1. Comparison of Observed Conditions with Stability Indices
LT Cathy McDougall
20 March 2001

2. Introduction
The forecasting of thunderstorms and related phenomena (hail, tornadoes, strong winds, heavy rain) is based primarily on the concepts of:
a) conditional instability.
b) convective instability.
c) forced lifting of air near the surface.

3. Conditional Instability
the lapse-rate definition (i.e., the environmental lapse rate lies between the dry- and the moist- adiabatic lapse rates) d >  > s d = -9.8 C/km s = (-7 +/- 3) C/km
the available-energy definition (i.e., a parcel possesses positive buoyant energy)
+ > - real latent instability
- < + pseudo latent instability
- only latent stability
saturated parcel = instability
unsaturated parcel = must
evaluate CAPE and CIN

4. Stability and Instability Indices
Showalter Index SI = T500 - Tp500
>= 3 No convective rain
1 to 3 Showers likely
-2 to 1 Thunderstorms probable
<= -3 Tornadoes probable
K Index K=(T850 - T500) + Td850 - (T700 - Td700 )
Mod K = (aveTsfc T500) + aveTdsfc (T700 - Td700 )
K THUNDERSTORM PROBABILITY
>

5. Instability Indices (Continued)
Total Totals Index TT = T Td T500
>44 Thunderstorms
>50 Thunderstorms with large hail and
tornadoes.
SWEAT Index 12D +20(TT-49) + 2v8 + v (S+0.2)
< 272: unlikely
273 to 299: general storms; slight risk of severe storms
300 to 400: storms approaching severe limits; moderate risk of severe storms
401 to 600: few severe storms with isolated tornadoes; strong risk
601 to 800: scattered tornadoes; high risk

6. Convective Instability
Convective instability was originally defined by Rossby (1932) to be when one of the three following equivalent conditions is met over a layer of the atmosphere:
(1) the lapse rate of wet-bulb temperature (Tw) exceeds the moist-adiabatic lapse rate (s)
(2) the equivalent potential temperature (e) decreases with height, or
(3) the wet-bulb potential temperature (w) decreases with height.

7. THUNDERSTORM EVENT 12 February 2001 at 4Z
Closest Sounding to the event was
12FEB01, 00Z

9. Instability Indices from 12/00Z
Showalter Index using -7 = Thunderstorms probable
Showalter Index using Skew T= No convective rain
Showalter Index using -4 = Tornadoes probable
KIndex = % probability of Thunderstorms
ModKIndex = % probability of Thunderstorms
TTIndex = Groups of tornadoes are feared - ref 1
Severe Weather Probability Moderate - ref 2
SWEAT = Usefulness low because < 250

10. Comparison with Stable Atmospheric Conditions
Sounding
05 Feb 2001
18 Z
Fog

11. In contrast with Thunderstorm case Theta E increases with height
in the lower levels indicating stability (no convective Instability).

12. Instability Indices from 05/18Z
Showalter Index using -7 = No convective rain
Showalter Index using Skew T= No convective rain
Showalter Index using -4 = No convective rain
KIndex = % probability of Thunderstorms
ModKIndex = % probability of Thunderstorms
TTIndex = Severe Weather Probability unlikely
SWEAT =No wind data available

13. Conclusions Stability indices are only a forecasting tool.
Don’t depend on a single index.
Severe weather indices only indicate the potential for convection.
Determination of the convective instability using e seemed to be a better indicator in our region.