Landspout=tornado..

[tom hanlon]

Interesting report today, May 28, 2007

See the SPC site today's storm reports.

A LANDSPOUT...TECHNICALLY A TORNADO BECAUSE IT WAS A ROTATING COLUMN OF AIR IN CONTACT WITH BOTH THE GROUND AND A CUMULIFORM CLOUD...CAUSED DAMAGE TO A COMMERCIAL BARN (BOX)

Nice to see that clarity.

Would they show the same clarity for a gustnado ?

Probably depends on the NWS office. Are they required to do a damage survey for every confirmed tor ? If so that workload would tend to favor reports in places where it would be a nice and rare chance to go out in the field, vs offices where it would be another hard day trying to find the damage path of a short lived gustnado in the middle of nowhere.

Just curious about the protocol and I thought the description was interesting.

--
Tom

 

[John Robinson]

Items appearing in the SPC storm report list come almost exclusively from reports submitted by local WFOs. Thus, the descriptiveness of a report depends on what the WFO wrote.

Per NWS directive 10-1605, gustnadoes are counted as thunderstorm wind events, not tornadoes, since they do not exhibit rotation at cloud base.

The same directive indicates that it is desirable that a WFO survey tornado damage if additional details are needed for the entry that the WFO has to submit for the Storm Data publication. Thus, there is no requirement that every tornado be surveyed. However, known or suspected EF4 or EF5 tornadoes will be surveyed, according to Directive 10-1604.

 

[Bob Schafer]

Per NWS directive 10-1605, gustnadoes are counted as thunderstorm wind events, not tornadoes, since they do not exhibit rotation at cloud base.

How does that differ from a landspout, then? What is meant by "rotation at cloud base"? Neither landspouts nor gustnadoes descend from rotating wall clouds, but both are often clearly "attached" to the cloud base, thus there is rotation "AT" the cloud base (though not mesocyclonic).

 

[Rick Smith]

I don't think the definition requires rotation at cloud base, just that the feature is in contact with the ground and extending from the cloud base.

Rick

 

[Skip Talbot]

How does that differ from a landspout, then? What is meant by "rotation at cloud base"? Neither landspouts nor gustnadoes descend from rotating wall clouds, but both are often clearly "attached" to the cloud base, thus there is rotation "AT" the cloud base (though not mesocyclonic).

Spouts, land and water, are driven by the updraft of the cumuliform tower they are connected to, right? Gustnadoes, however, are driven by interactions between ground level downdrafts and surface winds. Please correct me if I'm wrong, but even though a landspout producing storm may not have a deep mesocyclone, it can still produce weak tornadoes under its updraft region.

 

[scott r currens]

How does that differ from a landspout, then? What is meant by "rotation at cloud base"? Neither landspouts nor gustnadoes descend from rotating wall clouds, but both are often clearly "attached" to the cloud base, thus there is rotation "AT" the cloud base (though not mesocyclonic).

Doswell has a good document that covers this topic.
http://www.cimms.ou.edu/~doswell/a_tornado/atornado.html

Most gustnadoes are shallow, weak, and do not have any significant rotation at cloud base. This type of gustnado is never considered a tornado. The problem is that on occasion "gustnadoes" do become relatively strong (EF-0, EF-1, EF-2) with rotation to cloud base. When this happens I consider it a tornado because by definition it is one, although you will get many differing opinions from storm chasers and meteorologists alike. Nature doesn't always fit neatly into our categories (Mesocyclone tornado, Non-mesocyclone tornado, gustnado) and this gray area is where we often have disagreements in this community.

 

[Andrew Pritchard]

How does that differ from a landspout, then? What is meant by "rotation at cloud base"? Neither landspouts nor gustnadoes descend from rotating wall clouds, but both are often clearly "attached" to the cloud base, thus there is rotation "AT" the cloud base (though not mesocyclonic).

I've actually never seen a gustnado attached to a cloud base. They are not associated with any deep rotation in the cloud base, and are often just spun up on the leading edge of the storm's gust front. Gustnadoes and landspouts are two very different phenomena.

 

[Bob Schafer]

Thank you for the answers, though some kind of missed the crux of the question. I know the difference between a landspout and gustnado. The question centers on how the definition supposedly differentiates between the two.

Scott addressed that issue.

I have indeed seen gustnado rotation very obviously extending into the clouds.

I'm not (exactly) saying that anyone denied that gustnadoes ever extend into the clouds. There were a number of chasers present there invof Ulysses KS June 1, 2003. If anyone who was there wants to chime in, you're invited.

Pic attached. You can see a funnel (attached to the cloud base) above the debris, and they were clearly one and the same rotation.

View attachment 485

*edit I hope this doesn't descend into yet another "What, exactly, constitutes a tornado?" thread. Wasn't my intention!

