Soundings Definition: Upper Air Soundings provide an important means for determining the stability of the atmosphere above a specific location. The effects of lifting or lowering an air parcel can be determined by the conditions of the surrounding environment as measured by balloon sounding or wind profiling. Soundings can also reveal the existence of veering winds with altitude, a factor which can fuel the development of tornadoes.
Skew-t Theory: (1) (2) (3)   Skew-t Mastery: (1)   Skew-t Help: (1) (2)   Parameters: (1) (2) (3)
Links: JAVA Soundings Forecast Soundings:  Wx Machine  PSC  Tip Sheets:  WFO LMK  Alt  SPC  NSSL  RAP Meteograms
SPC Meso


CAPE
Definition: A cumulative measure of the positive buoyant energy (in J/kg) a rising parcel of air would have between its Level Of Free Convection (LFC) and the Equilibrium Level (EL). CAPE is a vertically integrated quantity as well as a fundamental indicator of the potential intensity of deep, moist convection.
Strength: Convective Available Potential Energy is the best thermodynamic parameter to assess buoyancy and the potential energy available for deep, moist convection.
Guidance Values:
Below 0 J/kg: Stable
0 to 2500 J/kg Moderately Unstable
2500 to 3500 J/kg Very Unstable
Over 3500 J/kg Extremely Unstable
Limitations: CAPE alone does not imply instability as capping can inhibit convection. CAPE depends on instability and depth of integration. Research has also shown that low-level CAPE may have relevance to tornado production
RUC2 SBCAPE:   0 Hr 3 Hr 6 Hr 9 Hr 12 Hr Loop
ETA SBCAPE:   0 Hr 3 Hr 6 Hr 9 Hr 12 Hr 15 Hr 18 Hr 21 Hr 24 Hr 27 Hr 30 Hr 33 Hr 36 Hr 39 Hr 42 Hr 45 Hr 48 Hr

Lifted
Index
Definition: The atmospheric stability parameter which estimates the tendency of a low-level parcel of air to continue to rise if it was 'lifted' to the middle of the atmosphere. Specifically, it is the difference between the 500 mb temperature and the temperature of a parcel lifted from the surface to 500 mb. The lift at the surface begins dry adiabatically until the LCL (point of intersection with the mixing ratio line) is reached and then moist adiabatically to 500 mb. Moisture and lapse rate (static stability) are therefore combined into one number.
Strength: Good stability parameter.
Guidance Values:
0 to -3 C Marginally Untable
-3 to -6 C Moderately Unstable
-6 to -9 C Very Unstable
> -9 C Extremely Unstable
Limitations: LI uses a temperature difference between rising parcels and the environment at a single level and therefore cannot account for details in the environmental temperature curve above the LCL and below 500 mb.
RUC2 LI:   0 Hr 3 Hr 6 Hr 9 Hr 12 Hr Loop
ETA LI:   0 Hr 3 Hr 6 Hr 9 Hr 12 Hr 15 Hr 18 Hr 21 Hr 24 Hr 27 Hr 30 Hr 33 Hr 36 Hr 39 Hr 42 Hr 45 Hr 48 Hr

Cap
Strength
Definition: Cap strength measures the ability of stable air aloft (a layer of relatively warm air) to inhibit low-level parcel ascent. Empirical studies show that a cap greater than 2C often precludes thunderstorms in the absence of a strong dynamical or forced lift. A strong cap prevents widespread convection from occurring. It also allows low level heat and moisture to increase over a period of time which in turn increases the amount of potential instability increasing the severe potential once the cap is broken. Additionally, a weak cap can also reveal where "warm air thunderstorms" will develop, assuming other conditions are favorable.
Strength: Good stability parameter.
Guidance Values:
> 2 C Convection Inhibited
< 2C
 Uncapped
 
Limitations: LI uses a temperature difference between rising parcels and the environment at a single level and therefore cannot account for details in the environmental temperature curve above the LCL and below 500 mb.
RUC2 Lid Strength:   0 Hr 3 Hr 6 Hr 9 Hr 12 Hr
ETA LI:   0 Hr 3 Hr 6 Hr 9 Hr 12 Hr 15 Hr 18 Hr 21 Hr 24 Hr 27 Hr 30 Hr 33 Hr 36 Hr 39 Hr 42 Hr 45 Hr 48 Hr 51 Hr 54 Hr 57 Hr 60 Hr

Lapse
Rate
Definition: The rate of temperature change with height in the atmosphere. A steep lapse rate is one in which the environmental temperature decreases rapidly with height. The steeper the environmental lapse rate, the more potentially unstable the atmosphere will be since rising parcels will tend to remain warmer than the environment (and continue to rise).

