Operational low-level buoyancy parameter guidelines associated with supercell tornadoes (see also description and key to examples)

Because supercell tornadoes are poorly understood and involve processes and interactions that are difficult to assess using simple "bulk" parameters, guidelines using CIN, LFC height, and low-level CAPE in potentially tornadic situations are not simple or easy to develop.   Since supercell tornadoes are associated with increased low-level and deep-layer wind shear, this shear (and general CAPE) should be evaluated before low-level thermodynamic parameters (such as LCL, CIN, and LFC) are assessed

===>Also, it is important to understand that the main benefit of assessing parameters is that they can help alert a forecaster to short-term forecast situations and environments that are more likely or less likely to support tornadoes, in a probabilistic sense.  They cannot provide definitive answers as to whether a storm in a certain environment will or won't produce a tornado.

The tables below show significant supercell tornado cases stratified by CIN, LFC, and low-level CAPE, respectively:

1)    Notice how CIN, LFC, and 0-3 km CAPE tend to vary as EHI (3rd column) and CAPE (last column) change in value.
2)    As CIN associated with significant tornadoes increases, so does EHI and total CAPE, on average.  This suggests that as CIN becomes larger (indicating a deeper low-level stable layer), larger shear-CAPE combinations are required to help generate strong low-level mesocyclones.
3)    As LFC heights associated with significant tornadoes increase, on average so do EHI and total CAPE, similar to CIN.
4)    As 0-3 km CAPE associated with significant tornadoes decreases, EHI and total CAPE increase, on average. 
5)    As in research done by Rasmussen and Blanchard, LCL heights average generally low (< 1000 m) for most significant tornado cases.
6)    Notice that most significant supercell tornadoes are associated with CIN less than 50-100 J/kg, and LFC heights below 2000m.
7)    Notice also that there are no significant tornadoes with CIN larger than around 150-160 J/kg and LFC height greater than 2500 m.

Using a different shear-CAPE parameter than EHI (VGP from Rasmussen and Blanchard's work), the scatterdiagrams below also suggest that significant supercell tornadoes generally don't occur with near-surface CIN greater than around 150  J/kg, or with low combinations of shear and CAPE (e.g., VGP values less than around .25 in these diagrams).  Using the parameter space below the lines in both scatterdiagrams eliminates half the nontornadic supercell cases while capturing nearly all the tornadic cases:
CIN vs 0-3 km VGP:    <F2-F5 tornadic supercell cases    <nontornadic supercell cases

This suggests that low-level thermodynamic characteristics such as CIN (and LFC height) are an important factor in a number of supercell tornado cases, and that supercells occurring in environments with CIN larger than 150-200 J/kg are significantly "elevated" (occurring over a deep stable layer in low-levels) and unlikely to support significant tornadoes.

Apart from LCL, low-level thermodynamic factors such as CIN, LFC, and low-level CAPE will likely be most useful operationally in determining environments that are significantly "elevated" and not conducive to significant tornadoes.   While many "elevated" situations on the cool side of well-defined surface fronts are easy to identify, a number of other scenarios are more subtle.   "Elevated" environments can even be located in the warm sector, with strong storms developing above an inversion located around 700-800 mb if the low-level moist layer is deep enough.   Parameters such as CIN and LFC height can be quite useful in diagnosing such environments.

A side note:  Don't assume that there is any organized relationship between total CAPE and low-level thermodynamic parameters.  The following diagrams with broad scatter show that there is really no way to infer low-level thermodynamic characteristics from a simple assessment of total CAPE alone:
total CAPE vs:  <LCL ht  <CIN  <LFC ht  <0-3 km CAPE

Low-level buoyancy parameter guidelines   (see also description and key to examples)

Based on the above information, the following general guidelines (please don't take values too literally!) are suggested for LCL, CIN, LFC, and low-level CAPE when cross-referencing potentially tornadic environments using shear-CAPE parameters.  Because AWIPS and the widely-used BUFKIT software do not use the virtual temperature correction, values shown below are based on non-virtual computations.  These guidelines generally apply to a mean-layer parcel in the lowest 50-100 mb.

