NoDak Gustnado deemed a Tornado

[Justin Turcotte]

May 20, 2005 a supercell thunderstorm passed some 15 miles south of Grand Forks, ND proper. The storm has a history of funnels and very large hail. The cell sent a potent downdraft to its north. This convergence boundary was clearly evident on radar. A cyclonic circulation developed on the western flank of the outflow and really got cranking. I was putting a forecast together at a private firm just a few blocks from the NWS office and heard the sirens blaze and couldn't help to wonder what the heck for as the most dangerous part of the storm was reasonably well south of town and moving east away from the area. I walked outside to take a look at the cell and saw the massive rotating dust plume crossing Interstate-29 and moving into town. Black dirt was lofted several hundred feet into the air giving the appearance of a nasty tornado. There was no storm updraft, no meso, no anything less the OFB. Apparently this was fetaure was also spotted by someone at the NWS who then pulled the trigger on the tornado warning. This beast of a gustnado passed through the south end of town causing some moderate tree damage.
Description on NCDC:

A tornado touched down on the south end of Grand Forks, travelling along county road 17 from one-half mile west of Interstate 29 to near county road 81. Trees were torn down on the north side of county road 17 near the intersection of Columbia Road. The tornado weakened as it crossed county road 17 off the south edge of Kings Walk golf course. A dust plume generated by the tornado pushed north across open fields and the golf course into housing and shopping areas in south Grand Forks. Top wind speeds were estimated to be around 75 mph.

This tornado was given an F1 rating. I've joked about sheriffnadoes but now we have NWSnadoes. I can see the logic of pulling the trigger for some sort of warning as this particular gustnado was the most potent I've witnessed. The question is why would NWS officially classify it as a tornado when the evidence clearly suggested otherwise? More cooking the books with verification? My local office is notorious for that. Perhaps the tornado climo record is littered with these circulations.

No pics or radar grabs but if someone digs in the archives it is a fun case to look at.

 

[Jeff Miller]

hmm

If it was rated F1, it was damaging. The general public do not know the difference between a gustnado and a tornado. I dont think the NWS is going to spend a great deal of time trying to write up a report to the general public about the differences of meso-induced tornadoes with shear-induced gustnadoes. A rotating column of air caused damage, therefore, they used the term "tornado" -- that would be my guess to this.

I dont think 99% of the general public know of the word "gustnado" or honestly even care.

 

[Scott Olson]

Looks to be at least 8-12 miles north of the Updraft. Boundary surging northward, probably some RFD interaction with wind shear this also occured near/right after the formation of the Meso low. From a preliminary look I would venture to say that the Grand Forks tornado wasn't the only tornado that day that wasn't associated with a mesocyclone. I'll try to take another look tommorow, map out the tornado tracks and post radar grabs.

I did chase that day and have really bad footage of the tornado NW of Hope.
I then tried to intercept the northern cell but wasn't quick enough so ended up on the southern cell. The tornado associated with the right mover seems to be the only one that showed a persistent mesocyclone and low level g2g.
By the time I reached it there was a slightly elevated sculpted updraft but it was weakening by then.

 

[James McCormick]

Tricky case.

I would be interested in hearing the NWS explanation of the tornado rating. It seems to me that the tornado label in the public information statement would have little if any effect on the public outlook. How often does the public get to hear word on whether or not the damage was officially tornadic or not?

Indeed given the time of the report it appears that this vortex formed as a result of the gust front. Not only would it appear to not be associated with the mesocyclone of the storm moving south of Grand Forks, it appears to not be intricately associated with the storm at all. Nothing in particular unusual there.

Given that it was a damaging weather producing vortex, what type of warning is appropriate? A high wind warning seems ineffective. A severe thunderstorm warning seems good at first but given the localized area of enhanced damage, I can see the logic in the tornado warning. However, if the event is not a tornado, why should it go into climatology as one? The warning was issued after the beginning of the event, so reporting the event as a tornado hurts the 'books' regarding lead time. Discussion of this event with a friend indicated that in one of the verification criteria of a tornado warning is in fact severe thunderstorm damage, which 75 MPH damage qualifies as. It seems that the verification process wasn't need in this particular case, which just makes things more clouded.

 

[rdale]

"The general public do not know the difference between a gustnado and a tornado"

During a warning, I agree. In post-event scientific analysis, for the climate record, I think a tornado <> gustnado.

 

[Dan Christianson]

I agree most people, see a gustnado/ landspout and will right there on the spot think its a tornado without even the knowledge of what it really is , some gustnado's are pretty strong but weak most of the time, do you have any pics of this beast? or links ?

Yeah sherrifnado's are common, what i like is how the NWS OFFICE knows theres a Supercell with strong rotation out in the country area and they dont give off a tornado warning, it kind of ticks me off that some farmsteads get destroyed, due to the fact no warning is issued till the storm get closer to towns and stuff" oh well

no one is perfect. :roll:

 

[rdale]

Because well less than half of supercells with strong rotation drop a tornado... There's much more to the equation.

- Rob

 

[Shane Adams]

I agree most people, see a gustnado/ landspout and will right there on the spot think its a tornado without even the knowledge of what it really is

A landspout, by definition, is a tornado.

 

[Dan Christianson]

Yeah i agree RDALE, i can see your point, also i know landspouts are a type of a tornado, just on occasions you can see a formation from ground to cloud, but most of the one's i have saw , didn't have a visible condesation funnel from cloud to groun, just dirt and debris swirling on the ground, we all have our opinions, and im still learning lol ;-)

 

[Scott Olson]

The interesting part here though is that this Tornado didn't have an updraft that vertically stretched and intensified the outflow boundary. The Left Mover was some distance away. The Surface vortex formed along the convergence line and that line does show up with some 30-35dbz returns on radar.. Most interesting is that it didn't seem to be associated with a thunderstorm cell but instead relied entirely on pre-existing voriticty and interaction.

The Forbes definition is pretty clear:
A vortex is classified as a tornado if 1) it produces at least FO damage or exhibits wind speeds capable of producing such damage, and if 2) it forms in association with the wind field of a thunderstorm or its accompanying mesoscale features, such as the gust front or flanking line.

An interesting day nonetheless!

-Scott Olson.

 

[Bill Tabor]

Originally posted by Scott Olson

The Forbes definition is pretty clear:
A vortex is classified as a tornado if 1) it produces at least FO damage or exhibits wind speeds capable of producing such damage, and if 2) it forms in association with the wind field of a thunderstorm or its accompanying mesoscale features, such as the gust front or flanking line.
-Scott Olson.

For purposes of warning the public I'd say that is a pretty good definition and distinguishes it from say..a dust devil. Of course wind speed would only be estimated upon spotting and wouldn't be known until after the event from studying damage. If a spotter sees a damaging vortex in association with a thunderstorm that appears to be endangering the public I'd say that a tornado warning should be legit regardless of the mechanism that produces it. I'd say distinguishing and classifying the type of tornado would be done after the fact for science purposes but isn't necessarily the same as the criteria for warning. Now that said we as chasers have slightly higher criteria for counting torns as most won't count gustnadoes. I consider landspouts as legitimate tornadoes along with meso torns for chaser counts. Still I suppose most of us would have to agree that a gustnado is a type of 'tornado' even though not created by standard mechanisms and typically very weak.

 

[Scott Olson]

I agree and im pretty sure now that this was a landspout. Justin did you see any cumuliform above or near the vortex? I wish someone had gotten a picture of it.