Let LR(e) = Environmental lapse rate
Let LR(p) = Parcel lapse rate
Let LR(da) = Dry adiabatic lapse rate (9.8 C/km)
Let LR(ma) = Moist adiabatic lapse rate (4 C/km to 9.8 C/km)

Absolute stability: LR(e) < LR(p) Parcel lapse rate steeper slope (cooler) than the environment
Neutral stability: LR(e) = LR(p) Parcel lapse rate same slope as the environment
Absolute instability: LR(e) > LR(p) Parcel lapse rate has less slope (warmer) than the environment
Conditional instability: LR(da) > LR(p) > LR(ma) Parcel lapse rate between the dry and moist rates
Strength: Good parcel static stability indicator.
Guidance Values:
6.3 C/km Severe Tstorms
7.0 C/km
 Tornadoes Possible
 
Limitations: Assessments of environmental lapse rates by themselves are insufficient to determine parcel buoyancies. Actual parcel instability leading to deep, moist convection is primarily associated with vertical parcel displacements.
RUC2 LR:   0 Hr 3 Hr 6 Hr 9 Hr 12 Hr Loop
ETA LR:   0 Hr 6 Hr 12 Hr 18 Hr 24 Hr 30 Hr 36 Hr 42 Hr 48 Hr 54 Hr 60 Hr 66 Hr 72 Hr 78 Hr 84 Hr

Storm
Relative
Helicity
(SRH)
Definition: Storm-relative helicity is an estimate of a thunderstorm's potential to acquire a rotating updraft given an environmental vertical wind shear profile, assuming thunderstorms are able to develop. It integrates the effects of S-R winds and the horizontal vorticity (generated by vertical shear of the horizontal wind) within the inflow layer of a storm.
Strength: Good estimate for supercell potential and possible tornado intensity.
Guidance Values:
150 m2s-2 Threshold For Supercell Development
150 - 299 m2s-2 Weak Tornadoes (F0 - F1) Possible
300 - 499 m2s-2 Strong Tornadoes (F2 - F3) Possible
> 450 m2s-2 Violent Tornadoes (F4 - F5) Possible
Limitations: SRH is very sensitive to changes in the horizontal wind vector and storm motion so algorithm requires frequent parameter update to maximize effectiveness.
RUC2 SRH:   0 Hr 3 Hr 6 Hr 9 Hr 12 Hr Loop
ETA SRH:   0 Hr 6 Hr 12 Hr 18 Hr 24 Hr 30 Hr 36 Hr 42 Hr 48 Hr 54 Hr 60 Hr 66 Hr 72 Hr 78 Hr 84 Hr

Storm
Motion
Definition: Storm motion is the average wind speed in knots a storm will move and the direction the storm will move from. It is calculated as 75% of the mean wind speed between the surface and 6 km. The storm moves slower than the ambient wind speed since a storm has a large mass of water that has to be pushed along. The turbulence within a storm also makes it more difficult to push along. Storms will move more quickly in cases where there is speed shear with height (wind speed increases with height).
Strength: Helps indicate direction and speed of individual storm cells.
RUC2 Storm Motion:   0 Hr 3 Hr 6 Hr 9 Hr 12 Hr
ETA Storm Motion:   0 Hr 6 Hr 12 Hr 18 Hr 24 Hr 30 Hr 36 Hr 42 Hr 48 Hr 54 Hr 60 Hr 66 Hr 72 Hr 78 Hr 84 Hr

850 mb
Winds
Definition: Strong low level winds will quickly advect warm and moist air into a region if it is associated with the low level jet. Temperatures and dewpoints can change rapidly during the day via a low level jet. If winds are light in the PBL, severe weather is not as likely.
Strength: Helps indicate likelihood of severe weather
Guidance Values:
70 kts Very fast advection
30-69 kts Strong Advection
20 to 29 kts Marginal advection
< 20 kts Weak advection
Limitations: While a low level jet facilitates the development of severe weather there are other convective-scale conditions are needed as well to support tornado development.
RUC2 850mB Winds:   0 Hr 3 Hr 6 Hr 9 Hr 12 Hr Loop
ETA 850mB Winds:   0 Hr 6 Hr 12 Hr 18 Hr 24 Hr 30 Hr 36 Hr 42 Hr 48 Hr 54 Hr 60 Hr 66 Hr 72 Hr 78 Hr 84 Hr

500 mb
Winds
Definition: Middle-level S-R winds are important in order to create a balance between the low-level storm inflow along the forward front flank baroclinic zone and the low-level outflow associated with the rear flank downdraft.
Strength: Help differentiate between tornadic and non-tornadic supercells
Guidance Values:
16 kts (8 m/s) Lower limit for tornadic supercells
40 kts (20 m/s)
 Upper limit for tornadic supercells
 
Limitations: While sufficient S-R winds at 500 mb appear to be needed for tornadic supercell storms to develop other convective-scale conditions are needed as well.
RUC2 500mB Winds:   0 Hr 3 Hr 6 Hr 9 Hr 12 Hr Loop
ETA 500mB Winds:   0 Hr 6 Hr 12 Hr 18 Hr 24 Hr 30 Hr 36 Hr 42 Hr 48 Hr 54 Hr 60 Hr 66 Hr 72 Hr 78 Hr 84 Hr

