10. SEVERE WEATHER STABILITY INDICES

Instability is a critical factor in severe weather development. Severe weather stability indices can be a useful tool when applied correctly to a given convective weather situation. However, great care should be used when applying these empirical indices because they simply cannot be applied to every weather situation and must always be applied in conjunction with other parameters.

A number of indices are tied to specific pressure levels which may (or may not) be representative of a particular convective weather situation. Soundings must be looked at as a whole. Stations at high elevations make some indices irrelevant. Local adaptations must be made at such stations and are not discussed here. One must also consider the fact that sometimes the upper air sounding itself may not even be representative of the overall synoptic situation.

Severe weather indices only indicate the potential for convection. There must still be sufficient forcing for upward motion to release the instability before thunderstorms can develop. A zero lifted index is sufficient for severe weather development if the dynamics are very strong. On the other hand, when the lifted indices are -8 or less, severe weather can occur with very weak upper air support Hales (1996). Also be aware of a strong capping inversion inhibiting updrafts.

A. K Index (K) (George 1960)

The K index is a measure of thunderstorm potential based on the vertical temperature lapse rate along with the amount and vertical extent of low-level moisture in the atmosphere.

 

(Units for all parameters are in °C)

 

 

K Index

STM Probability

<15 0%
15-20 <20%
21-25 20-40%
26-30 40-60%
31-35 60-80%
36-40 80-90%
>40 near 100%

 

B. Lifted Index (LI) (Galway 1956)

The LI is a measure of potential instability from the surface to 500 mb. Lift a parcel with an average mixing ratio and dry adiabat in the lowest 100 mb of the sounding. It is very similar to the Showalter Index, but better considers available low level moisture below 850mb.

 

Lifted Index (LI)

Instability

0 to 3 Stable. Weak convection possible with strong lifting or forcing mechanism
0 to -3 Marginally Unstable
-3 to -6 Moderately Unstable
-6 to -9 Very Unstable
< -9 Extremely Unstable

 

C. Showalter Stability Index (SSI) (Showalter 1953)

The SSI is a measure of the potential instability in the 850mb to 500 mb layer. The SSI is unrepresentative if the available low level moisture occurs below 850mb.

 

SSI

Stability

+1 to +2 Stable. Weak convection possible if strong lift present
0 to -3 Moderately Unstable
-4 to -6 Very Unstable
< -6 Extremely Unstable

 

D. Total Totals (TT) (Miller 1972)

The Total Totals Index consists of two components: Vertical Totals (VT) and Cross Totals (CT). VT represents static stability between 850 mb and 500 mb. The CT includes the 850 mb dew point. As a result, TT accounts for both static stability and 850 mb moisture. However, TT would be unrepresentative in situations where the low-level moisture resides below the 850 mb level. If a significant capping inversion is present, convection will be inhibited even with a high TT.

 

Total Totals

Thunderstorm Chances

45 to 50 Thunderstorms possible
50 to 55 Thunderstorms more likely (some severe)
55 to 60 Severe thunderstorms likely

 

E. Sweat Index (Severe Weather Threat Index–SWEAT) (Miller 1972)

The SWEAT Index evaluates the potential for severe weather by examining both kinematic and thermodynamic information into one index. Parameters include low-level moisture (850 mb dewpoint), instability (Total Totals Index), lower and middle-level (850 and 500 mb) wind speeds, and warm air advection (veering between 850 and 500 mb). Unlike the K Index, the SWEAT index should be used to assess severe weather potential, not ordinary thunderstorm potential.

The last term in the equation (the shear term) is set to zero if any of the following criteria are not met: 1) 850 mb wind direction ranges from 130 to 250 degrees, 2) 500 mb wind direction ranges from 210 to 310 degrees, 3) 500 mb wind direction minus the 850 mb wind direction is a positive number, and 4) both the 850 and 500 mb wind speeds are at least 15 kts. No term in the equation may be negative; if so, that term is set to zero.

 

 SWEAT over 300  Potential for severe thunderstorms
 SWEAT over 400  Potential for tornadoes

 


These are guidance values developed by the U.S. Air Force. Severe storms may still be possible for SWEAT values of 250-300 if strong lifting is present. In addition, tornadoes may occur with SWEAT values below 400, especially if convective cell and boundary interactions increase the local shear which would not be resolved in this index. The SWEAT value can increase significantly during the day, so low values based on 1200 UTC data may be unrepresentative if substantial changes in moisture, stability, and/or wind shear occur during the day.

F. Deep Convective Index (DCI) (Barlow 1993)

The DCI attempts to combine the properties of equivalent potential temperature (Qe) at 850 mb with instability.

Values of roughly 30 or higher indicate the potential for strong thunderstorms. Ridge axes of DCI seem to be a good indicator of location for thunderstorm development given the presence of forcing mechanisms.

11. ACKNOWLEDGMENTS

 


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