Ingredients necessary for tornadoes : 

 

In general, tornadoes are spawned by thunderstorms evolving in strong wind-sheared environments (i.e. by organized convection). More specifically, tornadoes thrive in environments with strong low-level vertical speed and directional wind shear. They are most often associated with supercell thunderstorms but can also be spawned by Quasi-Linear Convective Systems (QLCSs) (i.e. dynamic squall lines) composed of hybrid supercellular structures and/or containing Line Echo Wave Patterns (LEWPs).

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For a given environment favorable for supercell thunderstorm formation or more generally favorable for organized convection, the following atmospheric ingredients or airmass characteristics are known to favor the production of tornadoes :

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• Strong deep-layer shear - While not the most crucial ingredient, strong deep-layer (0-6 km) shear is nonetheless important given the fact that tornadoes are most often associated with supercell thunderstorms, which need elevated values of this ingredient (usually > 40 kts). Supercell thunderstorms are defined as storms containing a quasi-steady rotating updraft known as a mesocyclone. Mid-level mesocyclones can be the precursor to low-level mesocyclones (rotating wall clouds) and subsequent tornado formation if other atmospheric ingredients are present or come into play.

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• Strong low-level shear - For a given environment favorable for organized convection, the ones containing strong low-level shear are more apt to produce tornadoes than ones with weak low-level shear. Strong low-level speed shear can help increase the inflow and convergence into a storm and aid in tornadogenesis, especially if that inflow is streamwise (oriented parallel to the environmental or baroclinically-generated horizontal vorticity vectors/tubes) relative to the storm. High values of streamwise vorticity imply important directional wind shear with height, which can be measured with parameters such as Storm Relative Helicity (SRH) on a hodograph. Past studies have found that 0-1 km shear and 0-1 km SRH are more important for tornado formation than 0-3 km shear and 0-3 km SRH.  Recent studies have even shown that in some instances, an even shallower layer may be necessary to properly quantify tornadogenesis probability (i.e. over a 50 - 250 m layer close to the ground).

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• A low cloud base - A low thunderstorm cloud base will usually limit the evaporation potential in the sub-cloud layer and avoid that excessively cold and dense downdraft air undercuts any low-level mesocyclonic circulation (rotating wall cloud) associated with the updraft. A low cloud base usually necessitates the presence of a warm and humid boundary layer (i.e. high dew points). If the downdraft air located under a low-level mesocyclone is too cold and dense, it will have a harder time being sucked upwards into the low-level circulation and will often prevent inflow air from feeding into the circulation which will usually limit the tornado potential. Such storms will tend to become "outflow dominated" and tend to favor "bow-echo type" thunderstorm structures. If the downdraft air is more buoyant (warmer and less dense), this will tend to favor tornadogenesis, especially if coupled with strong low-level shear.

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• Rear-Flank Downdraft (RFD) air that is not too cold - This ingredient is strongly related to the last point. If the RFD of a supercell storm is too cold and dense, tornadogenesis will be hindered for the same reasons as mentioned previously. Cold and dense RFD air can be a result of a high cloud base and/or a result of the presence of dry layers at mid-levels due to the advection of an elevated mixed layer (EML) aloft, like with the Spanish Plume or Mexican Plume.  This dry air will cool the downdraft via evaporation which is one reason why low-precipitation supercells are less efficient in producing tornadoes than classic and high-precipitation supercells.

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• Backing of winds at the surface - Important backing of surface winds (anti-clockwise rotation) near the surface will tend to increase the streamwise vorticity and lead to higher 0-1 km SRH values. If this backing of winds is coupled to a strengthening of the thunderstorm inflow and/or a strengthening low-level jet, this can significantly increase the potential for tornadogenesis via low-level convergence, tilting and stretching of the environmental or baroclinically generated horizontal vorticity. 

