Gustnadoes

I thought about starting this in the discussion thread for yesterday's event, but thought it best to discuss them here.

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

Yesterday I was very careful with my wording to the NWS when I called them. Roughly quoted from memory...

Me: I'm calling in to report not a tornado, but a gustnado - or at least it's a very interesting looking dust cloud.
NWS: Is it rotating?
Me: <stopping the car, and getting out> Yes, it does have some slow rotation at the base. It's getting bigger.
NWS: How wide would you say it is?
Me: Umm, quarter of a mile to a half a mile - it started out as about 100 yard across at least.
NWS: How about height?
Me: The dust has reached all the way to the cloud base now.
NWS: Ok, hold on... ... ... We are seeing some rotation, so we're going to go ahead and issue the tornado warning.
 
It seems to me that your problem is a completely separate one to that of those who call gustnadoes tornadoes. In your case - the NWS took it on their own accord to issue a tornado warning for what YOU were calling a gustnado. At least YOU called it a GUSTnado - which is the correct term.

As far as everybody else seemingly calling every rotating column of dust (in the gust-front region of) yesterday's storms TORnadoes......I am truly baffled. I must have missed the official change of description for gustnado vs. tornado.

But I wasn't there so I don't really carry any weight. ;) (<that's a wink - not a grimace)

K.
 
I thought about starting this in the discussion thread for yesterday's event, but thought it best to discuss them here.

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

Yesterday I was very careful with my wording to the NWS when I called them. Roughly quoted from memory...

Me: I'm calling in to report not a tornado, but a gustnado - or at least it's a very interesting looking dust cloud.
NWS: Is it rotating?
Me: <stopping the car, and getting out> Yes, it does have some slow rotation at the base. It's getting bigger.
NWS: How wide would you say it is?
Me: Umm, quarter of a mile to a half a mile - it started out as about 100 yard across at least.
NWS: How about height?
Me: The dust has reached all the way to the cloud base now.
NWS: Ok, hold on... ... ... We are seeing some rotation, so we're going to go ahead and issue the tornado warning.
[/b]

Ask ten meteorologist and you will get ten different answers. I tend to follow Doswell's line of thinking on the subject.

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

Scott Currens
www.violentplains.com
 
I think to the general public...issuing TOR warnings for confirmed, long-lasting gustandoes and, sometimes, strong shear couplets on outflow near metropolitan areas, are good things. Gustandoes can do as much damage as a brief spinup from a mesocyclonic tornado...and the public doesn't know (or care) about the difference.

I guess in that sense, they have some validity for reports IF surveyable damage can be found. But they are inherently NOT tornadoes in the classical sense...they are not "pendant from a cumuloninbus cloud" as the definition suggests. Thus, I think they should be excluded...as folks who do research on tornado statistics wouldn't want them as part of the sample. If they do cause damage, maybe then they can go into the record books with a "*". :)
 
If all gustnadoes are to be counted as tornadoes, then I've seen a heck of a lot more tornadoes than I thought I had... Awesome though, since that means I've had a day in which I saw 20-25 tornadoes (6-10-04). Wow, I've even RUN through a tornado! The TIV 'ain't got nothin' on me', since I've driven through several tornadoes before.

News: 2006 sees record number of tornadoes -- Tornado count three times the previous record. Average lead-time drops to 2 minutes as many more "tornadoes" go unwarned.
 
If all gustnadoes are to be counted as tornadoes, then I've seen a heck of a lot more tornadoes than I thought I had... Awesome though, since that means I've had a day in which I saw 20-25 tornadoes (6-10-04). Wow, I've even RUN through a tornado! The TIV 'ain't got nothin' on me', since I've driven through several tornadoes before.

News: 2006 sees record number of tornadoes -- Tornado count three times the previous record.
[/b]

LOL! Even chaser stats would get skewed...

From Doswell's take:

"Some of the relatively intense vortices associated with a convective storm probably should not be considered tornadoes; e.g., circulations not extending to the surface, and true gustnadoes (see below), assuming we can identify them as such....My guess is that typically, they represent only a minor perturbation of essentially no significance, except in very rare examples."

Good stuff here..."very rare examples" would likely point to cases where damage occurs. Otherwise, the descripition of a "perturbation" along the edge of the cold outflow is a very good way of describing them.
 
I started a thread last year about one of these whirls... Last May 23 a gustnado (clearly a gustnado based on observation and radar) went across I-29 and through the far south side of Grand Forks, ND. It was given an F1 rating. The gustandao was 10 miles from any precip echo. One of the NWS mets saw this circulation and pulled the trigger for the torn warn. I wouldn't call it a tornado but it is an easy way to boost your PoD if you don't mind sacrificing some lead time. I guess I can understand the warning but why put it in Storm Data as a tornado when it was CLEARLY not one?
 
