Only supercells produce tornados?

[Allan Jeffers]

I got a question that has been bugging me for some time. A while back on my local news weather channel, I think it may have been channel 7 here in Denver said something, I can't remember quite what but what stuck in my mind was the following:

Can a tornado only be produced by a supercell? If that is the case, why the heck did they issue tornado warnings here in colorado for a severe thunderstorm?

I thought severe thunderstorms could produce tornados as well, but the way the weather man made it sound was only supercells produce tornados, while regular severe thunderstorms would produce only gustnados or landspouts?

Did I understand this right?

 

[Danny Neal]

A supercell is a severe thunderstorm with rotation. A supercell is a thunderstorm with a persistent rotating updraft. In essence, every thunderstorm has a weak rotation to it in one way or another. A severe thunderstorm can be anything from an elevated storm producing quarter hail to a bow echo producing 80 mph winds to a supercell producing a tornado. A lot of times you see a severe thunderstorm warning for the core of a supercell and a tornado warning for the business end. For example if you are in Wichita and there is a tornado in Mulvane, you may be under a severe thunderstorm warning because the area you are in is only getting hail and wind, while 10 miles to your south you may be getting a tornado. A severe thunderstorm has hail greater than 1 inch in diameter and winds 58 mph or greater. Many supercells have this as well as strong/long lived rotation.

I read it again and I think I know where you are coming from. Out in your area you guys get landspouts that don't form exactly the way a supercellular tornado may form. (E.G - meso - wall cloud - tornado) I am not exactly sure the processes behind landspout development but many form from high based storms in the High Plains. Landspouts are often difficult to detect because they don't have the impressive radar signatures associated with classic supercells.

"Can a tornado only be produced by a supercell?" I may be wrong on this and others may correct me but a true tornado I believe is produced only by a supercell because for there to be a tornado there has to be strong enough rotation at some point and in theory a strong/persistent rotation in a thunderstorm is normally categorized as a supercell. However other processes will conflict that (E.G - cell mergers, comma heads, inflow notches on the leading edge of a bow echo or squall line (which again could technically be characterized as a supercell in some cases))

I guess to conclude, when you normally think of tornadoes, a supercell comes to mind. However as I noted above landspouts can form from seemingly a cumulus cloud (much is the way waterspouts form over water) Gustnadoes are NOT considered tornadoes as they are due to interacts at the leading edge of a gust front and generally do not have cloud base rotation.

In my mind this reply was perfect, I am not sure if it translated down to my finger tips and I hope you aren't as confused as I am about what I just wrote lol

 

[Allan Jeffers]

Here are a couple crappy vids I took last season with my g1

first one, a siren was sounded for this and a tornado warning issued
the wall cloud:
http://www.youtube.com/watch?v=Cp-C5eo0Wdg

And as it crept over head (and me wishing I could get a better shot)
http://www.youtube.com/watch?v=AV9CruipEyQ

If I am not wrong, is that a RFD, or clear notch to the left of the wall cloud? (first video) Would this indicate that this was a supercell, or do gustnados form the same way?

I guess the other question is, do we actually get supercell thunderstorms in Colorado. BTW, these where taken up near Aurora, pretty much on 48th and tower road. This was NOT the tornado that touched down a few weeks ealier, although I did see that from a distance.

 

[Bart_Comstock]

Ok, first off lets look at the deffinition of a tornado. It is a violent, rapidly spinning column of air that connects the ground to the base of a storm's cloud. A landspout (or a non supercellular waterspout), while general not supercellular, is a tornado and can cause damage and thus merits a tornado warning.

Landspouts are generally formed in the growing stage of a thunderstorms development when already present spin on the surface of the earth gets pulled up into the storm and tightened up to create a tightly spinning column of air. This weak pre-existing ground spin is boundary layer vorticity. Landspouts are generally fairly short lived. (i hope o explained that correctly)

Gustnadoes are NOT tornadoes. They are breif spinups that are created by the outflow of a storm and are typicaly found along the gust front of a storm. These cirulations do not extend from the ground to the base of the storm. They are very brief but can be damaging although that is rare for them to do so.

Colorado does get many supercells. Infact the Denver metro was hit by a supercell spawned tornado last year, which is the one I believe you are referring to in your last post. The large Windsor, CO. wedge tornado was supercellular. The front range of Colorado is also in tornado alley.

I am at work right now and so I cant view your videos but do note that a wall cloud is not always present under a supercell and can be found under any thunderstorm and not all wall clouds rotate.

 

[MatthewCarman]

I have heard of showers producing weak tornadoes if there is enough spin in the air and conditions are right. I have not heard of any of these tornadoes actually doing any damage but it is possible considering there has been a dust devil or two that has done some damage before. Super cells have the best chance of producing a strong tornado but I think bowechoes and derechoes can do that to. Maybe there is just embeded super cells in those linear lines of storms that produce tornadoes. I am no expert so don't quote me on that.

 

[Skip Talbot]

Good points by all. A lot of this is just semantics. Remember that all supercells are thunderstorms, but not all thunderstorms are supercells. And "severe" is a label we affix to a thunderstorm if it has hail/wind/tornado reports above the criteria Danny outlined.

Tornadoes can be produced from thunderstorms that are not supercells. I think your weatherman meant to say that the vast majority of all strong/violent tornadoes are supercellular. As mentioned before, spouts do not need a supercell to form (although they can form under supercells as well). In fact they don't even need a thunderstorm, as a spout can spin up under a towering cumulus.