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• Some amount of surface layer based instability - While a certain amount of airmass instability is necessary for tornado development, large values are not needed. Tornadoes can be spawned with as little as 500-1000 J/kg  of CAPE with low-topped supercells during the cool season or even during the summer months (ex: CDF case with 600-1200 J/kg CAPE). When consulting CAPE values, it is better to use surface-based CAPE (SBCAPE) or mixed-layer CAPE (MLCAPE) rather than most-unstable CAPE (MUCAPE) since you want to estimate tornado potential with surface-based storms and not elevated storms that don't generate inflow close to the ground or that are de-coupled from the boundary layer. It is also a good idea to consult 0-3 km CAPE since large values of low-level CAPE can help tilt and stretch the low-level streamwise vorticity shear into the vertical under the updraft.

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• The presence of a surface convergence boundary - The presence of a surface convergence boundary on which a thunderstorm cell can initiate and/or remain anchored to, provides the storm with a vorticity rich environment (ex:  baroclinally-generated horizontal vorticity streamers that can be tilted into the vertical). This surface convergence boundary can be a front, a dryline, an outflow boundary or any other type of mesoscale/storm-scale feature (ex: sea-breeze front). If the convergence boundary is the storm's own outflow boundary (ex: a supercell's RFD gust front), tornadogenesis will usually only be favored if the outflow boundary does not undercut and outrun the storm and its updraft. If the convergence boundary remains anchored to the storm, the associated horizontal vorticity generated has a better chance of being tilted into the vertical beneath the updraft/mesocyclone. For this to happen, the storm's inflow usually has to be of a certain strength in order to push back or resist the outflow boundary. Storm inflow can be increased by the presence of a low-level jet for example or by strong pressure perturbations induced by the storm itself, especially when strong vertical wind shear is present (ex: with supercells) or when large airmass instability is present (ex: Jarrell. TX storm 1997).

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In a nowcasting environment, additional clues via radar that can alert forecasters on the imminence of tornadogenesis or that a tornado is on the ground in on-going convection :

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- The presence of a well-developped hook-echo with a supercell : this is a visual indication of the presence of a mesocyclone which is wrapping the precipitation around it's western and southern flank. When coupled with the development of a clear slot in the field directly behind the mesocyclonic updraft, this can be an indication of RFD strengthening and of an increased or immiment risk of tornadogenesis.

- The presence of a strong radial velocity couplet : this indicates the presence of a strong mesocyclone with the supercell. The stronger the gate to gate shear, the stronger the mesocyclone which tends to increase the tornado risk underneath it if storm-scale factors allow it.

- The presence of a tornado vortex signature (TVS) within the velocity couplet : this is usually a strong signal that a tornado is on the ground. TVSs are usually only visible rather close to the radar and with radars that sample the low-levels of the storm. They are apparent as a folded intense smaller velocity couplet within the broader mesocyclonic velocity couplet circulation.

- The trigger of a radar-based Mesocyclone Detection Algorithm : if it identifies a strong rotation with the storm, this provides additional support that the mesocyclonic circulation has vertical and temporal continuity and should be taken seriously/monitored.

- Low correlation coefficient (CC) values within the hook echo : low values hint at tornadic debris being lofted into the air and is a good indication that a tornado is on the ground and inflicting material damage. This is because tornadic debris is very heterogeneous in size and shape and therefore appears as a very de-correlated hydrometeor mass.

- The identification of a Descending Reflectivity Core (DRC) on radar reflectivity or velocity vertical cross-sections - When visible, this can be an indication that the precipitation cascade of a thunderstorm cell is in the process of collapsing to the ground. If this DRC collapses/descends around a supercell's mesocyclone and into the hook echo area of the storm, it can signify an imminent strengthening of the storm's Rear-Flank Downdraft (RFD) and lead to an RFD surge. This is usually observable in the field as a "clear slot formation" immediately behind the storm's mesocyclone and can aid in the tornadogenesis process via the tilting and stretching of baroclinic vorticity streamers along the RFD gustfront.

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