Gustnadoes

[Edward Ballou]

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.

 

[Karen Politte]

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.

 

[Scott Currens]

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

 

[Chad Ringley]

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 "*".

 

[Jeff Snyder]

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.

 

[Chad Ringley]

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.

 

[Justin Turcotte]

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?

 

[Robert Dewey]

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...

 

[Scott Currens]

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

 

[mikegeukes]

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

 

[Mike Peregrine]

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.

 

[MatthewCarman]

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.

 

[Ben Baranowski]

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

 

[Scott Currens]

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.

 

[Chad Ringley]

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.