I agree with the NWS met... I'm thinking that if a gustnado is long lasting, almost 1/2 mile wide, and extends up to the cloud base - it's pretty serious (and remember, there was rotation on doppler). I think a TOR was a wise decision in this case, even if it wasn't a tornado...
 
1. How should gustnadoes be classified?[/b]

True gustnadoes have limited vertical growth and should not be considered tornadoes. On the other hand, if what you observed near Mitchell meets the following criteria then it was a tornado.

1. The vortex at the surface is F0 or greater intensity (capable of producing damage).
2. The vortex is relatively long-lived (not a brief spin-up).
3. The vortex is vertically connected to deep moist convection (rotation at cloud base).

I think we need to face the fact that a tornado doesn't always fit neatly into a any one category
2. Should their damage (if any) cause for an F rating?[/b]
Yes, if it is a true tornado.

3. At what point are they reportable?[/b]
At the point you think it is a threat to persons of property.

Because I decided not to chase today, I had time to work on a tornado classification diagram. It's a work in progress.

I think most tornadoes fit somewhere in the diagram. A tornado may develop in one classification and move to another during its lifecycle. An example would be the Jarrell, TX tornado that by some accounts, formed as a landspout before the thunderstorm developed mesocyclone.

Scott Currens
www.violentplains.com
 
AMS Glossary:
gustnado—Colloquial expression for a short-lived, shallow, generally weak tornado found along a gust front. Gustnadoes are usually visualized by a rotating dust or debris cloud.
See nonsupercell tornado.[/b]

NWS Glossary:
Gustnado
(or Gustinado) - [Slang], gust front tornado. A small tornado, usually weak and short-lived, that occurs along the gust front of a thunderstorm. Often it is visible only as a debris cloud or dust whirl near the ground. Gustnadoes are not associated with storm-scale rotation (i.e. mesocyclones); they are more likely to be associated visually with a shelf cloud than with a wall cloud.[/b]
Mike
 
Being my opinionated self, I'm going to jump in here ... I don't know why. Because I love abuse, I guess. And because there isn't anything better to do this month.

Edward - sounds to me like you handled this nicely, first of all. You called it in without making hard conclusions. You told them exactly what you saw. Sounds like a good deal. Even now you are reluctant to make hard-and-fast statements. This is good practice, IMO.

Now ... I'll agree that there are many events that are tough to classify (see Doswell) - - and technically a tornado is a violently rotating column of air in contact with both the cloud base and the ground. Damage can start to occur at pretty low wind speeds ... like even 40-50 mph. So even strong dust devils can reach the low end of Fujita limits. And gustnadoes definitely can. No question.

Having said all this - - everyone still knows that there is a DIFFERENCE between mesocyclone-spawned, supercell associated tornadoes and gustnadoes. You can usually tell the difference by their appearance (though some are pretty tough to distinguish), and where they are located in the storm. If you have a long shelf cloud, for example ... that is spinning up eddies in multiple locations, you pretty much know you've got a gustnado situation. This happened with us a couple years ago in Nebraska. One spinup was huge ... like a quarter mile wide ... and it really looked tornado-ish. But it wasn't a tornado. It was a gustnado. It wasn't associated with a mesocyclone, and we knew it ... even though there definitely was rotation in the cloud base that we could clearly see and got on film. But I still knew that it wasn't the same. It was definitely cool, and probably even produced some damage. But not a true tornado. Also last year near St. Joe I saw another one along a gust front that was clearly rotating. Cars were stopped to watch it ... it looked like a tornado in every way. But it wasn't. The nearest meso was probably in Iowa somewhere that day.

I don't know all the details yesterday and wasn't watching that closely to see what kind of signatures these storms were producing ... (by the way, the one near I-29 should have been bringing back some pretty well-defined radar sweeps - it's not far from the radar tower in Sioux Falls). So it might help to go back through the imagery, etc. But if it were me, I would probably do just what Edward did ... call in something that looked like a tornado that was kicking up dust, but then letting the radar gurus make the final call. I hate events like that, personally - just because we can feel this need to seek validation. It's not that big of a deal, though. Just say what you saw and leave it at that. You can personally come to your own conclusions that might be different from everyone else on the subject, including the final say from the rating team. No big whoop. We still dig the photos and the accounts. On these types of events, I'd always clarify the reports as much as possible by using caveats and disclaimers, just like Edward did.
 
