Gustnadoes

[Tony Laubach]

The gustnado our group observed in South Dakota was on the ground for well over 5 minutes with constant circulation. It covered some ground as well over its trek eastward. I've counted it tentatively as a tornado in my chasing log (marked by *) simply because of its duration and intensity as we approached it. We never reported this to any office, and especially not a true tornado.

 

[Edward Ballou]

Thanks all for the comments. I think anything like this can and most definitely SHOULD be reported. After skirting the edge of the gust front and seeing first hand the damage it can do, I am glad they warned the cell. It was most certainly a dangerous situation. The gust front was knocking down 2-3 inch branches and spewing dust and corn stalks all over the place, making for very hazardous driving conditions.

I am not sure of the gustnado's duration as later it seemed to combine with the front but I watched as it crossed Highway 37 and after that, a line of cars (10-15 cars) came out of the dust cloud, moving very slowly. I don't know if they heard the warning but I think it was justified caution that they exhibited. I'm thinking they might have waited on the north side of it to let it cross the road, then they continued.

On that note though, I saw police heading towards the gustnado but no lights or anything. They weren't preventing people from driving through it. I saw others who drove right up to it when it started out and then stopped in the road. I looked back one more time to see a car parked horizontally on the road, but it might have just been turning around.

I think the ambiguity that arises is when the NWS wants to get out the message that this is more than just your average Severe Thunderstorm. To some people, they take it as there's no tornado so we can drive right through it. Judging from even the small eddy of a gustnado that I drove through on I-90, it pushed my car pretty hard...I'll have to go check the video again to see how much of an impact it had on the semi that was in front of me.

I did manage to get some later video of it, but it wasn't the best as I think the gust front had absorbed it back in by that time. The only three photos I got of what I reported in chronological order:

 

[Scott Olson]

1. How should gustnadoes be classified?
2. Should their damage (if any) cause for an F rating?
3. At what point are they reportable?


To me it's when the damage from the gustnado is greater than the straight line winds that are there and/or when it causes F0 tornadic damage along it's path. I don't think it's a question of process but rather wind speed/damage and rotation.

BTW, i've been looking at some radar imagery and one thing that stands out is the cell I was viewing from the N. This was the cell with where I witnessed a huge cloud of dirt from a protruding and lowered cloud base and several strong and brief gustnadoes just to the W of it. Scott Currens witnessed a tornado just 5-10minutes before this radar scan.




Looking S

 

[Jeff Snyder]

1. How should gustnadoes be classified?[/b]

I don't know as if there is an official NWS position... I know I've seen statements from Tulsa NWSFO do not count gustnadoes as official tornadoes. On one hand, some environments and storms can produce 10-20-30 gustnadoes, and most are weak, shallow, and short-lived. On the other hand, surely we can't discount all non-mesocyclone tornadoes. Most tornadoes derive some of their rotation from stretching of vertical vorticity... For example, landspouts are non-supercell tornadoes, but, with many of them, their rotation is driven by strong stretching (in a TCu, etc) of pre-existing vertical vorticity near or of a misocyclone. In contrast, many gustnadoes are caused, I suspect, from heterogeneities along a strong horizontal shear zone (leading edge of outflow, etc).

2. Should their damage (if any) cause for an F rating?
3. At what point are they reportable?[/b]

Don't know about the former, but the latter -- whenever they persist long enough for your report to be given. I question whether we should now start calling all gustnadoes tornadoes (to "artificially boost" our stats?). That said, I think there are certainly reportable phenomena as long as you report just what you are seeing... The original poster and other involved did exactly the right thing IMO -- report what you see, and let the NWS decide what to do about it. I don't think it's ever a bad thing to report an 'abnormal' feature that may cause damage or injury (high winds, large hail, tornadoes, etc), as long as you report it like you see it (lol don't say "wow, this tornado is destroying everything!" -- i.e. don't exaggerate). Again, I agree with reporting this type of feature regardless of whether you think it's a tornado.