Energy
Helicity
Index
(EHI)
Definition: Combines CAPE and S-R helicity into one index to assess the potential for supercell and mesocyclone development. High EHI values represent an environment possessing high CAPE and/or high S-R helicity. Both CAPE and SRH are very important in the formation of a strongly rotating convective updraft.
Strength: Useful for supercell and tornado forecasting
Guidance Values:
< 1 Supercells & Tornadoes unlikely
1 - 2 Supercells & Tornadoes possible
2 - 2.4 Supercells likely & Tornadoes possible
2.5 - 2.9 Supercell Tornadoes likely
3 - 3.9 Strong Tornadoes possible
> 4 Violent Tornadoes possible
Limitations: Does not always differentiate between tornadic and non-tornadic storms. High CAPE can inflate EHI and render it ineffective.
ETA EHI (0Z & 12Z:   0 Hr 3 Hr 6 Hr 9 Hr 12 Hr 15 Hr 18 Hr 21 Hr 24 Hr 27 Hr 30 Hr 33 Hr 36 Hr 39 Hr 42 Hr 45 Hr 48 Hr

Theta-E
Definition: THETA-E (Equivalent Potential Temperature) is the temperature that results after all latent heat is released in a parcel of air and the then brought adiabatically to the 1000 mb level. THETA-E increases as dewpoint and/or temperature increases.
Strength: High THETA-E regions often are most instable and the focus of activity
Guidance Values:
Instability Region with relatively high Theta-E are more unstable
Low Level Jet Moisture advection increases instability
Theta-E Ridge Regions with highest values are often burst point of convective activity
Limitations: If a strong cap is in place, convective storms will not occur even if THETA-E is high
RUC2 Theta-e Convergance:   0 Hr 3 Hr 6 Hr 9 Hr 12 Hr
ETA 2m Theta-e:   0 Hr 6 Hr 12 Hr 18 Hr 24 Hr 30 Hr 36 Hr 42 Hr 48 Hr

Supercell
Composite
Parameter
Definition: Supercell Composite Parameter is a multi-parameter index that includes 0-3 km storm-relative helicity, CAPE, and BRN shear. Each parameter is normalized to supercell "threshold" values. 0-6 km shear is divided by 40 kt, CAPE is divided by 1000 J/kg, and BRN shear is divided by 40 m2/s2
Strength:
Used by SPC forecasters for the past several years. Increasing values appear to be associated with an increased potential for supercells and tornadoes.
Guidance Values:
10 Sig Tornado
5.7 Weak Tornado
3.2 Nontornadic
1.2 Marginal
Limitations: As with any index, a high value should warrant further checking of environmental parameters to confirm whether severe conditions may occur
RUC2 SCP:   0 Hr 3 Hr 6 Hr 9 Hr 12 Hr All
ETA SCP:   0 Hr 3 Hr 6 Hr 9 Hr 12 Hr 15 Hr 18 Hr 21 Hr 24 Hr 27 Hr 30 Hr 33 Hr 36 Hr 39 Hr 42 Hr 45 Hr 48 Hr

Craven
Significant
Severe
Definition: The simple product of 100mb MLCAPE and 0-6km magnitude of the vector difference (m/s; often referred to as "deep layer shear") accounts for the compensation between instability and shear magnitude
Strength: Useful index for predicting severe events
Guidance Values:
> 20 Sig Severe
Limitations: As with any index, a high value should warrant further checking of environmental parameters to confirm whether severe conditions may occur
RUC2 SigSvr:   0 Hr 3 Hr 6 Hr 9 Hr 12 Hr All
ETA SigSvr:   0 Hr 3 Hr 6 Hr 9 Hr 12 Hr 15 Hr 18 Hr 21 Hr 24 Hr 27 Hr 30 Hr 33 Hr 36 Hr 39 Hr 42 Hr 45 Hr 48 Hr

Significant
Tornado
Definition: The Significant Tornado Parameter is a multi-parameter index that includes 0-6-km shear magnitude, 0-1-km storm-relative helicity, 100-mb mean parcel CAPE, and 100-mb mean parcel LCL height.
Strength: Useful index for predicting tornado potential
Guidance Values:
> 1 Tornadic
< 1 Non-Tornadic
Limitations: As with any index, a high value should warrant further checking of environmental parameters to confirm whether severe conditions may occur
RUC2 SigTor:   0 Hr 3 Hr 6 Hr 9 Hr 12 Hr All
ETA SigTor:   0 Hr 3 Hr 6 Hr 9 Hr 12 Hr 15 Hr 18 Hr 21 Hr 24 Hr 27 Hr 30 Hr 33 Hr 36 Hr 39 Hr 42 Hr 45 Hr 48 Hr
Other
Severe
Weather
Indices
Convective Season Environmental Paramaters & Indices (Louisville NWS)
Severe Weather Parameters (CAPS)
Severe Weather Cookbook (Jeff Haby)