LCL height (lifting condensation level): 

• The work of Rasmussen, Markowski, and others shows supercell tornadoes to be more likely with lower LCL heights (e.g., below 1000-1200 m) and smaller surface dew point depressions suggesting large low-level humidity.  A situation to be careful about is when LCL heights are low, but CIN is large (> 150-200 J/kg) and LFC heights are high, which implies an "elevated" environment less conducive to tornadoes.   It should also be noted that supercells in the high plains of the U.S. (west of 99 deg W lon) occasionally seem to be able to support significant tornadoes with LCL heights as high as 1500-2000 m.   The reasons for this are unclear, but may have to do with increased low-level heating over this elevated terrain.

CIN (convective inhibition):

• Near-surface CIN values less than roughly 50 J/kg are preferred for tornadic environments.  But in some cases in the plains, significant tornadoes can occur when CIN appears relatively large, up to around 150 J/kg.   In such cases with relatively large CIN, total CAPE, SRH and vertical shear are also typically very large (look for 0-1 km EHI values of 4.0 or 5.0 or greater, or 0-3 km VGP values of .40-.50 or greater).  Such shear-CAPE combinations probably help to generate a mesocyclone strong enough to develop within the relatively stable layer near the ground.  See discussion about relatively large CIN associated with some significant tornadoes. 
• Significant tornadoes are unlikely with near-surface CIN values larger than 150-200 J/kg, even with large vertical shear and shear-CAPE combinations.   Such large CIN values indicate that the environment is significantly "elevated" (see examples of elevated environments) with a deep stable layer in the lowest 3 km.

LFC height (level of free convection): 

• LFC heights below 2000 m are most associated with supercell tornadoes, but in the plains heights in the 2000 to 2400 m range can also be associated with significant tornadoes if total CAPE, SRH and vertical shear are large (the guidelines here are similar to those with tornado cases involving relatively large CIN).  As with CIN > 150-200 J.kg, LFC heights above 2400-2500 m are unlikely to support significant tornadoes because they imply a deep stable layer in the lowest 3 km.   LFC heights should be cross-referenced with CIN values when possible.

0-3 km CAPE (positive buoyancy below 3 km):

• Recent experience shows that 0-3 km CAPE is a parameter very sensitive to model inaccuracies and lifting parcel, more so than parameters such as CIN.  As a result, this parameter should not be assessed without reference to CIN and LFC height.  Use this parameter with caution and not in isolation!
• Significant tornado environments typically have some amount of low-level CAPE, but this can vary from as little as 30-40 J/kg to more than 200 J/kg.
• Values of 0-3 km CAPE around 200 J/kg or more are quite large, and in weaker shear environments may contribute to weak or low-end significant tornadoes, particularly if storms form near low-level boundaries with pre-existing vertical vorticity.  See discussion about unusually large low-level CAPE and supercell tornadoes.  Unfortunately, recent experience has shown that inaccuracies in forecast models' handling of boundary layer data often result in exaggerated and excessively large low-level CAPE depictions, particularly across the eastern 1/3rd of the United States. 
• When there is no CAPE below 3 km, and CIN is large (>150-200 J/kg) and LFC heights are high (>2400-2500 m), this indicates a deep stable layer in the lowest 3 km that is unlikely to support significant tornadoes.

Important note:  When assessing low-level thermodynamic parameters such as CIN, LFC, and low-level CAPE, be sure to look at all available surface observations, including mesonet observations, for largest credible temperature and dewpoint.  See discussion about local surface observations in computing CIN, LFC, etc.

Examples (compare the following to the guidelines above):     (see also description and key to examples)

^ This environment appears quite capable of supporting significant tornadoes because the shear-CAPE combinations are large (EHI and VGP), and near-surface CIN, while relatively large, is not prohibitive given the large vertical shear and shear-CAPE combinations.  See discussion about relatively large CIN associated with some significant tornadoes. 

^ This environment is also quite capable of supporting significant tornadoes because there is large 0-3 km CAPE and very little CIN, while VGP, SRH, and vertical shear are at least moderate in value.  See other examples of significant tornadic environments.

^ Because of the very large area of near-surface CIN that extends throughout the bottom 3 km, this environment is quite "elevated" and unlikely to support significant tornadoes because of the deep low-level stable layer, even though shear-CAPE combinations and vertical shear are large.  See other examples of elevated environments.

Unfortunately, environments with marginal combinations of shear, CAPE, and low-level thermodynamic parameters occur relatively frequently in the warm season, and are often difficult to assess (see examples of "gray area" environments that are difficult to assess).  While these situations are in general less likely to produce significant tornadoes than environments where there is larger shear and total CAPE, they will still require careful monitoring using all available data.

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