To me it's when the damage from the gustnado is greater than the straight line winds that are there and/or when it causes F0 tornadic damage along it's path. I don't think it's a question of process but rather wind speed/damage and rotation.[/b]

It should be noted that using the "F0" criterion for determining whether a gustnado is a tornado would mean that every single gustnado is a tornado. Remember, there is no lower bound for F0 -- it's winds are <73mph. Therefore, every single gustnado with winds <73mph is "F0-caliber".

I just think the tornado count would increase by twice or thrice as much if we count all gustnadoes as tornadoes. I guess it just comes down to determining the process by which the vortex developed... Then, what about vortices that form downstream of sharp edges in high winds? We've all seen 'vortices' downwind of the edges of buildings when winds are strong... What about vortices that form downstream of natural phenomena like groups of trees? I've seen 'wake vortices' downwind of a dense grove of trees before.... Do we count these? I agree, this is getting a little far-fetched, but that's relative, yeah?

 

[Scott Olson]

Im not saying that we should count gustnadoes as tornadoes. But in the case of the other day we had large, persistent, rotating colums of air with at least two being near an updraft (and low level rotation on Mitchell). Now by my own interuptation I wouldn't count them as tornadoes either, since they didn't cause damage (BTW I meant F1 threshold not F0 LOL). For instance some of the brief but strong gustnadoes I saw were actually not on the leading edge of the outflow but just to the west near where the updraft was located according to what I saw.

 

[Simon Brewer]

Strong, long-lived gustnadoes are counted as such in my book. I will never classify a gustnado within the same class as a land spout, a dust devil, or a mesocyclone tornado no matter how intense the wirlwind in each respective vortex classification. In rare cases a gustnado vortex may be ingested into an updraft of some sort, whether it be a surface based shelf cloud along a gustfront or a new rainfree base ahead or along the gust front, but the vortex is still generated and initially fueled by outflow winds; the stronger the outflow winds the stronger the gustnado in most cases.

None of the many gustnadoes I witnessed in South Dakota (two of which went down as tornado reports; not pointing fingers or saying it was wrong to report it as such) appeared to be ingested into an updraft. The closest those respective vorticies came to being ingested into an updraft was the small 'fingers' of scud clouds developing on the edge of the same very strong and cool pools of outflow generated by brief relatively more intense downdrafts and microbursts along the squall line, which generated and fueled the gustnadoes themselves.

I would lose sleep at night if I were to ever consider a true gustnado to be a tornado unless there was no doubt in my mind the gustnado was ingested into an updraft and stretched by the updraft, but I have yet to see this first hand in the past 7 years of chasing. I've seen some incredible gustnadoes (one passed over my car, while chasing and shifted my vehicle rather quickly off the road and into a ditch, but we kept a close eye on the gust front over head and noted no rotation) and I will refer to them as such, but never will they be counted as tornadoes in my book.

 

[Paul Knightley]

I agree - a gustnado per se is not a tornado. Alternatively, a landspout is a tornado, and as such, should just be called a tornado, IMO. While I'm on the subject, waterspouts are simply tornadoes too, but that's a different debate!

As far as gustnadoes being ingested into the updraught goes - can the vorticies which appear to develop on the leading edge of the RFD (cyclonic and anticyclonic) initially be called gustnadoes? I'm just hypothesising here, but it appears to me that the RFD surges forward, with cyclonic vertical vorticity on its left flank (looking in its direction of motion, in the N Hem), and anti-cyc vorticity on its right flank. The vorticity on its left flank then gets stretched by the strong updraught created by the mesocyclone, and can become a tornado. Occasionally, the vorticity on the right flank gets stretched by a passing updraught along the flanking line (April anti-cyc tornado at El Reno, OK?). I guess what I am saying is that could this possible mechanism of tornado formation be classed as "gustnadoes" being turned into tornadoes?!

 

[Dave Kaplow]

I agree that vortices on the leading edge of the outflow are not tornadoes, not unless they are clearly ingested into an updraft and a condensation funnel forms. Even then it can be argued that the vortex is not a true tornado until that transformation occurs.

