Shelf clouds tornadoes causes...

I know that shelf cloud tornadoes are possible but are not so much frequent like wall cloud tornadoes, and overall are less strong than these.
Here in Italy are not so common but sometimes I've seen some pics about them. Anyway I'm not speaking about gustnadoes but I 'm speaking about a real tornado that descends from a shelf cloud.
If someone have some pics, please post it :wink:
Besides, if someone wants, let's try to anlyze synoptic pattern that could lead to the formation of a shelf cloud tornado.
As to me, the first condition is that a strong squall line has to move through a high sheared ground with elevated value of storm relative helicity where it could from a good contrast between gust front and inflow wind: in this way it could produces the right orizontal vorticity.
 
Well this post was overlooked, and I'm sure on purpose! Nobody wants to say that they believe a tornado can come out of a shelf cloud. :) 151 views of this post and nobody wanted to bite.

Well I tend to be a bit rash in my comments, so I'll bite and get the heat for it. Why not.

A shelf cloud tornado... hrm... I won't discount the idea. Something that a lot of people don't really comprehend is that a shelf cloud itself is an updraft area. Sure you have the outflow below at surface level, but what makes a shelf cloud a shelf cloud is the fact that there is this inflow aloft, just above the surface which gives it the shelfy appearance. This air rides up above the surface outflow and enters the storm and rises once it gets to the tower.

Think of a squall line. There may be a great shelf out front, and the reflectivity has the deepest reds on the "eastern" side of an eastbound squall line. That's because the inflow is coming in through the shelf cloud and the updraft is on the front side of the storm.

The problem with tornadoes in these situations is that you generally have the strong outflow at the surface. This undercuts anything that is trying to get surface based. Therefore, with this cold and strong outflow (which as a rule does not get caught back into the updraft by going up through the underbelly of the shelf cloud), nothing of any significant buoyancy is able to get rooted below the shelf to create .... you know... what none of us want to talk about.

So for the vast majority of the time, a tornado here is quite unlikely.

What you often get are gustnadoes or spinups simply caused by the strong outflow -- NOT rooted into any cloud base --. Or perhaps you'll get a 'smokenado' like the one I took in the picture below where the smoke is caught at the intersection of the outflow speeding out from the left and meeting the rising inflow from the right which is heading into the shelf aloft. This causes the air within this boundary (gust front) to rise to the nose of the shelf.

[Broken External Image]:http://www.stormtours.com/2003/61-Meade, KS Smokenado 2.jpg

This obviously is another example that is NOT a tornado. Far from it.

So could a true tornado (not a gustnado or rising smoke) occur under a shelf.... I too, am too chicken to answer that question. :)
 
Andrea, can you offer some link to Italian shelf cloud tornado photos? I'm sure many people would like to see them, if they exist. Or... are all these shelf cloud tornados in fact scudnados? Like in these photos of mine:

[Broken External Image]:http://www.myrsky.com:81/albums/shelfcloud-05082003/109_0938_IMG.jpg

[Broken External Image]:http://www.myrsky.com:81/albums/shelfcloud-05082003/109_0941_IMG.jpg
In this case a local farmer cycled past me, stopped and yelled: "Is that tornado? What happens to my forest?!

:D

More photos of this one here: http://www.myrsky.com/gallery/shelfcloud-05082003

Regards, Olli
 
I've seen some interesting tornado look-alikes coming from shelf clouds (one instance while I was up on the roof, looking between two groves of trees at what looked at first like a wall cloud and strong funnel cloud coming from the southwest). However, I haven't seen or remembered any pictures of tornadoes from shelf clouds, since it is a rather rare occurrance, but that doesn't mean they're not possible :).

Nice pictures, Olli.
 
Nice photo Olli!!
I'm sorry but I've no italian pics about, but I remember some month ago I saw in a meteorological forum a shelf cloud tornado photographed in Italy...Anyway it was so weak, because ,as Andrew Revering says, a shelf cloud cannot contain a strong buoyancy to generate a strong tornado. But I'm sure that if the contest is so much sheared, inside the shelf cloud the relative boyancy can spawn a weak tornado (I don't think more than F0).
By the way it seems to me that 1 or 2 months ago, I saw here, on storm track, a video with a shelf cloud tornado....Does nobody confirm?
 
It has been well-documented by the BAMEX study and others that tornadoes do occur along the leading edge of bow echoes. Often this occurs when the bow echo interacts with some boundary, such as an outflow boundary from earlier convection. Most often the tornadoes occur from the apex of the bow echo northward. Examples of research documenting this can be found at:

http://www.crh.noaa.gov/lsx/science/mcstype1b.htm

Scroll about halfway down and follow the links to the case studies.

In many of these cases, it is likely that the tornado would form under the shelf cloud on the leading edge of the bow echo. I saw an example of this last July 5 in southwest Illinois. I was actually on the way home from a chase as I had been ovetaken by a cluster of supercells that had quickly morphed into a bow echo. It had been a bow echo for at least a half hour before the experience I am about to describe. I was in a congested area a short distance from my home when I saw what looked like a funnel cloud under the shelf cloud to my SE shortly after it passed over me. I thought, no, it looks like a funnel but in that location it has to be a scud finger. Wrong! A tornado touched down briefly in that location, and radar documented rotation in the storm. Between my disbelief, the congested location I was in, and the quickness of the event, I got no photos or video. This event was not a gustnado, but a true tornado, with rotation detectable on radar. It does not occur commonly, but with bow echoes it is possible for tornadoes to occur under shelf clouds.
 
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