Only supercells produce tornados?

[Paul Knightley]

Supercells produce the most violent tornadoes, but many tornadoes don't come from classic supercell structures. Tornadoes may be embedded in lines, form on surging cold fronts in association with misocyclones, develop beneath rapidly growing cumulus, etc.

I think because supercells spawn the most deadly tornadoes, research has obviously focussed on these storms and perhaps led to a perception that only supercells produce "true" tornadoes (whatever they might be!) - in reality, tornadoes develop from a broad spectrum of storm types, and what is important are the storm-scale processes which allow them to form, rather than whether or not a storm is a supercell.

 

[Scott Olson]

Your short answer is no but you can parse the definition and semantics of supercell and tornado quite a bit. Even so, with landspouts, waterspouts from fair weather cumulus that move onto land and vortices associated with a mesovortex/bow echo.

Tornado casts a rather wide net on purpose to not have to differentiate between the convective or atmospheric mode producing the vortex.

 

[Mike Johnson]

Tornadoes can absolutely form from non-supercellular storms. All it requires is stretching and tilting of existing horizontal vorticity that can be present along most boundaries or in the ambient atmosphere. Supercellular tornadoes are typically stronger and have more longevity, thus are more well known.

 

[Bobby Prentice]

Can a tornado only be produced by a supercell?

First, a little background.

supercell—An often dangerous convective storm that consists primarily of a single, quasi-steady rotating updraft, which persists for a period of time much longer than it takes an air parcel to rise from the base of the updraft to its summit (often much longer than 10–20 min).

Most rotating updrafts are characterized by cyclonic vorticity (see mesocyclone). The supercell typically has a very organized internal structure that enables it to propagate continuously. It may exist for several hours and usually forms in an environment with strong vertical wind shear. Supercells often propagate in a direction and with a speed other than indicated by the mean wind in the environment. Such storms sometimes evolve through a splitting process, which produces a cyclonic, right-moving (with respect to the mean wind), and anticyclonic, left-moving, pair of supercells. Severe weather often accompanies supercells, which are capable of producing high winds, large hail, and strong, long-lived tornadoes.

Research indicates about 26% of all supercells produce a tornado report.

supercell tornado—A tornado that occurs within supercells that contain well-established midlevel mesocyclones.

However, a large percentage of tornadoes are not produced by supercells, mainly the weak ones (F0/F1).

nonsupercell tornado—A tornado that occurs with a parent cloud in its growth stage and with its vorticity originating in the boundary layer. The parent cloud does not contain a preexisting midlevel mesocyclone. Landspouts and gustnadoes are examples of the nonsupercell tornado.

Here is a schematic which explains the nonsupercell tornado process:

Nonsupercells over land are called "Landspouts."

landspout—1. (Rare.) A tornado. 2. Colloquial expression describing tornadoes occurring with a parent cloud in its growth stage and with its vorticity originating in the boundary layer. The parent cloud does not contain a preexisting midlevel mesocyclone. The landspout was so named because it looks like a weak, Florida Keys waterspout over land. See nonsupercell tornado.

Waterspouts can be either supercell or nonsupercell tornadoes.

waterspout—1. In general, any tornado over a body of water. 2. In its most common form, a nonsupercell tornado over water.

Such events consist of an intense columnar vortex (usually containing a funnel cloud) that occurs over a body of water and is connected to a cumuliform cloud. Waterspouts exhibit a five- stage, discrete life cycle observable from aircraft: 1) dark-spot stage; 2) spiral pattern stage; 3) spray-ring stage; 4) mature or spray-vortex stage; and 5) decay stage. Waterspouts occur most frequently in the subtropics during the warm season; more are reported in the lower Florida Keys than in any other place in the world. Funnel diameters range from a few up to 100 m or more; lifetimes average 5–10 minutes, but large waterspouts can persist for up to one hour.

Well-known nonsupercell tornado hot-spots include the Denver Cyclone and Denver Convergence Vorticity Zone (DCVZ) (landspouts) and the Florida Keys (waterspouts).

More information can be found at:
NSSL: Tornado Basics

 

[Bill Tabor]

I believe it was Jon Davies some years back that had an article and mentioned a landspout that was up to F4 in strength. So, yes, landspouts are true tornadoes. A true tornado is associated directly with cloud base and a deep convection process within a storm. Supercellular tornadoes form in association with a rotating inflow updraft which on radar is known as a 'mesocyclone'. Landspouts typically form on a leading convergent frontal part of a storm (but not directly associated with the mesocyclone). Severe storms can and do produce tornadoes. A supercell is not required in order to qualify for a severe storm it just has to meet criteria such as associated wind speed, hail size, etc. Last I heard and researched a gustnado was not considered a 'true' tornado because it is not directly associated with deep convection and connection to a storm / cloud base. Gustnadoes typically form out, in front, and along shear interfaces of gust fronts of storms and are not connected to the cloud. However sometimes gustnadoes have formed and been ingested by a storm, attaching and creating a landspout tornado...or so I have heard. Keep in mind that a tornado need not have visible condensation cloud, funnel material to exist and be classified as a tornado. It must have violent motion, and it must be connected / associated with the storm cloud above. This is where it sometimes gets difficult as a chaser to determine if the vortex is connected to the cloud and not just a dust whirl beneath. This can be hard in dry climates such as New Mexico, as well as in weak, or newly forming tornadoes.

 

[Allan Jeffers]

Thanks for the info everyone.

 

[BBauer]

Correct me if I am wrong but it seems a good example of a fairly strong tornado (strong F2) that was non-supercellular was the tornado that hit Salt Lake City in 1999. Here is the summary given by the NWS...

"Synoptic Analysis: On the morning of August 11, 1999, an upper level trough of colder air moved into northern Utah from Nevada. In advance, warm breezy southerly winds blew over the Salt Lake Valley. By Noon, there was evidence that either an old frontal boundary existed or a convergence zone had developed across the Salt Lake Valley due to breezes from the Great Salt Lake meeting up with the southerly winds that prevailed through the majority of the valley. The Salt Lake morning sounding indicated some vertical shearing of the winds (differences in wind speeds) along with the jet-stream over northern Utah. As this happened, thunderstorms began to form over the Oquirrhs in the Herriman area and over the south end of the Great Salt Lake/north end of the Oquirrhs in the Magna area. By 12:35 PM, there was a thunderstorm over the north portion of the Salt Lake Valley–with clouds tops extending up to 41,000 feet high–that rapidly intensified and generated a rare F2 tornado."

It seems this tornado formed from a convergence zone as depicted in the diagram shown in an earlier post. It doesn't seem that there is evidence that the SLC tornado had a mesocyclone but I am a total newb so I could be wrong.

 

[Paul Knightley]

From what I understand this was a non-supercell tornado. NSTs also develop within strong cold-frontal rainbands, with LEWP-type behaviour, and can reach EF2 no problem!