Tuesday, April 5, 2011

Big gustnado near Perry KS on 4/3/10 - not a tornado!






Sunday's (4/3/10) storms over northeast KS featured a supercell that developed near Manhattan around 6 pm CDT (2300 UTC) north of a surface front. There was enough of a "cap" present that it took northward lift over the frontal surface to initate storms, even with temepratures near 90 F south of the front. As the Manhattan supercell passed N of Topeka and moved into Jefferson County northwest of Lawrence, the gust front extending south of the cell managed to tap very warm surface air south of the front, and a large well-organized gustnado (see photos above at top) formed near the intersection of the surface front and gustfront, east of Perry KS and well south of the supercell's mid-level mesocyclone.

Scott Blair has excellent photos and analysis at www.targetarea.net/apr311.html, and NWS Topeka has a good discussion here.

The large gustnado (a relatively shallow gust-front vortex compared to a tornado; see diagram above) probably would not have happened if the supercell gust front had not been able to access the steep low-level lapse rate air south of the surface front. On the radar images above (and a radar loop on the Topeka NWS site), you can see where the supercell was intially all north of the surface front. But as it moved east, the extreme south end along the gust front began to pull in true surface-based air near the frontal boundary-gust front intersection. Note that the observed sounding at Topeka prior to 7 pm CDT (0000 UTC, see skewT graphic above), before both the surface front and gust front passage, showed a very steep temperature drop off in the lowest 2-3 km, a lapse rate that meteorologists would call nearly "dry-adiabatic". Such lapse rates are associated with dust devils and rapidly rising thermals on hot sunny days. In this case, it appears that this air helped with low-level stretching along the gust front near the boundary intersection area, creating a rather intense vortex on the gust front.

Note on the radar image above that the gustnado location was well removed from the mid-level mesocyclone area of the supercell to the north (the wide video image by Connor McCreary above also shows the shallow gustnado occuring along the flanking line/gust front). I've heard some people online claim this to be a "true" tornado, but real understanding of the cell's geometry shows that it was _not_ a tornado, according to accepted definitions of gustnadoes. Gustnadoes can also often occur on days with LCL heights above 1600-2000m (like on 4/3/10), while true supercell tornadoes generally do not.

I've also heard some people claim Sunday's gustnado was definitely a "tornado" because it had multiple vortices. Well... that means nothing; many gustnadoes have multiple vortices. See the last set of images above taken on a storm chase south of Lubbock last June 14th with my wife Shawna and also Anna and Bill Stromberg. Dozens of gustnadoes occured along a storm gust front that hot Texas day, including some with multiple vortices (see the first 6/14/10 photo by Anna and Bill). Some of the gustnadoes were relatively large, too, with organized rear inflow jets, as can be seen in the 6/14 images.

This particular gustnado on 4/3/10 was large enough and well-organized enough to do some damage, and has certainly gotten plenty of air play on TV in view of the lack of real tornadoes that day. Gust front vortices can be very interesting!

- Jon Davies 4/5/11

9 comments:

brian said...

Very nice analysis. You nailed it.
Brian Inman
Meteorologist
WGEM Quincy, Il

nathan.morris92 said...

Hi Jon.

What do you make of this? I noticed this rotation at the base of an advancing gust front.

For it to be a gustnado, does it have be coming from the ground up?

Pics (in chronological order)
http://i215.photobucket.com/albums/cc198/apocalypse1992/DSC_0055.jpg

http://i215.photobucket.com/albums/cc198/apocalypse1992/DSC_0057.jpg

http://i215.photobucket.com/albums/cc198/apocalypse1992/DSC_0070.jpg

Radar (pics taken at Wagga Wagga in the NE corner)
http://i215.photobucket.com/albums/cc198/apocalypse1992/Radar5-2-114-50pm.png

Synoptics (Wagga is in NSW near the Victorian border, on the eastern side of the trough)
http://www.bom.gov.au/archive/charts/2011/02/IDX0102.201102050000.gif

Any help would be appreciated. :)

Scott said...

Hi Jon,

It is clear the vortex was not a mesocyclone tornado but can a non-mesocyclone tornado really be ruled out? The real determining factor is how deep the vortex was. Dust did not reach cloud base but the vortex may have still extended into the convection above.

None the less it was an interesting vortex. Unfortunately I was a few miles east of the storm at that time.

I always enjoy your write-ups, keep them coming.

Scott C.

brandon said...

John, is it at all possible that combination of rfd,gust front & frontal boundary could have pulled or stretched horizontal vorticity that obviously would have been wrapping back in to the main meso(or rfd) pulled the horizontal tube forward away from wrapping and or main meso and downforcing pushed the tube verticle...and putting this on the ground...even if for seconds (kind of a backwards effect if your understanding what i am saying. This down forcing of elevated tube along frontal boundary had a ribbon/vacuum effect...which would be to hold a ribbon or string to a vacuum hose allowing it to suck thru...thus at one moment a tornado becoming a gustnado as the top end was pulled down and away. I say this for a couple of reasons..first, in analyzing radar, there is a clear meso albeit elevated...this does not discount the probabilty of a horizontal vortex above the cloude base unseen ..and also not at all neccesarily seen by radar.Secondly instead of wrapping...when it hit the two boundaries, was unable to wrap in and instead pulled to the ground...because of the clear image of funnel vortex...and very distinct and SHARP hazy/milky vortex reaching from lowering to ground.This can not be discounted....its so vivid...and lastly..as you can see in photos and certain areas of video...the disconnect from cloud base happening as vortex is pulled through and pushed to the ground...towards the end you see a pushing out 'splattering' effect and then dissipation.Also see effect of 'wagging tail' of rfd...a clear sign of suction. If you have time..Brandon McDaniel stormchaser7777@yahoo.com Thanks Jon.

brandon said...

Jon, if you get the chance, please go to my fb page thru Shawna..Please see the (1:30am posted) photo that i am talking about plus the discussion...because i have added thoughts to the raambling statement on here.I am just wanting your thoughts on viability here...as i have thought about this for a few years.~Brandon

Jon Davies said...

Hi Scott C:

You may be right... see my more recent gustnado vs. tornado write-up. I can't rule out that this had some characteristics of a non-mesocyclone tornado, somewhat similar to a landspout. I do think the dominant processes were those associated with gustnadoes because of the sloped advancing gust front. But when a tight vortex associated with a thunderstorm stays well-organized for 5-10 minutes and does EF1 damage, it falls into a gray area that doesn't entirely fit an AMS/NWS definition, and maybe we should start thinking about calling it a tornado.

Jon Davies said...

Brian:

Thanks for the positive comment... I don't know that I nailed it, but I appreciate your feedback.

Jon Davies said...

Nathan:

I looked through your photos... I don't think what you photographed was a gustnado, but can't say for sure. The definition of a gustnado is that it forms from the bottom up through stretching along a gust front. You feature looks like it was just behind the shelf and might have been associated with a broad eddy or indentation in the gust front. Gust front features are often hard to identify from still images.

Jon Davies said...

Brandon:

Given the AMS definition, I'll concede that Sunday's vortex doesn't entirely fit the definition of a gustnado. See my more recent write-up on gustnadoes and tornadoes.