually still ongoing in the rope out stage to the west of the Conway Springs tornado(es) in the first 3 pics above.

Mike U
 
I was just down the road from Mike that day and I agree, this situation was different than your typical tornado cycle. I think this had something to do with how the event unfolded. Whether you consider it a single tornado or multiple ones, the storm behaved quite differently than any I've ever seen.

Instead producing a tornado followed by a quick occlusion then another tornado, or producing multiple tornadoes simultaneously, it produced (depending how you judge it) either one continous, very large multiple vortex with only a large, single trunk of condensation visible at any one time, or a string of individual tornadoes, all originating from the same meso (though different spots in the meso). Bottom line, there was no sign of imminent occlusion through any of the tornadoes or vortices.

The main distinguishing factor though IMO was the way the tornadoes or vortices followed the parent rotation around in a circle, riding the LL meso's flow. A very impressive and amazing event to witness, probably a once-in-a-decade type scene.
 
Originally posted by Mike Hollingshead
I'm working on something and before I assume something that perhaps I shouldn't I figured I'd \"poll\" the crowd. I'm trying to recall a tornadic event I've seen in person that has not had a visible RFD notch(one where I couldn't find one). Is it safe to assume this to be the norm or typical? That an RFD/Clear notch is visible >90% of the time a tornado occurs? One must realize sometimes this \"clear\" notch is full of precip. It is still an RFD notch and I think in most cases it should be able to be noted. Even further east where structure isn't always so clear I imagine most of the time one can find the RFD notch. Thoughts? Safe to say it is typical to have one and see it before or during tornadogenisis around 90% of the time?

Thanks...

I'm probably not the best qualified to answer these as I don't have exact numbers - but I suspect the % is much less than 90 for all tornado events, as I think supercell tornadoes are only about 60% of all events, but that number could be way off. Then, the visual clear slot occlusion that you are referring to is generally visible only with classic supercells, which are of course a subset. I've seen tornadic LPs that didn't display an occlusion, as well as HPs where the occlusion is essentially a rain curtain that wraps around the tornado obscurring the view. Research has suggested that even though the occlusion in classic supercells appears clear - there are actually large raindrops in there. Also, worth mentioning that tornadoes have also been documented to occur in the flanking line, and the visual occlusion process occurs much more frequently than tornadoes occur associated with it. So, my guess is that for the type of events you typically chase, then among those the numbers for tornadic event where a clear slot develops may be as high as you suggest - but those numbers don't transfer to all tornado events as far as I know.

Hope that helps.
Glen
 
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