Bart, is right that Colorado is a great place for supercells. In fact, when I think of amazing supercell structure, Colorado is one of the places that comes to mind as the upslope flow helps to initiate supercells and the high elevation leads to very clear and dramatic views of them.

 

[Bart_Comstock]

[...]Super cells have the best chance of producing a strong tornado but I think bowechoes and derechoes can do that to. Maybe there is just embeded super cells in those linear lines of storms that produce tornadoes. I am no expert so don't quote me on that.

The primary way a tornado forms in a bow is from the cyclonic spin on the "top" of the bow.

Think of it this way. The air moves and behaves like water. If you have ever been canoeing and watched as your paddle moves through the water you will notice that on the tips of the paddle's blade there are little whirlpools that will form. The same thing happens with a bow echo. as it surges forward through the air the ends of it will form "whirlpools" in the atmosphere with the one on the north end being cyclonic and the one on the south being anti-cyclonic (if storm motion is west to east in the northern hemisphere). The anti-cyclonic rotation to the south generally will not take hold and form a supercell but the cyclonic rotation on the north side will often result in a HP supercell which in turn can result in tornadoes. These are often difficult to chase as the mesocylones in these book end vortexes are almost always embedded in rain.

Tornadoes embedded in a line of storms are usually the result of a LEWP which is an embedded area of low pressure (a mesocyclone) and thus the updraft of an embedded supercell. A LEWP is typically going to look like an embedded and elongated S curve in a line of storms. or a big < between bows (like in a serial derecho).

Here is a poor example. I didn't have any better radar grabs on my laptop and I didn't feel like drawing one on paint because I am lazy.

You can also have a broken line of supercells that will have a gap of either weak or no reflectivity between each cell on radar. Some the cell on the southern end (in the northern hemisphere) of a line of storms can be a more classic looking supercell. These are cells are commonly referred to as the "tail end charley."

 

[Jeremy Perez]

Bart, thanks very much for that post. It looks like something similar just occurred over Phoenix. It was very interesting to watch the notch quickly form on radar and the warning box pop up (Jan 22, 2010 - 03:27 UT).

 

[Jason Harris]

Speaking of non-supercell tornadoes, Florida NWS put out a severe weather pdf that includes this mention:
--

Smaller tornadoes
can also develop when cold air rushing
out from underneath one thunderstorm
collides with cold air rushing out from
another thunderstorm. As these winds
merge from different directions, rotation
takes place, and that rotation produces a tornado.

From: Florida Severe Weather Awareness Guide 2010
download: http://www.srh.noaa.gov/mlb/

What more precisely is this referring to? Sounds a bit like Bart's description of bownadoes up above.

 

[Skip Talbot]

That sounds like they are trying to explain to the public that intersecting outflow boundaries enhances storm relative helicity. Although they made it sound like it produces vorticity (which it might?) and that directly results in a tornado (not really). This doesn't necessarily mean its a nonsupercellular tornado either, as the interaction between supercells (and other thunderstorms) and intersecting outflow boundaries is often noted before tornadogenesis.

 

[Jeff Duda]

Speaking of non-supercell tornadoes, Florida NWS put out a severe weather pdf that includes this mention:
--

From: Florida Severe Weather Awareness Guide 2010
download: http://www.srh.noaa.gov/mlb/

What more precisely is this referring to? Sounds a bit like Bart's description of bownadoes up above.

Random spinups? A lot of chaotic motion can occur in the wake of a thunderstorm, and especially when outflow boundaries collide. Not only do colliding OFBs produce massive convergence and upward motion needed to tilt and stretch vorticity into the vertical, but there is also much baroclinically generated horizontal vorticity at the leading edge of the cold pools. Therefore, when they collide, there is vorticity there to tilt and stretch, possibly producing a short lived tornado.

 

[Charles Kuster]

Great points by everyone! Tornadoes can definitely form without the presence of a supercell. Landspouts (non-supercell tornadoes) provide the best example of this occurrence. All you really need is a vigorous updraft to stretch the pre-existing ground spin and you can get a non-supercell tornado. They are particularly hard to warn for, because it may look like you have a small non-impressive storm on radar, but there is still a tornado on the ground. One such example occurred on October 4, 2004 near Denver International Airport. Eleven tornadoes were produced in thirty five minutes!

13 minutes before the first tornado.

17 minutes after the first tornado. A tornado is on the ground at this time.

Some video from that event can be found here: http://www.youtube.com/watch?v=3FuuP7uJVEk

I have heard of showers producing weak tornadoes if there is enough spin in the air and conditions are right.

This reminded me of a tornado that I saw on May 5, 2007. The storm was definitely a supercell, but it was not very impressive. We drove right through the "core" and only experienced light to moderate rain. It did not look very impressive on radar either, but the storm produced an EF-2 tornado near Osborne, KS.

Five minutes after a tornado struck Osborne, KS. Tornado is still on the ground at time of radar image.

 

[Kevin Bowman]

The August 4th Chicago derecho produced tornadoes on its forward flank. Check out Video number 2

http://www.wgntv.com/weather/wgntv-fermilab-2009,0,652841.htmlstory

http://www.youtube.com/watch?v=jSJiucjn7GA

 

[MatthewCarman]

Thanks for that information Bart. I am reading it right now.

 

[Justin Turcotte]

Had a nonsupercell tornado in my back yard a couple years ago. Cold core version.
http://www.plainschase.com/photos/2008/storm06092008/2.JPG