However, let's not forget that gustnados can form on a storm's inflow as well. Here the case becomes much murkier. In Kansas in '02 I witnessed a very large and intense dust-whirl type vortex that spun up on the strong inflow of a supercell. Everyone that saw it recognized it for what it clearly was: a big gustnado. However, the next day I was watching some video of the event that was shown on local TV, and at the end of the clip the camera pans up to the clouds and reveals a small but unmistakable nub funnel poking down out of the cloud base. Can this event still be classified as a gustnado? A large and strong vortex that forms on the inflow, under a funnel - you could make a good case that this was a tornado.

I guess there are always going to be some in-between cases that are hard to classify.

 

[John Farley]

I agree that vortices on the leading edge of the outflow are not tornadoes, not unless they are clearly ingested into an updraft and a condensation funnel forms. Even then it can be argued that the vortex is not a true tornado until that transformation occurs.
[/b]

A condensation funnel is not a necessary condition for a tornado. A tornado is wind, not cloud, and some tornadoes - along the leading edge of outflow and elsewhere - occur without funnels. I do agree that there must be a connection between an updraft and the ground in order to classify a swirl on the ground as a tornado - but rotation in the clouds above the swirl is a sign of this, as is a funnel.

In some parts of the country, notably Illinois and nearby areas, a significant minority of tornadoes occur along the leading edge of storms, usually bow echos and usually at or north of the apex of the bow. This has been documented by BAMEX and other research. In a fair number of cases, the rotation in the clouds is strong enough to be detected by doppler radar. Hence, the fact that it is along the leading edge of a storm does not mean that it is not a true tornado.

Now, before everybody jumps me, let me make it clear that I agree that it is not a tornado unless there is evidence of a connection between a storm updraft and the ground. Either a funnel or rotation in the clouds is evidence of this if it is above a dust swirl on the ground. OTOH, if it is just a dust swirl without evidence of either a funnel or cloud-level rotation, then it's a gustnado.

 

[Dave Kaplow]

A condensation funnel is not a necessary condition for a tornado. A tornado is wind, not cloud, and some tornadoes - along the leading edge of outflow and elsewhere - occur without funnels. I do agree that there must be a connection between an updraft and the ground in order to classify a swirl on the ground as a tornado - but rotation in the clouds above the swirl is a sign of this, as is a funnel.[/b]

I realize of course that condensation need not be present for a vortex to be classified as a tornado. I specified a funnel simply because this in my mind would be absolute proof that there is significant rotation in the cloud base. Simply seeing diffuse rotation in some low clouds above a classic gustnado-like dust whirl would not be enough for me to call it a tornado. There has to be no doubt at all that a connection exists between the swirl on the ground and rotation in the updraft base. I have often seen ragged outflow scud take on rotation for a short time, but there is no real updraft, it's just a temporary eddy with little or no vertical motion, probably caused by much the same mechanism as a gustnado. I believe this type of cloud base rotation is insufficient, even if it occurs directly over a strong whirl of debris, to certify something as a true tornado. It may not be in the official definition of a tornado, but I think there should be some kind of updraft involved before I would call a given vortex a true tornado.

 

[Joe Nield]

In some parts of the country, notably Illinois and nearby areas, a significant minority of tornadoes occur along the leading edge of storms, usually bow echos and usually at or north of the apex of the bow. This has been documented by BAMEX and other research. In a fair number of cases, the rotation in the clouds is strong enough to be detected by doppler radar. Hence, the fact that it is along the leading edge of a storm does not mean that it is not a true tornado.[/b]

I believe Jeff Trapp and others on the BAMEX project have calculated that as many as half of all Indiana tornadoes are non-supercell tornadoes.

 

[Paul Knightley]

Many of our tornadoes in the UK, which are mostly weak, could be classed as "non-supercell" tornadoes, especially those of high summer - I suppose they would be classed as, "landspouts" in the USA.