 

[Bill Tabor]

Gustnadoes aren't supposed to be attached to the cloud base. That's what makes them gustnadoes. If they were and it was rotation not associated with a meso it would be considered a landspout.

The definition of a tornado requires "pendant from a cumuliform cloud", and violently rotating. As another stated gustnadoes are caused by storm shear typically on the gustfront, but I believe near the inflow intersection. Landspouts are caused by the storm ingesting vorticity or moving into an area that has increased vorticity. Sometimes a gustnado can become a landspout if the storms overruns it and uses it to create the landspout.

 

[Ilya Neyman]

Very interesting thread and one that seems to ask for clarity. I was not there for the Ulysses, KS event on June 1, 2003 and have not even looked at storm reports/radar but after looking at Bob's photo I would have to say I would classify that as at the very least a landspout. I can't see whether or not the funnel is extending out of a wall cloud region as the top part of the photo is just above the bottom of the funnel cloud but if it did then it would probably be a mesocyclonically-induced tornado. I can't classify it as a gustnado as by definition gustnadoes are spun up typically along gust fronts and are ground-level circulations (eddies) that do not connect with the parent thunderstorm base, as if they did they would be tornadic and no need to set aside a seperate class then.

On May 22nd of this year a bit before the the Graham County, KS tornado we were just north of I-70 several miles from the meso when we observed a compact ground level dust swirl just to our west. This was right underneath the rotating base of the supercell. There was no evident funnel of either condensation or dust emanating from cloud base but for the short time that the dust was visible it began going upward, as it moved from south to north. I'm not quite sure what EXACTLY that was, but if I had to choose I would classify that feature as a gustnado. It was to the east of the developing RFD in a region probably experiencing SE-E inflow. Whatever the case it was a brief spin up but not in your traditional "along the gust-front" sense.

 

[Bob Schafer]

...Bob's photo I would have to say I would classify that as at the very least a landspout. I can't see whether or not the funnel is extending out of a wall cloud region as the top part of the photo is just above the bottom of the funnel cloud but if it did then it would probably be a mesocyclonically-induced tornado.

Sorry I couldn't/didn't offer better pics. I have only that one and this new attachment vidcapped. It was from about the same time. I'd have to go back and dig out the tape and re-vidcap.

We were chasing ahead of a linear complex, and a whole bunch (5 or 6) of these gustnadoes/landspouts (whatever) formed in the space of a few minutes at the gust front. A couple lasted for quite a while (by gustnado standards)... a couple of minutes or so. No way were they beneath wall clouds. Mesoscale rotation? Hmmm, I doubt it, but there may have been some minor rotation embedded in the line.

On the tape, I've got Jay Antel (sp?) talking to Mike U @ DDC saying "Mike, we've got the gustnadoes from hell here!!" LOL

 

[cdcollura]

I made this diagram a few months back, refined from a drawing Tony Laubach and Scott Currens and myself drew in the dusty rear window of one of our chase vehicles!

 

[Doug Lee]

In 11 years of spotter classes, I have always been told that gustnadoes are not tornadoes, but they can still be dangerous. They usually form on the boundary between inflow and outflow winds, and usually in front of the parent thunderstorm. They are never attached to the cloud base, so they are never classified as tornadoes. The NWS regards them as tornado look-alikes.

I can see Bill Tabor's point about storms overtaking a gustnado and the gustnado becoming a landspout. In the Nova episode a few years back, a researcher took data from a real tornado event and fed it into a 3D modelling program. Before the model storm produced a tornado, the program indicated the formation of a vortex at the surface. I believe that coupling might occur between the surface vortex and the rotating updraft, and this produces the tornado. I can see a gustnado, as a surface vortex, coupling with a rotating updraft and becoming a tornado.

Remember, a tornado is a violently rotating column of AIR extending from the base of a storm and extending all the way to the ground. The condensation funnel does not have to reach the ground. If you see a funnel extending part way to the ground and a dust or debris cloud on the ground, it is a tornado.

 

[Glen Romine]

In the Nova episode a few years back, a researcher took data from a real tornado event and fed it into a 3D modelling program. Before the model storm produced a tornado, the program indicated the formation of a vortex at the surface. I believe that coupling might occur between the surface vortex and the rotating updraft, and this produces the tornado. I can see a gustnado, as a surface vortex, coupling with a rotating updraft and becoming a tornado.

For the record, this was not a gustnado in the model simulation. The surface circulation was very large in scale (though much smaller than the mesocyclone) and with relatively weak (non-tornadic) winds. Such surface circulation features have been observed in both tornadic and nontornadic storms. This circulation was preceded by others that were non-tornadic, though eventually one did become a tornado-like vortice. The simulation also had small, shallow eddy circulations that developed along the gust front edge, as well as along the occluded boundary feeding back to the center of the surface circulation. These may have been features similar to gustnadoes, but were too poorly resolved by the simulation to call them that.