I think you made the right call edward. You saw what appeard to be a gustnado or weak tornado and made the call. The NWS made the final call and put it in the records as a tornado. They are the ones who called it a tornado not you. I think it was wrong of some people who I wont name to post on here and put you down and attack you for your descision without even getting your side of the story. Saying you sent in a false report was wrong and if they had asked you they would know this. You did the right thing. -MatthewCarman.
 
From the posted sources above, I hear two real criteria for differentiating tornadoes from other rotating phenomena. 1. The vortex should be attached to cloud base. More specifically a thunderstorm cloud base and 2. The vortex must be capable of damage. From an NWS standpoint I would say winds in excess of 58mph. This gets into a gray area already because I'm sure there are some vorticies that are truly tornadoes but less than 58mph. None-the-less, this basic classification allows all landspouts/waterspouts to be in the category but also allows for some wiggle room for large gustnadoes that form around thunderstorms. Again...gray area because any long lasting vortex around a thunderstorm should most likely be reported.

For a real fun experience see: H.J. Lugt, The Dilemma of Defining a Vortex, Recent Developments in Theoretical and Experimental Fluid Mechanics, pp. 309-321, 1979.
Then we can debate why all vorticies aren't tornadoes, or I suppose why all tornadoes aren't vorticies.

Anyway, my hat off to Edward and the NWS operator. This in my mind is a great case of excellent communication between chasers and the NWS to identify a potential hazardous situation.

Ben
 
Hey Mike, I’m glad you decided to jump into the fire. LOL

Having said all this - - everyone still knows that there is a DIFFERENCE between mesocyclone-spawned, supercell associated tornadoes and gustnadoes. You can usually tell the difference by their appearance (though some are pretty tough to distinguish), and where they are located in the storm.
[/b]

Yes, but does everyone know the difference between a gustnado and a non-mesocyclone tornado?

I get the feeling that every vortex on a gustfront is automatically labeled a gustnado. I don't think anyone claimed that any of the 5-23 tornadoes were mesocyclone-spawned, supercell associated tornadoes. I believe there were several non-mesocyclone tornadoes with significant vertical depth on the leading edge of the outflow, and that they are not the same phenomena as the countless shallow vortices that occurred (gustnadoes).

If you have a long shelf cloud, for example ... that is spinning up eddies in multiple locations, you pretty much know you've got a gustnado situation. [/b]

Does this mean that any vortex that forms on the leading edge of outflow is a gustnado regardless of vertical growth and organized rotation at cloudbase? If that is the case then a gustnado should be defined as any vortex that develops on the leading edge of outflow period.

If all gustnadoes are to be counted as tornadoes, then I've seen a heck of a lot more tornadoes than I thought I had... Awesome though, since that means I've had a day in which I saw 20-25 tornadoes (6-10-04). Wow, I've even RUN through a tornado! The TIV 'ain't got nothin' on me', since I've driven through several tornadoes before.

News: 2006 sees record number of tornadoes -- Tornado count three times the previous record. Average lead-time drops to 2 minutes as many more "tornadoes" go unwarned.
[/b]

I don't think any gustnadoes should be recorded as tornadoes. The only ones that should be recorded are mesocyclone and non-mesocyclone tornadoes. It would also be nice if non-mesocyclone tornadoes were recorded as such in the database.
 
I get the feeling that every vortex on a gustfront is automatically labeled a gustnado. I don't think anyone claimed that any of the 5-23 tornadoes were mesocyclone-spawned, supercell associated tornadoes. I believe there were several non-mesocyclone tornadoes with significant vertical depth on the leading edge of the outflow, and that they are not the same phenomena as the countless shallow vortices that occurred (gustnadoes).
Does this mean that any vortex that forms on the leading edge of outflow is a gustnado regardless of vertical growth and organized rotation at cloudbase? If that is the case then a gustnado should be defined as any vortex that develops on the leading edge of outflow period.
[/b]

Interesting point here. I think the key is "organized rotation at cloud base" more than "vertical depth". Dust devils can have very deep circulations, but would never be considered a "tornado". However, dust devils can cause damage and injury (Recent ND injury: http://www.stormtrack.org/forum/index.php?...0&#entry126225) as some gustnadoes do.

Makes you wonder if events where injuries and significant damage occur from "gustnadoes" and "dust devils" should be reported and archived, because these events are as noteworthy as a brief spinup under a meso in an open field.
 
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.
 
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:
5-23-06-13.JPG


5-23-06-14.JPG


5-23-06-15.JPG
 
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
 
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?
 
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.
 
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.
 
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?!
 
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.
 
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.
 
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.
 
Back
Top