I suspect many tornadoes worldwide are non-supercell in "type"...however, one thing intrigues me:

What is so special about a "mesocyclone-spawned" tornado? I ask this for the following reasons:

1) Is the rotation of the mesocyclone itself key in producing/spawning the tornado, or is it just key in producing a strong-violent updraught which can then stretch vertical vorticity (probably derived from the RFD), and sustain this for (sometimes) long periods of time?

2) If similar vorticity existed near a non-supercell updraught, is it likely that it too would be stretched into a tornado, but it would be weaker/shorter in duration, due to the weaker/shorter duration updraught?

I wonder if research has been skewed towards tornadoes which are associated with deep, persistent mesocyclones (for obvious reasons, as they do the most damage), and this has somewhat distorted the view of how tornadoes develop? I am only saying this from the point of view of someone who has looked into tornadoes across here in the UK, which often occur in non-supercell type storms, but are still worthy of research, as they sometimes produce (not-insignificant) damage.

 

[Robert Dewey]

Tornado -- 1. A violently rotating column of air, in contact with the ground, either pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud.[/b]

http://www.cimms.ou.edu/~doswell/a_tornado/atornado.html

 

[jladue]

I agree that vortices on the leading edge of the outflow are not tornadoes, not unless they are clearly ingested into an updraft and a condensation funnel forms. Even then it can be argued that the vortex is not a true tornado until that transformation occurs.[/b]

Consider that the definition of a tornado is pretty vague. It's only requirement is to be underneath a convective updraft and be violent at ground level. However there's no mention of tornado tilt. How many times have you seen a roping tornado where the ground contact has been been physically pushed away from the overlying updraft base by outflow? Does that mean the vortex cannot be called a tornado? Clearly being underneath an updraft is not a requirement. Some connection to updraft is a requirement whether or not that connection is sloped. However, there is a lot of uncertainty as to what a connection means. The below quote makes sense....

I agree that vortices on the leading edge of the outflow are not tornadoes, not unless they are clearly ingested into an updraft and a condensation funnel forms. Even then it can be argued that the vortex is not a true tornado until that transformation occurs.[/b]

...unless you compare two identical gustnado type vortices of similar origins, one being associated with a lower LCL. I'm confident to say that all gustnadoes have updraft associated with them to stretch available vorticity. But what if both gustnadoes were 2000' tall, though one of them occurred underneath a 1000' LCL, and the other under a 5000' LCL? You would see obvious cloud base rotation and a funnel with the lower LCL case. What are the chances the lower LCL case would be classified as a tornado? I'd say it would be pretty likely. Would it be wrong to call the lower LCL case a tornado?

I bet many Illinois squall line tornado events and high plains gustnado events originate in similar ways. Both events are damaging surface vortices. The lower LCL case shows a more obvious connection with some kind of updraft above the LCL. But both low and high LCL cases may have continuous updraft between the ground vortex location and the LCL; only the low LCL case shows visualization of updraft at the same level as the vortex (hence the funnel).

I thought of these questions when I saw a damaging gustnado partially deroof a house 1/4 mi away underneath a flanking line of a developing high-based supercell. I didn't see any cloud base rotation, however a spotter reported it as a tornado and LBB issued a tornado warning. The gustnado lasted 2 minutes, longer than some tornadoes I've seen, and it did damage. More of them could've followed given that several were already observed. I call it gustnado but what aspect of it prevents the vortex from being called a tornado given the definition? Does the definition of tornado need to be changed or should we be documenting more gustnadoes as tornadoes?

Jim

 

[Paul Knightley]

I suppose what differentiates a "gustnado" (I prefer the term "eddy whirlwind"), and a tornado is that the former is created along the gust front as a result of strong horizontal shear across the gust front, whereas the latter is the result of strong vortex stretching by an updraught.

I have seen whirlwinds along the leading edge of outflow many miles from the storm - are these then tornadoes too? No, so why should whirlwinds along the gust front closer to the storm be classed as tornadoes? They shouldn't! (unless of course they get ingested/stretched by the updraught).